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authorRasmus Villemoes <linux@rasmusvillemoes.dk>2018-03-01 00:19:21 +0100
committerAl Viro <viro@zeniv.linux.org.uk>2018-03-19 01:07:04 -0400
commit1c94984396dc7bc40b4f6899674eaa41f29a4f6e (patch)
treea111ee94fffcd116066eb92572e2658bd2a9d101 /fs/namei.c
parent304ec482f562885b178b370cd50340447585d1c0 (diff)
vfs: make sure struct filename->iname is word-aligned
I noticed that offsetof(struct filename, iname) is actually 28 on 64 bit platforms, so we always pass an unaligned pointer to strncpy_from_user. This is mostly a problem for those 64 bit platforms without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned accesses carry a penalty. A user-space microbenchmark doing nothing but strncpy_from_user from the same (aligned) source string runs about 5% faster when the destination is aligned. That number increases to 20% when the string is long enough (~32 bytes) that we cross a cache line boundary - that's for example the case for about half the files a "git status" in a kernel tree ends up stat'ing. This won't make any real-life workloads 5%, or even 1%, faster, but path lookup is common enough that cutting even a few cycles should be worthwhile. So ensure we always pass an aligned destination pointer to strncpy_from_user. Instead of explicit padding, simply swap the refcnt and aname members, as suggested by Al Viro. Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/namei.c')
-rw-r--r--fs/namei.c2
1 files changed, 2 insertions, 0 deletions
diff --git a/fs/namei.c b/fs/namei.c
index 921ae32dbc80..5a66e7ca5d60 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -39,6 +39,7 @@
#include <linux/bitops.h>
#include <linux/init_task.h>
#include <linux/uaccess.h>
+#include <linux/build_bug.h>
#include "internal.h"
#include "mount.h"
@@ -130,6 +131,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
struct filename *result;
char *kname;
int len;
+ BUILD_BUG_ON(offsetof(struct filename, iname) % sizeof(long) != 0);
result = audit_reusename(filename);
if (result)