[PATCH] s390x: Fix segfault in wcsncmp [BZ #31934]

[Carlos O'Donell]

On 7/11/24 5:28 AM, Stefan Liebler wrote:
> The z13/vector-optimized wcsncmp implementation segfaults if n=1
> and there is only one character (equal on both strings) before
> the page end.  Then it loads and compares one character and misses
> to check n again.  The following load fails.

LGTM. Thank you for fixing this!

Please feel free to push since we're currently in bug fixing phase and this has
no ABI impact.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> This patch removes the extra load and compare of the first character
> and just start with the loop which uses vector-load-to-block-boundary.
> This code-path also checks n.
> 
> With this patch both tests are passing:
> - the simplified one mentioned in the bugzilla 31934
> - the full one in Florian Weimer's patch:
> "manual: Document a GNU extension for strncmp/wcsncmp"
> (https://patchwork.sourceware.org/project/glibc/patch/874j9eml6y.fsf@oldenburg.str.redhat.com/):
> On s390x-linux-gnu (z16), the new wcsncmp test fails due to bug 31934.
> ---
>  sysdeps/s390/wcsncmp-vx.S | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/sysdeps/s390/wcsncmp-vx.S b/sysdeps/s390/wcsncmp-vx.S
> index bf6dfa6bc2..8b081567a2 100644
> --- a/sysdeps/s390/wcsncmp-vx.S
> +++ b/sysdeps/s390/wcsncmp-vx.S
> @@ -59,14 +59,7 @@ ENTRY(WCSNCMP_Z13)
>  	sllg	%r4,%r4,2	/* Convert character-count to byte-count.  */
>  	locgrne	%r4,%r1		/* Use max byte-count, if bit 0/1 was one.  */
>  
> -	/* Check first character without vector load.  */
> -	lghi	%r5,4		/* current_len = 4 bytes.  */
> -	/* Check s1/2[0].  */
> -	lt	%r0,0(%r2)
> -	l	%r1,0(%r3)
> -	je	.Lend_cmp_one_char
> -	crjne	%r0,%r1,.Lend_cmp_one_char

OK. Removes the unrolled processing of just 1 character.

> -
> +	lghi	%r5,0		/* current_len = 0 bytes.  */

OK. Current progress is set to 0 at the start (as expected).

>  .Lloop:
>  	vlbb	%v17,0(%r5,%r3),6 /* Load s2 to block boundary.  */
>  	vlbb	%v16,0(%r5,%r2),6 /* Load s1 to block boundary.  */

... and we use vlbb to avoid crossing the block boundary (page boundary).

> @@ -167,7 +160,6 @@ ENTRY(WCSNCMP_Z13)
>  	srl	%r4,2		/* And convert it to character-index.  */
>  	vlgvf	%r0,%v16,0(%r4)	/* Load character-values.  */
>  	vlgvf	%r1,%v17,0(%r4)
> -.Lend_cmp_one_char:

OK. Label no longer needed.

>  	cr	%r0,%r1
>  	je	.Lend_equal
>  	lghi	%r2,1

-- 
Cheers,
Carlos.