Re: [PATCH v4 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy()

[Catalin Marinas <catalin.marinas@arm.com>]

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@arm.com> 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