s390x: Fix segfault in wcsncmp [BZ #31934]

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.

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

diff --git a/sysdeps/s390/wcsncmp-vx.S b/sysdeps/s390/wcsncmp-vx.S
index bf6dfa6bc2b904b5778ba7a7702dd13438d45885..8b081567a2a5732ce6e349624f53ea689cf3ea1c 100644 (file)
--- 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
-
+       lghi    %r5,0           /* current_len = 0 bytes.  */
 .Lloop:
        vlbb    %v17,0(%r5,%r3),6 /* Load s2 to block boundary.  */
        vlbb    %v16,0(%r5,%r2),6 /* Load s1 to block 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:
        cr      %r0,%r1
        je      .Lend_equal
        lghi    %r2,1