linux-kernel - [PATCH] lib/strscpy: remove word-at-a-time optimization.

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Date:   Tue,  9 Jan 2018 19:37:45 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Eryu Guan <eguan@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chris Metcalf <metcalf@...m.mit.edu>,
        David Laight <David.Laight@...LAB.COM>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        stable@...r.kernel.org
Subject: [PATCH] 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@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@...tuozzo.com>
Cc: <stable@...r.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