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 <v...@zeniv.linux.org.uk>
Signed-off-by: Will Deacon <will.dea...@arm.com>
---
v1 -> v2: moved shifting out into a macro to avoid inline the #ifdefs
I didn't bother checking for CONFIG_DCACHE_WORD_ACCESS, since
the macros are harmless enough as they are.
fs/dcache.c | 2 +-
fs/namei.c | 7 +------
include/linux/dcache.h | 2 ++
3 files changed, 4 insertions(+), 7 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 4bdb300b16e2..6055d61811d3 100644
--- a/fs/dcache.c
+++ b/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));
}
diff --git a/fs/namei.c b/fs/namei.c
index c53d3a9547f9..3531deebad30 100644
--- a/fs/namei.c
+++ b/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);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 57e87e749a48..bf72e9ac6de0 100644
--- a/include/linux/dcache.h
+++ b/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
/*
--
1.8.2.2
--
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