The patch titled
Subject: lib/strscpy: remove word-at-a-time optimization.
has been added to the -mm tree. Its filename is
lib-strscpy-remove-word-at-a-time-optimization.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/lib-strscpy-remove-word-at-a-time-optimization.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/lib-strscpy-remove-word-at-a-time-optimization.patch
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------------------------------------------------------
From: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Subject: lib/strscpy: remove word-at-a-time optimization.
strscpy() performs the word-at-a-time optimistic reads. So it may may
access the memory past the end of the object, which is perfectly fine
since strscpy() doesn't use that (past-the-end) data and makes sure the
optimistic read won't cross a page boundary.
But KASAN doesn't know anything about that so it will complain. There are
several possible ways to address this issue, but none are perfect. See
https://lkml.kernel.org/r/9f0a9cf6-51f7-cd1f-5dc6-6d510a7b8ec4@xxxxxxxxxxxxx
It seems the best solution is to simply disable word-at-a-time
optimization. My trivial testing shows that byte-at-a-time could be up to
x4.3 times slower than word-at-a-time. It may seems like a lot, but it's
actually ~1.2e-10 sec per symbol vs ~4.8e-10 sec per symbol on modern
hardware. And we don't use strscpy() in a performance critical paths to
copy large amounts of data, so it shouldn't matter anyway.
Link: http://lkml.kernel.org/r/20180109163745.3692-1-aryabinin@xxxxxxxxxxxxx
Fixes: 30035e45753b7 ("string: provide strscpy()")
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Eryu Guan <eguan@xxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Chris Metcalf <metcalf@xxxxxxxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/string.c | 38 --------------------------------------
1 file changed, 38 deletions(-)
diff -puN lib/string.c~lib-strscpy-remove-word-at-a-time-optimization lib/string.c
--- a/lib/string.c~lib-strscpy-remove-word-at-a-time-optimization
+++ a/lib/string.c
@@ -29,7 +29,6 @@
#include <linux/errno.h>
#include <asm/byteorder.h>
-#include <asm/word-at-a-time.h>
#include <asm/page.h>
#ifndef __HAVE_ARCH_STRNCASECMP
@@ -177,45 +176,8 @@ EXPORT_SYMBOL(strlcpy);
*/
ssize_t strscpy(char *dest, const char *src, size_t count)
{
- const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
- size_t max = count;
long res = 0;
- if (count == 0)
- return -E2BIG;
-
-#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
- /*
- * If src is unaligned, don't cross a page boundary,
- * since we don't know if the next page is mapped.
- */
- if ((long)src & (sizeof(long) - 1)) {
- size_t limit = PAGE_SIZE - ((long)src & (PAGE_SIZE - 1));
- if (limit < max)
- max = limit;
- }
-#else
- /* If src or dest is unaligned, don't do word-at-a-time. */
- if (((long) dest | (long) src) & (sizeof(long) - 1))
- max = 0;
-#endif
-
- while (max >= sizeof(unsigned long)) {
- unsigned long c, data;
-
- c = *(unsigned long *)(src+res);
- if (has_zero(c, &data, &constants)) {
- data = prep_zero_mask(c, data, &constants);
- data = create_zero_mask(data);
- *(unsigned long *)(dest+res) = c & zero_bytemask(data);
- return res + find_zero(data);
- }
- *(unsigned long *)(dest+res) = c;
- res += sizeof(unsigned long);
- count -= sizeof(unsigned long);
- max -= sizeof(unsigned long);
- }
-
while (count) {
char c;
_
Patches currently in -mm which might be from aryabinin@xxxxxxxxxxxxx are
kasan-makefile-support-llvm-style-asan-parameters.patch
lib-strscpy-remove-word-at-a-time-optimization.patch