Browse Source

dcache: allow word-at-a-time name hashing with big-endian CPUs

When explicitly hashing the end of a string with the word-at-a-time
interface, we have to be careful which end of the word we pick up.

On big-endian CPUs, the upper-bits will contain the data we're after, so
ensure we generate our masks accordingly (and avoid hashing whatever
random junk may have been sitting after the string).

This patch adds a new dcache helper, bytemask_from_count, which creates
a mask appropriate for the CPU endianness.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
master
Will Deacon 8 years ago
committed by Linus Torvalds
parent
commit
a5c21dcefa
  1. 2
      fs/dcache.c
  2. 7
      fs/namei.c
  3. 2
      include/linux/dcache.h

2
fs/dcache.c

@ -192,7 +192,7 @@ static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char
if (!tcount)
return 0;
}
mask = ~(~0ul << tcount*8);
mask = bytemask_from_count(tcount);
return unlikely(!!((a ^ b) & mask));
}

7
fs/namei.c

@ -1598,11 +1598,6 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
* do a "get_unaligned()" if this helps and is sufficiently
* fast.
*
* - Little-endian machines (so that we can generate the mask
* of low bytes efficiently). Again, we *could* do a byte
* swapping load on big-endian architectures if that is not
* expensive enough to make the optimization worthless.
*
* - non-CONFIG_DEBUG_PAGEALLOC configurations (so that we
* do not trap on the (extremely unlikely) case of a page
* crossing operation.
@ -1646,7 +1641,7 @@ unsigned int full_name_hash(const unsigned char *name, unsigned int len)
if (!len)
goto done;
}
mask = ~(~0ul << len*8);
mask = bytemask_from_count(len);
hash += mask & a;
done:
return fold_hash(hash);

2
include/linux/dcache.h

@ -29,8 +29,10 @@ struct vfsmount;
/* The hash is always the low bits of hash_len */
#ifdef __LITTLE_ENDIAN
#define HASH_LEN_DECLARE u32 hash; u32 len;
#define bytemask_from_count(cnt) (~(~0ul << (cnt)*8))
#else
#define HASH_LEN_DECLARE u32 len; u32 hash;
#define bytemask_from_count(cnt) (~(~0ul >> (cnt)*8))
#endif
/*

Loading…
Cancel
Save