Re: [PATCH v4] x86: Fix bug in strncmp-evex and strncmp-avx2 [BZ #28895]

Re: [PATCH v4] x86: Fix bug in strncmp-evex and strncmp-avx2 [BZ #28895]

[Sunil Pandey]

On Tue, Feb 15, 2022 at 6:29 PM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Tue, Feb 15, 2022 at 6:27 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > Logic can read before the start of `s1` / `s2` if both `s1` and `s2`
> > are near the start of a page. To avoid having the result contimated by
> > these comparisons the `strcmp` variants would mask off these
> > comparisons. This was missing in the `strncmp` variants causing
> > the bug. This commit adds the masking to `strncmp` so that out of
> > range comparisons don't affect the result.
> >
> > test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass as
> > well a full xcheck on x86_64 linux.
> > ---
> >  string/test-strncmp.c                  | 23 +++++++++++++++++++++++
> >  sysdeps/x86_64/multiarch/strcmp-avx2.S |  1 +
> >  sysdeps/x86_64/multiarch/strcmp-evex.S |  1 +
> >  3 files changed, 25 insertions(+)
> >
> > diff --git a/string/test-strncmp.c b/string/test-strncmp.c
> > index 831cb77893..df7cea4068 100644
> > --- a/string/test-strncmp.c
> > +++ b/string/test-strncmp.c
> > @@ -423,6 +423,28 @@ check3 (void)
> >         }
> >  }
> >
> > +static void
> > +check4 (void)
> > +{
> > +  /* To trigger bug 28895; We need 1) both s1 and s2 to be within 32 bytes of
> > +     the end of the page. 2) For there to be no mismatch/null byte before the
> > +     first page cross. 3) For length (`n`) to be large enough for one string to
> > +     cross the page. And 4) for there to be either mismatch/null bytes before
> > +     the start of the strings.  */
> > +
> > +  size_t size = 10;
> > +  size_t addr_mask = (getpagesize () - 1) ^ (sizeof (CHAR) - 1);
> > +  CHAR *s1 = (CHAR *)(buf1 + (addr_mask & 0xffa));
> > +  CHAR *s2 = (CHAR *)(buf2 + (addr_mask & 0xfed));
> > +  int exp_result;
> > +
> > +  STRCPY (s1, L ("tst-tlsmod%"));
> > +  STRCPY (s2, L ("tst-tls-manydynamic73mod"));
> > +  exp_result = SIMPLE_STRNCMP (s1, s2, size);
> > +  FOR_EACH_IMPL (impl, 0)
> > +  check_result (impl, s1, s2, size, exp_result);
> > +}
> > +
> >  static void
> >  check_overflow (void)
> >  {
> > @@ -546,6 +568,7 @@ test_main (void)
> >    check1 ();
> >    check2 ();
> >    check3 ();
> > +  check4 ();
> >
> >    printf ("%23s", "");
> >    FOR_EACH_IMPL (impl, 0)
> > diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> > index 99e5349be8..07a5a2c889 100644
> > --- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
> > +++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> > @@ -661,6 +661,7 @@ L(ret8):
> >  # ifdef USE_AS_STRNCMP
> >         .p2align 4,, 10
> >  L(return_page_cross_end_check):
> > +       andl    %r10d, %ecx
> >         tzcntl  %ecx, %ecx
> >         leal    -VEC_SIZE(%rax, %rcx), %ecx
> >         cmpl    %ecx, %edx
> > diff --git a/sysdeps/x86_64/multiarch/strcmp-evex.S b/sysdeps/x86_64/multiarch/strcmp-evex.S
> > index 6f42e3155a..56d8c118e4 100644
> > --- a/sysdeps/x86_64/multiarch/strcmp-evex.S
> > +++ b/sysdeps/x86_64/multiarch/strcmp-evex.S
> > @@ -689,6 +689,7 @@ L(ret8):
> >  # ifdef USE_AS_STRNCMP
> >         .p2align 4,, 10
> >  L(return_page_cross_end_check):
> > +       andl    %r10d, %ecx
> >         tzcntl  %ecx, %ecx
> >         leal    -VEC_SIZE(%rax, %rcx, SIZE_OF_CHAR), %ecx
> >  #  ifdef USE_AS_WCSCMP
> > --
> > 2.25.1
> >
>
> LGTM.
>
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
>
> Thanks.
>
> --
> H.J.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil