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vfs: make sure struct filename->iname is word-aligned

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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>
This commit is contained in:
Rasmus Villemoes 2018-03-01 00:19:21 +01:00 committed by Al Viro
parent 304ec482f5
commit 1c94984396
2 changed files with 3 additions and 1 deletions
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2
fs/namei.c
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@ -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)

2
include/linux/fs.h
View File

@ -2380,8 +2380,8 @@ struct audit_names;
struct filename {
const char *name; /* pointer to actual string */
const __user char *uptr; /* original userland pointer */
struct audit_names *aname;
int refcnt;
struct audit_names *aname;
const char iname[];
};