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Subject[PATCH] lib/strscpy: remove word-at-a-time optimization.
Date
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@virtuozzo.com

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.

Fixes: 30035e45753b7 ("string: provide strscpy()")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: <stable@vger.kernel.org>
---
lib/string.c | 38 --------------------------------------
1 file changed, 38 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 64a9e33f1daa..6205dd71aa0f 100644
--- a/lib/string.c
+++ b/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;

--
2.13.6
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