Commit a5c21dce authored by Will Deacon's avatar Will Deacon Committed by Linus Torvalds
Browse files

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: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 319720f5

fs/dcache.c

+1 −1
Original line number Diff line number Diff line
@@ -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));

}

fs/namei.c

+1 −6
Original line number Diff line number Diff line
@@ -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);

include/linux/dcache.h

+2 −0
Original line number Diff line number Diff line
@@ -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



/*

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