Re: [PATCH v1] x86: Fix wcsnlen-avx2 page cross length comparison [BZ #29591]

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Sep 20, 2022 at 5:58 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Previous implementation was adjusting length (rsi) to match
> bytes (eax), but since there is no bound to length this can cause
> overflow.
>
> Fix is to just convert the byte-count (eax) to length by dividing by
> sizeof (wchar_t) before the comparison.
>
> Full check passes on x86-64 and build succeeds w/ and w/o multiarch.
> ---
>  string/test-strnlen.c                  | 70 +++++++++++++++-----------
>  sysdeps/x86_64/multiarch/strlen-avx2.S |  7 +--
>  2 files changed, 43 insertions(+), 34 deletions(-)
>
> diff --git a/string/test-strnlen.c b/string/test-strnlen.c
> index 4a9375112a..5cbaf4b734 100644
> --- a/string/test-strnlen.c
> +++ b/string/test-strnlen.c
> @@ -73,7 +73,7 @@ do_test (size_t align, size_t len, size_t maxlen, int max_char)
>  {
>    size_t i;
>
> -  align &= 63;
> +  align &= (getpagesize () / sizeof (CHAR) - 1);
>    if ((align + len) * sizeof (CHAR) >= page_size)
>      return;
>
> @@ -90,38 +90,50 @@ do_test (size_t align, size_t len, size_t maxlen, int max_char)
>  static void
>  do_overflow_tests (void)
>  {
> -  size_t i, j, len;
> +  size_t i, j, al_idx, repeats, len;
>    const size_t one = 1;
>    uintptr_t buf_addr = (uintptr_t) buf1;
> +  const size_t alignments[] = { 0, 1, 7, 9, 31, 33, 63, 65, 95, 97, 127, 129 };
>
> -  for (i = 0; i < 750; ++i)
> +  for (al_idx = 0; al_idx < sizeof (alignments) / sizeof (alignments[0]);
> +       al_idx++)
>      {
> -      do_test (1, i, SIZE_MAX, BIG_CHAR);
> -
> -      do_test (0, i, SIZE_MAX - i, BIG_CHAR);
> -      do_test (0, i, i - buf_addr, BIG_CHAR);
> -      do_test (0, i, -buf_addr - i, BIG_CHAR);
> -      do_test (0, i, SIZE_MAX - buf_addr - i, BIG_CHAR);
> -      do_test (0, i, SIZE_MAX - buf_addr + i, BIG_CHAR);
> -
> -      len = 0;
> -      for (j = 8 * sizeof(size_t) - 1; j ; --j)
> -        {
> -          len |= one << j;
> -          do_test (0, i, len - i, BIG_CHAR);
> -          do_test (0, i, len + i, BIG_CHAR);
> -          do_test (0, i, len - buf_addr - i, BIG_CHAR);
> -          do_test (0, i, len - buf_addr + i, BIG_CHAR);
> -
> -          do_test (0, i, ~len - i, BIG_CHAR);
> -          do_test (0, i, ~len + i, BIG_CHAR);
> -          do_test (0, i, ~len - buf_addr - i, BIG_CHAR);
> -          do_test (0, i, ~len - buf_addr + i, BIG_CHAR);
> -
> -          do_test (0, i, -buf_addr, BIG_CHAR);
> -          do_test (0, i, j - buf_addr, BIG_CHAR);
> -          do_test (0, i, -buf_addr - j, BIG_CHAR);
> -        }
> +      for (repeats = 0; repeats < 2; ++repeats)
> +       {
> +         size_t align = repeats ? (getpagesize () - alignments[al_idx])
> +                                : alignments[al_idx];
> +         align /= sizeof (CHAR);
> +         for (i = 0; i < 750; ++i)
> +           {
> +             do_test (align, i, SIZE_MAX, BIG_CHAR);
> +
> +             do_test (align, i, SIZE_MAX - i, BIG_CHAR);
> +             do_test (align, i, i - buf_addr, BIG_CHAR);
> +             do_test (align, i, -buf_addr - i, BIG_CHAR);
> +             do_test (align, i, SIZE_MAX - buf_addr - i, BIG_CHAR);
> +             do_test (align, i, SIZE_MAX - buf_addr + i, BIG_CHAR);
> +
> +             len = 0;
> +             for (j = 8 * sizeof (size_t) - 1; j; --j)
> +               {
> +                 len |= one << j;
> +                 do_test (align, i, len, BIG_CHAR);
> +                 do_test (align, i, len - i, BIG_CHAR);
> +                 do_test (align, i, len + i, BIG_CHAR);
> +                 do_test (align, i, len - buf_addr - i, BIG_CHAR);
> +                 do_test (align, i, len - buf_addr + i, BIG_CHAR);
> +
> +                 do_test (align, i, ~len - i, BIG_CHAR);
> +                 do_test (align, i, ~len + i, BIG_CHAR);
> +                 do_test (align, i, ~len - buf_addr - i, BIG_CHAR);
> +                 do_test (align, i, ~len - buf_addr + i, BIG_CHAR);
> +
> +                 do_test (align, i, -buf_addr, BIG_CHAR);
> +                 do_test (align, i, j - buf_addr, BIG_CHAR);
> +                 do_test (align, i, -buf_addr - j, BIG_CHAR);
> +               }
> +           }
> +       }
>      }
>  }
>
> diff --git a/sysdeps/x86_64/multiarch/strlen-avx2.S b/sysdeps/x86_64/multiarch/strlen-avx2.S
> index 0593fb303b..b9b58ef599 100644
> --- a/sysdeps/x86_64/multiarch/strlen-avx2.S
> +++ b/sysdeps/x86_64/multiarch/strlen-avx2.S
> @@ -544,14 +544,11 @@ L(return_vzeroupper):
>  L(cross_page_less_vec):
>         tzcntl  %eax, %eax
>  #  ifdef USE_AS_WCSLEN
> -       /* NB: Multiply length by 4 to get byte count.  */
> -       sall    $2, %esi
> +       /* NB: Divide by 4 to convert from byte-count to length.  */
> +       shrl    $2, %eax
>  #  endif
>         cmpq    %rax, %rsi
>         cmovb   %esi, %eax
> -#  ifdef USE_AS_WCSLEN
> -       shrl    $2, %eax
> -#  endif
>         VZEROUPPER_RETURN
>  # endif
>
> --
> 2.34.1
>

LGTM.

Thanks.

-- 
H.J.