original development tree for Linux kernel GTP module; now long in mainline.
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CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h percpu.h is included by sched.h and module.h and thus ends up being included when building most .c files. percpu.h includes slab.h which in turn includes gfp.h making everything defined by the two files universally available and complicating inclusion dependencies. percpu.h -> slab.h dependency is about to be removed. Prepare for this change by updating users of gfp and slab facilities include those headers directly instead of assuming availability. As this conversion needs to touch large number of source files, the following script is used as the basis of conversion. http://userweb.kernel.org/~tj/misc/slabh-sweep.py The script does the followings. * Scan files for gfp and slab usages and update includes such that only the necessary includes are there. ie. if only gfp is used, gfp.h, if slab is used, slab.h. * When the script inserts a new include, it looks at the include blocks and try to put the new include such that its order conforms to its surrounding. It's put in the include block which contains core kernel includes, in the same order that the rest are ordered - alphabetical, Christmas tree, rev-Xmas-tree or at the end if there doesn't seem to be any matching order. * If the script can't find a place to put a new include (mostly because the file doesn't have fitting include block), it prints out an error message indicating which .h file needs to be added to the file. The conversion was done in the following steps. 1. The initial automatic conversion of all .c files updated slightly over 4000 files, deleting around 700 includes and adding ~480 gfp.h and ~3000 slab.h inclusions. The script emitted errors for ~400 files. 2. Each error was manually checked. Some didn't need the inclusion, some needed manual addition while adding it to implementation .h or embedding .c file was more appropriate for others. This step added inclusions to around 150 files. 3. The script was run again and the output was compared to the edits from #2 to make sure no file was left behind. 4. Several build tests were done and a couple of problems were fixed. e.g. lib/decompress_*.c used malloc/free() wrappers around slab APIs requiring slab.h to be added manually. 5. The script was run on all .h files but without automatically editing them as sprinkling gfp.h and slab.h inclusions around .h files could easily lead to inclusion dependency hell. Most gfp.h inclusion directives were ignored as stuff from gfp.h was usually wildly available and often used in preprocessor macros. Each slab.h inclusion directive was examined and added manually as necessary. 6. percpu.h was updated not to include slab.h. 7. Build test were done on the following configurations and failures were fixed. CONFIG_GCOV_KERNEL was turned off for all tests (as my distributed build env didn't work with gcov compiles) and a few more options had to be turned off depending on archs to make things build (like ipr on powerpc/64 which failed due to missing writeq). * x86 and x86_64 UP and SMP allmodconfig and a custom test config. * powerpc and powerpc64 SMP allmodconfig * sparc and sparc64 SMP allmodconfig * ia64 SMP allmodconfig * s390 SMP allmodconfig * alpha SMP allmodconfig * um on x86_64 SMP allmodconfig 8. percpu.h modifications were reverted so that it could be applied as a separate patch and serve as bisection point. Given the fact that I had only a couple of failures from tests on step 6, I'm fairly confident about the coverage of this conversion patch. If there is a breakage, it's likely to be something in one of the arch headers which should be easily discoverable easily on most builds of the specific arch. Signed-off-by: Tejun Heo <tj@kernel.org> Guess-its-ok-by: Christoph Lameter <cl@linux-foundation.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
12 years ago
CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
FS-Cache: Simplify cookie retention for fscache_objects, fixing oops Simplify the way fscache cache objects retain their cookie. The way I implemented the cookie storage handling made synchronisation a pain (ie. the object state machine can't rely on the cookie actually still being there). Instead of the the object being detached from the cookie and the cookie being freed in __fscache_relinquish_cookie(), we defer both operations: (*) The detachment of the object from the list in the cookie now takes place in fscache_drop_object() and is thus governed by the object state machine (fscache_detach_from_cookie() has been removed). (*) The release of the cookie is now in fscache_object_destroy() - which is called by the cache backend just before it frees the object. This means that the fscache_cookie struct is now available to the cache all the way through from ->alloc_object() to ->drop_object() and ->put_object() - meaning that it's no longer necessary to take object->lock to guarantee access. However, __fscache_relinquish_cookie() doesn't wait for the object to go all the way through to destruction before letting the netfs proceed. That would massively slow down the netfs. Since __fscache_relinquish_cookie() leaves the cookie around, in must therefore break all attachments to the netfs - which includes ->def, ->netfs_data and any outstanding page read/writes. To handle this, struct fscache_cookie now has an n_active counter: (1) This starts off initialised to 1. (2) Any time the cache needs to get at the netfs data, it calls fscache_use_cookie() to increment it - if it is not zero. If it was zero, then access is not permitted. (3) When the cache has finished with the data, it calls fscache_unuse_cookie() to decrement it. This does a wake-up on it if it reaches 0. (4) __fscache_relinquish_cookie() decrements n_active and then waits for it to reach 0. The initialisation to 1 in step (1) ensures that we only get wake ups when we're trying to get rid of the cookie. This leaves __fscache_relinquish_cookie() a lot simpler. *** This fixes a problem in the current code whereby if fscache_invalidate() is followed sufficiently quickly by fscache_relinquish_cookie() then it is possible for __fscache_relinquish_cookie() to have detached the cookie from the object and cleared the pointer before a thread is dispatched to process the invalidation state in the object state machine. Since the pending write clearance was deferred to the invalidation state to make it asynchronous, we need to either wait in relinquishment for the stores tree to be cleared in the invalidation state or we need to handle the clearance in relinquishment. Further, if the relinquishment code does clear the tree, then the invalidation state need to make the clearance contingent on still having the cookie to hand (since that's where the tree is rooted) and we have to prevent the cookie from disappearing for the duration. This can lead to an oops like the following: BUG: unable to handle kernel NULL pointer dereference at 000000000000000c ... RIP: 0010:[<ffffffff8151023e>] _spin_lock+0xe/0x30 ... CR2: 000000000000000c ... ... Process kslowd002 (...) .... Call Trace: [<ffffffffa01c3278>] fscache_invalidate_writes+0x38/0xd0 [fscache] [<ffffffff810096f0>] ? __switch_to+0xd0/0x320 [<ffffffff8105e759>] ? find_busiest_queue+0x69/0x150 [<ffffffff8110ddd4>] ? slow_work_enqueue+0x104/0x180 [<ffffffffa01c1303>] fscache_object_slow_work_execute+0x5e3/0x9d0 [fscache] [<ffffffff81096b67>] ? bit_waitqueue+0x17/0xd0 [<ffffffff8110e233>] slow_work_execute+0x233/0x310 [<ffffffff8110e515>] slow_work_thread+0x205/0x360 [<ffffffff81096ca0>] ? autoremove_wake_function+0x0/0x40 [<ffffffff8110e310>] ? slow_work_thread+0x0/0x360 [<ffffffff81096936>] kthread+0x96/0xa0 [<ffffffff8100c0ca>] child_rip+0xa/0x20 [<ffffffff810968a0>] ? kthread+0x0/0xa0 [<ffffffff8100c0c0>] ? child_rip+0x0/0x20 The parameter to fscache_invalidate_writes() was object->cookie which is NULL. Signed-off-by: David Howells <dhowells@redhat.com> Tested-By: Milosz Tanski <milosz@adfin.com> Acked-by: Jeff Layton <jlayton@redhat.com>
9 years ago
CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
FS-Cache: Simplify cookie retention for fscache_objects, fixing oops Simplify the way fscache cache objects retain their cookie. The way I implemented the cookie storage handling made synchronisation a pain (ie. the object state machine can't rely on the cookie actually still being there). Instead of the the object being detached from the cookie and the cookie being freed in __fscache_relinquish_cookie(), we defer both operations: (*) The detachment of the object from the list in the cookie now takes place in fscache_drop_object() and is thus governed by the object state machine (fscache_detach_from_cookie() has been removed). (*) The release of the cookie is now in fscache_object_destroy() - which is called by the cache backend just before it frees the object. This means that the fscache_cookie struct is now available to the cache all the way through from ->alloc_object() to ->drop_object() and ->put_object() - meaning that it's no longer necessary to take object->lock to guarantee access. However, __fscache_relinquish_cookie() doesn't wait for the object to go all the way through to destruction before letting the netfs proceed. That would massively slow down the netfs. Since __fscache_relinquish_cookie() leaves the cookie around, in must therefore break all attachments to the netfs - which includes ->def, ->netfs_data and any outstanding page read/writes. To handle this, struct fscache_cookie now has an n_active counter: (1) This starts off initialised to 1. (2) Any time the cache needs to get at the netfs data, it calls fscache_use_cookie() to increment it - if it is not zero. If it was zero, then access is not permitted. (3) When the cache has finished with the data, it calls fscache_unuse_cookie() to decrement it. This does a wake-up on it if it reaches 0. (4) __fscache_relinquish_cookie() decrements n_active and then waits for it to reach 0. The initialisation to 1 in step (1) ensures that we only get wake ups when we're trying to get rid of the cookie. This leaves __fscache_relinquish_cookie() a lot simpler. *** This fixes a problem in the current code whereby if fscache_invalidate() is followed sufficiently quickly by fscache_relinquish_cookie() then it is possible for __fscache_relinquish_cookie() to have detached the cookie from the object and cleared the pointer before a thread is dispatched to process the invalidation state in the object state machine. Since the pending write clearance was deferred to the invalidation state to make it asynchronous, we need to either wait in relinquishment for the stores tree to be cleared in the invalidation state or we need to handle the clearance in relinquishment. Further, if the relinquishment code does clear the tree, then the invalidation state need to make the clearance contingent on still having the cookie to hand (since that's where the tree is rooted) and we have to prevent the cookie from disappearing for the duration. This can lead to an oops like the following: BUG: unable to handle kernel NULL pointer dereference at 000000000000000c ... RIP: 0010:[<ffffffff8151023e>] _spin_lock+0xe/0x30 ... CR2: 000000000000000c ... ... Process kslowd002 (...) .... Call Trace: [<ffffffffa01c3278>] fscache_invalidate_writes+0x38/0xd0 [fscache] [<ffffffff810096f0>] ? __switch_to+0xd0/0x320 [<ffffffff8105e759>] ? find_busiest_queue+0x69/0x150 [<ffffffff8110ddd4>] ? slow_work_enqueue+0x104/0x180 [<ffffffffa01c1303>] fscache_object_slow_work_execute+0x5e3/0x9d0 [fscache] [<ffffffff81096b67>] ? bit_waitqueue+0x17/0xd0 [<ffffffff8110e233>] slow_work_execute+0x233/0x310 [<ffffffff8110e515>] slow_work_thread+0x205/0x360 [<ffffffff81096ca0>] ? autoremove_wake_function+0x0/0x40 [<ffffffff8110e310>] ? slow_work_thread+0x0/0x360 [<ffffffff81096936>] kthread+0x96/0xa0 [<ffffffff8100c0ca>] child_rip+0xa/0x20 [<ffffffff810968a0>] ? kthread+0x0/0xa0 [<ffffffff8100c0c0>] ? child_rip+0x0/0x20 The parameter to fscache_invalidate_writes() was object->cookie which is NULL. Signed-off-by: David Howells <dhowells@redhat.com> Tested-By: Milosz Tanski <milosz@adfin.com> Acked-by: Jeff Layton <jlayton@redhat.com>
9 years ago
CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
CacheFiles: A cache that backs onto a mounted filesystem Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a backing store for the cache. CacheFiles uses a userspace daemon to do some of the cache management - such as reaping stale nodes and culling. This is called cachefilesd and lives in /sbin. The source for the daemon can be downloaded from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.c And an example configuration from: http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf The filesystem and data integrity of the cache are only as good as those of the filesystem providing the backing services. Note that CacheFiles does not attempt to journal anything since the journalling interfaces of the various filesystems are very specific in nature. CacheFiles creates a misc character device - "/dev/cachefiles" - that is used to communication with the daemon. Only one thing may have this open at once, and whilst it is open, a cache is at least partially in existence. The daemon opens this and sends commands down it to control the cache. CacheFiles is currently limited to a single cache. CacheFiles attempts to maintain at least a certain percentage of free space on the filesystem, shrinking the cache by culling the objects it contains to make space if necessary - see the "Cache Culling" section. This means it can be placed on the same medium as a live set of data, and will expand to make use of spare space and automatically contract when the set of data requires more space. ============ REQUIREMENTS ============ The use of CacheFiles and its daemon requires the following features to be available in the system and in the cache filesystem: - dnotify. - extended attributes (xattrs). - openat() and friends. - bmap() support on files in the filesystem (FIBMAP ioctl). - The use of bmap() to detect a partial page at the end of the file. It is strongly recommended that the "dir_index" option is enabled on Ext3 filesystems being used as a cache. ============= CONFIGURATION ============= The cache is configured by a script in /etc/cachefilesd.conf. These commands set up cache ready for use. The following script commands are available: (*) brun <N>% (*) bcull <N>% (*) bstop <N>% (*) frun <N>% (*) fcull <N>% (*) fstop <N>% Configure the culling limits. Optional. See the section on culling The defaults are 7% (run), 5% (cull) and 1% (stop) respectively. The commands beginning with a 'b' are file space (block) limits, those beginning with an 'f' are file count limits. (*) dir <path> Specify the directory containing the root of the cache. Mandatory. (*) tag <name> Specify a tag to FS-Cache to use in distinguishing multiple caches. Optional. The default is "CacheFiles". (*) debug <mask> Specify a numeric bitmask to control debugging in the kernel module. Optional. The default is zero (all off). The following values can be OR'd into the mask to collect various information: 1 Turn on trace of function entry (_enter() macros) 2 Turn on trace of function exit (_leave() macros) 4 Turn on trace of internal debug points (_debug()) This mask can also be set through sysfs, eg: echo 5 >/sys/modules/cachefiles/parameters/debug ================== STARTING THE CACHE ================== The cache is started by running the daemon. The daemon opens the cache device, configures the cache and tells it to begin caching. At that point the cache binds to fscache and the cache becomes live. The daemon is run as follows: /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>] The flags are: (*) -d Increase the debugging level. This can be specified multiple times and is cumulative with itself. (*) -s Send messages to stderr instead of syslog. (*) -n Don't daemonise and go into background. (*) -f <configfile> Use an alternative configuration file rather than the default one. =============== THINGS TO AVOID =============== Do not mount other things within the cache as this will cause problems. The kernel module contains its own very cut-down path walking facility that ignores mountpoints, but the daemon can't avoid them. Do not create, rename or unlink files and directories in the cache whilst the cache is active, as this may cause the state to become uncertain. Renaming files in the cache might make objects appear to be other objects (the filename is part of the lookup key). Do not change or remove the extended attributes attached to cache files by the cache as this will cause the cache state management to get confused. Do not create files or directories in the cache, lest the cache get confused or serve incorrect data. Do not chmod files in the cache. The module creates things with minimal permissions to prevent random users being able to access them directly. ============= CACHE CULLING ============= The cache may need culling occasionally to make space. This involves discarding objects from the cache that have been used less recently than anything else. Culling is based on the access time of data objects. Empty directories are culled if not in use. Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying filesystem. There are six "limits": (*) brun (*) frun If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off. (*) bcull (*) fcull If the amount of available space or the number of available files in the cache falls below either of these limits, then culling is started. (*) bstop (*) fstop If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again. These must be configured thusly: 0 <= bstop < bcull < brun < 100 0 <= fstop < fcull < frun < 100 Note that these are percentages of available space and available files, and do _not_ appear as 100 minus the percentage displayed by the "df" program. The userspace daemon scans the cache to build up a table of cullable objects. These are then culled in least recently used order. A new scan of the cache is started as soon as space is made in the table. Objects will be skipped if their atimes have changed or if the kernel module says it is still using them. =============== CACHE STRUCTURE =============== The CacheFiles module will create two directories in the directory it was given: (*) cache/ (*) graveyard/ The active cache objects all reside in the first directory. The CacheFiles kernel module moves any retired or culled objects that it can't simply unlink to the graveyard from which the daemon will actually delete them. The daemon uses dnotify to monitor the graveyard directory, and will delete anything that appears therein. The module represents index objects as directories with the filename "I..." or "J...". Note that the "cache/" directory is itself a special index. Data objects are represented as files if they have no children, or directories if they do. Their filenames all begin "D..." or "E...". If represented as a directory, data objects will have a file in the directory called "data" that actually holds the data. Special objects are similar to data objects, except their filenames begin "S..." or "T...". If an object has children, then it will be represented as a directory. Immediately in the representative directory are a collection of directories named for hash values of the child object keys with an '@' prepended. Into this directory, if possible, will be placed the representations of the child objects: INDEX INDEX INDEX DATA FILES ========= ========== ================================= ================ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400 cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry If the key is so long that it exceeds NAME_MAX with the decorations added on to it, then it will be cut into pieces, the first few of which will be used to make a nest of directories, and the last one of which will be the objects inside the last directory. The names of the intermediate directories will have '+' prepended: J1223/@23/+xy...z/+kl...m/Epqr Note that keys are raw data, and not only may they exceed NAME_MAX in size, they may also contain things like '/' and NUL characters, and so they may not be suitable for turning directly into a filename. To handle this, CacheFiles will use a suitably printable filename directly and "base-64" encode ones that aren't directly suitable. The two versions of object filenames indicate the encoding: OBJECT TYPE PRINTABLE ENCODED =============== =============== =============== Index "I..." "J..." Data "D..." "E..." Special "S..." "T..." Intermediate directories are always "@" or "+" as appropriate. Each object in the cache has an extended attribute label that holds the object type ID (required to distinguish special objects) and the auxiliary data from the netfs. The latter is used to detect stale objects in the cache and update or retire them. Note that CacheFiles will erase from the cache any file it doesn't recognise or any file of an incorrect type (such as a FIFO file or a device file). ========================== SECURITY MODEL AND SELINUX ========================== CacheFiles is implemented to deal properly with the LSM security features of the Linux kernel and the SELinux facility. One of the problems that CacheFiles faces is that it is generally acting on behalf of a process, and running in that process's context, and that includes a security context that is not appropriate for accessing the cache - either because the files in the cache are inaccessible to that process, or because if the process creates a file in the cache, that file may be inaccessible to other processes. The way CacheFiles works is to temporarily change the security context (fsuid, fsgid and actor security label) that the process acts as - without changing the security context of the process when it the target of an operation performed by some other process (so signalling and suchlike still work correctly). When the CacheFiles module is asked to bind to its cache, it: (1) Finds the security label attached to the root cache directory and uses that as the security label with which it will create files. By default, this is: cachefiles_var_t (2) Finds the security label of the process which issued the bind request (presumed to be the cachefilesd daemon), which by default will be: cachefilesd_t and asks LSM to supply a security ID as which it should act given the daemon's label. By default, this will be: cachefiles_kernel_t SELinux transitions the daemon's security ID to the module's security ID based on a rule of this form in the policy. type_transition <daemon's-ID> kernel_t : process <module's-ID>; For instance: type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t; The module's security ID gives it permission to create, move and remove files and directories in the cache, to find and access directories and files in the cache, to set and access extended attributes on cache objects, and to read and write files in the cache. The daemon's security ID gives it only a very restricted set of permissions: it may scan directories, stat files and erase files and directories. It may not read or write files in the cache, and so it is precluded from accessing the data cached therein; nor is it permitted to create new files in the cache. There are policy source files available in: http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2 and later versions. In that tarball, see the files: cachefilesd.te cachefilesd.fc cachefilesd.if They are built and installed directly by the RPM. If a non-RPM based system is being used, then copy the above files to their own directory and run: make -f /usr/share/selinux/devel/Makefile semodule -i cachefilesd.pp You will need checkpolicy and selinux-policy-devel installed prior to the build. By default, the cache is located in /var/fscache, but if it is desirable that it should be elsewhere, than either the above policy files must be altered, or an auxiliary policy must be installed to label the alternate location of the cache. For instructions on how to add an auxiliary policy to enable the cache to be located elsewhere when SELinux is in enforcing mode, please see: /usr/share/doc/cachefilesd-*/move-cache.txt When the cachefilesd rpm is installed; alternatively, the document can be found in the sources. ================== A NOTE ON SECURITY ================== CacheFiles makes use of the split security in the task_struct. It allocates its own task_security structure, and redirects current->act_as to point to it when it acts on behalf of another process, in that process's context. The reason it does this is that it calls vfs_mkdir() and suchlike rather than bypassing security and calling inode ops directly. Therefore the VFS and LSM may deny the CacheFiles access to the cache data because under some circumstances the caching code is running in the security context of whatever process issued the original syscall on the netfs. Furthermore, should CacheFiles create a file or directory, the security parameters with that object is created (UID, GID, security label) would be derived from that process that issued the system call, thus potentially preventing other processes from accessing the cache - including CacheFiles's cache management daemon (cachefilesd). What is required is to temporarily override the security of the process that issued the system call. We can't, however, just do an in-place change of the security data as that affects the process as an object, not just as a subject. This means it may lose signals or ptrace events for example, and affects what the process looks like in /proc. So CacheFiles makes use of a logical split in the security between the objective security (task->sec) and the subjective security (task->act_as). The objective security holds the intrinsic security properties of a process and is never overridden. This is what appears in /proc, and is what is used when a process is the target of an operation by some other process (SIGKILL for example). The subjective security holds the active security properties of a process, and may be overridden. This is not seen externally, and is used whan a process acts upon another object, for example SIGKILLing another process or opening a file. LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request for CacheFiles to run in a context of a specific security label, or to create files and directories with another security label. This documentation is added by the patch to: Documentation/filesystems/caching/cachefiles.txt Signed-Off-By: David Howells <dhowells@redhat.com> Acked-by: Steve Dickson <steved@redhat.com> Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com> Acked-by: Al Viro <viro@zeniv.linux.org.uk> Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
13 years ago
  1. /* CacheFiles extended attribute management
  2. *
  3. * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
  4. * Written by David Howells (dhowells@redhat.com)
  5. *
  6. * This program is free software; you can redistribute it and/or
  7. * modify it under the terms of the GNU General Public Licence
  8. * as published by the Free Software Foundation; either version
  9. * 2 of the Licence, or (at your option) any later version.
  10. */
  11. #include <linux/module.h>
  12. #include <linux/sched.h>
  13. #include <linux/file.h>
  14. #include <linux/fs.h>
  15. #include <linux/fsnotify.h>
  16. #include <linux/quotaops.h>
  17. #include <linux/xattr.h>
  18. #include <linux/slab.h>
  19. #include "internal.h"
  20. static const char cachefiles_xattr_cache[] =
  21. XATTR_USER_PREFIX "CacheFiles.cache";
  22. /*
  23. * check the type label on an object
  24. * - done using xattrs
  25. */
  26. int cachefiles_check_object_type(struct cachefiles_object *object)
  27. {
  28. struct dentry *dentry = object->dentry;
  29. char type[3], xtype[3];
  30. int ret;
  31. ASSERT(dentry);
  32. ASSERT(dentry->d_inode);
  33. if (!object->fscache.cookie)
  34. strcpy(type, "C3");
  35. else
  36. snprintf(type, 3, "%02x", object->fscache.cookie->def->type);
  37. _enter("%p{%s}", object, type);
  38. /* attempt to install a type label directly */
  39. ret = vfs_setxattr(dentry, cachefiles_xattr_cache, type, 2,
  40. XATTR_CREATE);
  41. if (ret == 0) {
  42. _debug("SET"); /* we succeeded */
  43. goto error;
  44. }
  45. if (ret != -EEXIST) {
  46. kerror("Can't set xattr on %*.*s [%lu] (err %d)",
  47. dentry->d_name.len, dentry->d_name.len,
  48. dentry->d_name.name, dentry->d_inode->i_ino,
  49. -ret);
  50. goto error;
  51. }
  52. /* read the current type label */
  53. ret = vfs_getxattr(dentry, cachefiles_xattr_cache, xtype, 3);
  54. if (ret < 0) {
  55. if (ret == -ERANGE)
  56. goto bad_type_length;
  57. kerror("Can't read xattr on %*.*s [%lu] (err %d)",
  58. dentry->d_name.len, dentry->d_name.len,
  59. dentry->d_name.name, dentry->d_inode->i_ino,
  60. -ret);
  61. goto error;
  62. }
  63. /* check the type is what we're expecting */
  64. if (ret != 2)
  65. goto bad_type_length;
  66. if (xtype[0] != type[0] || xtype[1] != type[1])
  67. goto bad_type;
  68. ret = 0;
  69. error:
  70. _leave(" = %d", ret);
  71. return ret;
  72. bad_type_length:
  73. kerror("Cache object %lu type xattr length incorrect",
  74. dentry->d_inode->i_ino);
  75. ret = -EIO;
  76. goto error;
  77. bad_type:
  78. xtype[2] = 0;
  79. kerror("Cache object %*.*s [%lu] type %s not %s",
  80. dentry->d_name.len, dentry->d_name.len,
  81. dentry->d_name.name, dentry->d_inode->i_ino,
  82. xtype, type);
  83. ret = -EIO;
  84. goto error;
  85. }
  86. /*
  87. * set the state xattr on a cache file
  88. */
  89. int cachefiles_set_object_xattr(struct cachefiles_object *object,
  90. struct cachefiles_xattr *auxdata)
  91. {
  92. struct dentry *dentry = object->dentry;
  93. int ret;
  94. ASSERT(dentry);
  95. _enter("%p,#%d", object, auxdata->len);
  96. /* attempt to install the cache metadata directly */
  97. _debug("SET #%u", auxdata->len);
  98. ret = vfs_setxattr(dentry, cachefiles_xattr_cache,
  99. &auxdata->type, auxdata->len,
  100. XATTR_CREATE);
  101. if (ret < 0 && ret != -ENOMEM)
  102. cachefiles_io_error_obj(
  103. object,
  104. "Failed to set xattr with error %d", ret);
  105. _leave(" = %d", ret);
  106. return ret;
  107. }
  108. /*
  109. * update the state xattr on a cache file
  110. */
  111. int cachefiles_update_object_xattr(struct cachefiles_object *object,
  112. struct cachefiles_xattr *auxdata)
  113. {
  114. struct dentry *dentry = object->dentry;
  115. int ret;
  116. ASSERT(dentry);
  117. _enter("%p,#%d", object, auxdata->len);
  118. /* attempt to install the cache metadata directly */
  119. _debug("SET #%u", auxdata->len);
  120. ret = vfs_setxattr(dentry, cachefiles_xattr_cache,
  121. &auxdata->type, auxdata->len,
  122. XATTR_REPLACE);
  123. if (ret < 0 && ret != -ENOMEM)
  124. cachefiles_io_error_obj(
  125. object,
  126. "Failed to update xattr with error %d", ret);
  127. _leave(" = %d", ret);
  128. return ret;
  129. }
  130. /*
  131. * check the consistency between the backing cache and the FS-Cache cookie
  132. */
  133. int cachefiles_check_auxdata(struct cachefiles_object *object)
  134. {
  135. struct cachefiles_xattr *auxbuf;
  136. enum fscache_checkaux validity;
  137. struct dentry *dentry = object->dentry;
  138. ssize_t xlen;
  139. int ret;
  140. ASSERT(dentry);
  141. ASSERT(dentry->d_inode);
  142. ASSERT(object->fscache.cookie->def->check_aux);
  143. auxbuf = kmalloc(sizeof(struct cachefiles_xattr) + 512, GFP_KERNEL);
  144. if (!auxbuf)
  145. return -ENOMEM;
  146. xlen = vfs_getxattr(dentry, cachefiles_xattr_cache,
  147. &auxbuf->type, 512 + 1);
  148. ret = -ESTALE;
  149. if (xlen < 1 ||
  150. auxbuf->type != object->fscache.cookie->def->type)
  151. goto error;
  152. xlen--;
  153. validity = fscache_check_aux(&object->fscache, &auxbuf->data, xlen);
  154. if (validity != FSCACHE_CHECKAUX_OKAY)
  155. goto error;
  156. ret = 0;
  157. error:
  158. kfree(auxbuf);
  159. return ret;
  160. }
  161. /*
  162. * check the state xattr on a cache file
  163. * - return -ESTALE if the object should be deleted
  164. */
  165. int cachefiles_check_object_xattr(struct cachefiles_object *object,
  166. struct cachefiles_xattr *auxdata)
  167. {
  168. struct cachefiles_xattr *auxbuf;
  169. struct dentry *dentry = object->dentry;
  170. int ret;
  171. _enter("%p,#%d", object, auxdata->len);
  172. ASSERT(dentry);
  173. ASSERT(dentry->d_inode);
  174. auxbuf = kmalloc(sizeof(struct cachefiles_xattr) + 512, cachefiles_gfp);
  175. if (!auxbuf) {
  176. _leave(" = -ENOMEM");
  177. return -ENOMEM;
  178. }
  179. /* read the current type label */
  180. ret = vfs_getxattr(dentry, cachefiles_xattr_cache,
  181. &auxbuf->type, 512 + 1);
  182. if (ret < 0) {
  183. if (ret == -ENODATA)
  184. goto stale; /* no attribute - power went off
  185. * mid-cull? */
  186. if (ret == -ERANGE)
  187. goto bad_type_length;
  188. cachefiles_io_error_obj(object,
  189. "Can't read xattr on %lu (err %d)",
  190. dentry->d_inode->i_ino, -ret);
  191. goto error;
  192. }
  193. /* check the on-disk object */
  194. if (ret < 1)
  195. goto bad_type_length;
  196. if (auxbuf->type != auxdata->type)
  197. goto stale;
  198. auxbuf->len = ret;
  199. /* consult the netfs */
  200. if (object->fscache.cookie->def->check_aux) {
  201. enum fscache_checkaux result;
  202. unsigned int dlen;
  203. dlen = auxbuf->len - 1;
  204. _debug("checkaux %s #%u",
  205. object->fscache.cookie->def->name, dlen);
  206. result = fscache_check_aux(&object->fscache,
  207. &auxbuf->data, dlen);
  208. switch (result) {
  209. /* entry okay as is */
  210. case FSCACHE_CHECKAUX_OKAY:
  211. goto okay;
  212. /* entry requires update */
  213. case FSCACHE_CHECKAUX_NEEDS_UPDATE:
  214. break;
  215. /* entry requires deletion */
  216. case FSCACHE_CHECKAUX_OBSOLETE:
  217. goto stale;
  218. default:
  219. BUG();
  220. }
  221. /* update the current label */
  222. ret = vfs_setxattr(dentry, cachefiles_xattr_cache,
  223. &auxdata->type, auxdata->len,
  224. XATTR_REPLACE);
  225. if (ret < 0) {
  226. cachefiles_io_error_obj(object,
  227. "Can't update xattr on %lu"
  228. " (error %d)",
  229. dentry->d_inode->i_ino, -ret);
  230. goto error;
  231. }
  232. }
  233. okay:
  234. ret = 0;
  235. error:
  236. kfree(auxbuf);
  237. _leave(" = %d", ret);
  238. return ret;
  239. bad_type_length:
  240. kerror("Cache object %lu xattr length incorrect",
  241. dentry->d_inode->i_ino);
  242. ret = -EIO;
  243. goto error;
  244. stale:
  245. ret = -ESTALE;
  246. goto error;
  247. }
  248. /*
  249. * remove the object's xattr to mark it stale
  250. */
  251. int cachefiles_remove_object_xattr(struct cachefiles_cache *cache,
  252. struct dentry *dentry)
  253. {
  254. int ret;
  255. ret = vfs_removexattr(dentry, cachefiles_xattr_cache);
  256. if (ret < 0) {
  257. if (ret == -ENOENT || ret == -ENODATA)
  258. ret = 0;
  259. else if (ret != -ENOMEM)
  260. cachefiles_io_error(cache,
  261. "Can't remove xattr from %lu"
  262. " (error %d)",
  263. dentry->d_inode->i_ino, -ret);
  264. }
  265. _leave(" = %d", ret);
  266. return ret;
  267. }