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

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 <>
Signed-off-by: default avatarWill Deacon <>
Signed-off-by: default avatarLinus Torvalds <>
parent 319720f5
......@@ -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));
......@@ -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;
return fold_hash(hash);
......@@ -29,8 +29,10 @@ struct vfsmount;
/* The hash is always the low bits of hash_len */
#define HASH_LEN_DECLARE u32 hash; u32 len;
#define bytemask_from_count(cnt) (~(~0ul << (cnt)*8))
#define HASH_LEN_DECLARE u32 len; u32 hash;
#define bytemask_from_count(cnt) (~(~0ul >> (cnt)*8))
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