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

[Linus Torvalds]

On Thu, Jan 25, 2018 at 12:32 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> So I'd *much* rather have some way to tell KASAN that word-at-a-time
>> is going on. Because that approach definitely makes a difference in
>> other places.
>
> The other option was to use READ_ONCE_NOCHECK().

How about just using the same accessor that we do for the dcache case.
That gives a reasonable example of the whole word-at-a-time model, and
should be good.

COMPLETELY UNTESTED patch attached. This needs to be verified.

It does limit the word-at-a-time code to the architectures that select
both HAVE_EFFICIENT_UNALIGNED_ACCESS and DCACHE_WORD_ACCESS, but that
seems a reasonable choice anyway.

               Linus
 lib/string.c | 28 +++++++---------------------
 1 file changed, 7 insertions(+), 21 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 64a9e33f1daa..25a3c200307e 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -177,33 +177,18 @@ 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)) {
+#if CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS && CONFIG_DCACHE_WORD_ACCESS
+{
+	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
+	while (count >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-		c = *(unsigned long *)(src+res);
+		c = load_unaligned_zeropad(src+res);
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
@@ -213,8 +198,9 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 		*(unsigned long *)(dest+res) = c;
 		res += sizeof(unsigned long);
 		count -= sizeof(unsigned long);
-		max -= sizeof(unsigned long);
 	}
+}
+#endif
 
 	while (count) {
 		char c;