[glibc/release/2.34/master] x86: Fix __wcsncmp_evex in strcmp-evex.S [BZ# 28755]

[H.J. Lu <hjl@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=08beb3a3f4f46e306fffe184a08c5664bf0e13d6

commit 08beb3a3f4f46e306fffe184a08c5664bf0e13d6
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Sun Jan 9 16:02:28 2022 -0600

    x86: Fix __wcsncmp_evex in strcmp-evex.S [BZ# 28755]
    
    Fixes [BZ# 28755] for wcsncmp by redirecting length >= 2^56 to
    __wcscmp_evex. 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 7e08db3359c86c94918feb33a1182cd0ff3bb10b)

Diff:
---
 sysdeps/x86_64/multiarch/strcmp-evex.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/sysdeps/x86_64/multiarch/strcmp-evex.S b/sysdeps/x86_64/multiarch/strcmp-evex.S
index 459eeed09f..d5aa6daa46 100644
--- a/sysdeps/x86_64/multiarch/strcmp-evex.S
+++ b/sysdeps/x86_64/multiarch/strcmp-evex.S
@@ -97,6 +97,16 @@ 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_evex
+#  endif
 	/* Convert units: from wide to byte char.  */
 	shl	$2, %RDX_LP
 #  endif