[PATCH v3 1/7] x86: Fix __wcsncmp_avx2 in strcmp-avx2.S [BZ# 28755]

[H.J. Lu]

On Mon, Jan 10, 2022 at 1:36 PM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Fixes [BZ# 28755] for wcsncmp by redirecting length >= 2^56 to
> __wcscmp_avx2. For x86_64 this covers the entire address range so any
> length larger could not possibly be used to bound `s1` or `s2`.
>
> test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass.
>
> Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
> ---
>  sysdeps/x86_64/multiarch/strcmp-avx2.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> index a45f9d2749..9c73b5899d 100644
> --- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
> +++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> @@ -87,6 +87,16 @@ ENTRY (STRCMP)
>         je      L(char0)
>         jb      L(zero)
>  #  ifdef USE_AS_WCSCMP
> +#  ifndef __ILP32__
> +       movq    %rdx, %rcx
> +       /* Check if length could overflow when multiplied by
> +          sizeof(wchar_t). Checking top 8 bits will cover all potential
> +          overflow cases as well as redirect cases where its impossible to
> +          length to bound a valid memory region. In these cases just use
> +          'wcscmp'.  */
> +       shrq    $56, %rcx
> +       jnz     __wcscmp_avx2
> +#  endif
>         /* Convert units: from wide to byte char.  */
>         shl     $2, %RDX_LP
>  #  endif
> --
> 2.25.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.