x86: Fix __wcsncmp_avx2 in strcmp-avx2.S [BZ# 28755]

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>
(cherry picked from commit ddf0992cf57a93200e0c782e2a94d0733a5a0b87)

diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
index c9f246415faba2ad1feb4814bdc72ffed4c14508..360370cb7add27637c6c11687bf380be7d05ff28 100644 (file)
--- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
+++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
@@ -83,6 +83,16 @@ 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