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/[email protected] 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 <[email protected]> Cc: <[email protected]> --- 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
