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

[Linus Torvalds]

On Thu, Jan 25, 2018 at 12:32 AM, Dmitry Vyukov <dvyu...@google.com> wrote:
> On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
> <torva...@linux-foundation.org> 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;