On Wed, Apr 02, 2025 at 05:08:51PM -0700, Peter Collingbourne wrote: > On Wed, Apr 2, 2025 at 1:10 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > 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. > > The return value is the same with/without the patch, it's the number > of bytes copied before the null terminator (i.e. not including the > extra nulls now written). I was thinking of the -E2BIG return but you are right, the patch wouldn't change this. If, for example, you read 8 bytes across a page boundary and it faults, load_unaligned_zeropad() returns fewer characters copied, implying the source was null-terminated. read_word_at_a_time(), OTOH, panics in the next byte-at-a-time loop. But it wouldn't return -E2BIG either, so it doesn't matter for the caller. > > 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. > > So do you think that the code should continue to panic if the source > string is unterminated because of a page boundary? I don't have a > strong opinion but maybe that's something that we should only do if > some error checking option is turned on? It's mostly about keeping the current behaviour w.r.t. page boundaries. Not a strong opinion either. The change would be to not read across page boundaries. > > 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. > > By "padding" do you mean the extra (up to sizeof(unsigned long)) nulls > now written to the destination? No, I meant the padding of the source when a fault occurs. The write to the destination would only be a single '\0' byte. It's the destination safe termination vs. panic above. > > 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. > > Yes, that works, and I added a test case for that in v5. The stack > trace looks like this: Thanks for checking. -- Catalin