commit b50d5b746cc0af5ad52164dcb0d3628f08b05a0d
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Sun Jan 9 16:02:21 2022 -0600

    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 40333010a65650f9..3dfcb1bf803cf9ec 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