[glibc] x86-64 strncpy: Properly handle the length parameter [BZ# 29839]

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

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e5672763c44f16ddbc42809f5def7c6a962602bd

commit e5672763c44f16ddbc42809f5def7c6a962602bd
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Dec 1 16:36:02 2022 -0800

    x86-64 strncpy: Properly handle the length parameter [BZ# 29839]
    
    On x32, the size_t parameter may be passed in the lower 32 bits of a
    64-bit register with the non-zero upper 32 bits.  The string/memory
    functions written in assembly can only use the lower 32 bits of a
    64-bit register as length or must clear the upper 32 bits before using
    the full 64-bit register for length.
    
    This pach fixes strncpy for x32.  Tested on x86-64 and x32.  On x86-64,
    libc.so is the same with and without the fix.
    Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

Diff:
---
 sysdeps/x86_64/multiarch/strncpy-avx2.S | 4 ++++
 sysdeps/x86_64/multiarch/strncpy-evex.S | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/sysdeps/x86_64/multiarch/strncpy-avx2.S b/sysdeps/x86_64/multiarch/strncpy-avx2.S
index e9afd8fbed..3e6350ce4a 100644
--- a/sysdeps/x86_64/multiarch/strncpy-avx2.S
+++ b/sysdeps/x86_64/multiarch/strncpy-avx2.S
@@ -52,6 +52,10 @@
 
 	.section SECTION(.text), "ax", @progbits
 ENTRY(STRNCPY)
+# ifdef __ILP32__
+	/* Clear the upper 32 bits.  */
+	movl	%edx, %edx
+# endif
 	/* Filter zero length strings and very long strings.  Zero
 	   length strings just return, very long strings are handled by
 	   just running rep stos{b|l} to zero set (which will almost
diff --git a/sysdeps/x86_64/multiarch/strncpy-evex.S b/sysdeps/x86_64/multiarch/strncpy-evex.S
index 49eaf4cbd9..dec8cccc2b 100644
--- a/sysdeps/x86_64/multiarch/strncpy-evex.S
+++ b/sysdeps/x86_64/multiarch/strncpy-evex.S
@@ -80,6 +80,10 @@
 
 	.section SECTION(.text), "ax", @progbits
 ENTRY(STRNCPY)
+# ifdef __ILP32__
+	/* Clear the upper 32 bits.  */
+	movl	%edx, %edx
+# endif
 	/* Filter zero length strings and very long strings.  Zero
 	   length strings just return, very long strings are handled by
 	   just running rep stos{b|l} to zero set (which will almost