On Fri, Mar 28, 2025 at 05:03:36PM -0700, Peter Collingbourne wrote:
> diff --git a/lib/string.c b/lib/string.c
> index eb4486ed40d25..b632c71df1a50 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -119,6 +119,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
> if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
> return -E2BIG;
>
> +#ifndef CONFIG_DCACHE_WORD_ACCESS
> #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> /*
> * If src is unaligned, don't cross a page boundary,
> @@ -133,12 +134,14 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
> /* If src or dest is unaligned, don't do word-at-a-time. */
> if (((long) dest | (long) src) & (sizeof(long) - 1))
> max = 0;
> +#endif
> #endif
>
> /*
> - * read_word_at_a_time() below may read uninitialized bytes after the
> - * trailing zero and use them in comparisons. Disable this optimization
> - * under KMSAN to prevent false positive reports.
> + * load_unaligned_zeropad() or read_word_at_a_time() below may read
> + * uninitialized bytes after the trailing zero and use them in
> + * comparisons. Disable this optimization under KMSAN to prevent
> + * false positive reports.
> */
> if (IS_ENABLED(CONFIG_KMSAN))
> max = 0;
> @@ -146,7 +149,11 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
> while (max >= sizeof(unsigned long)) {
> unsigned long c, data;
>
> +#ifdef CONFIG_DCACHE_WORD_ACCESS
> + c = load_unaligned_zeropad(src+res);
> +#else
> c = read_word_at_a_time(src+res);
> +#endif
> if (has_zero(c, &data, &constants)) {
> data = prep_zero_mask(c, data, &constants);
> data = create_zero_mask(data);
Kees mentioned the scenario where this crosses the page boundary and we
pad the source with zeros. It's probably fine but there are 70+ cases
where the strscpy() return value is checked, I only looked at a couple.
Could we at least preserve the behaviour with regards to page boundaries
and keep the existing 'max' limiting logic? If I read the code
correctly, a fall back to reading one byte at a time from an unmapped
page would panic. We also get this behaviour if src[0] is reading from
an invalid address, though for arm64 the panic would be in
ex_handler_load_unaligned_zeropad() when count >= 8.
Reading across tag granule (but not across page boundary) and causing a
tag check fault would result in padding but we can live with this and
only architectures that do MTE-style tag checking would get the new
behaviour.
What I haven't checked is whether a tag check fault in
ex_handler_load_unaligned_zeropad() would confuse the KASAN logic for
MTE (it would be a second tag check fault while processing the first).
At a quick look, it seems ok but it might be worth checking.
--
Catalin
