runtime/msan: use different msan routine for copying

__msan_memmove records the fact that we're copying memory, and
actually does the copy. Use instead __msan_copy_shadow, which
records the fact that we're copying memory, but doesn't actually
do the copy itself.

We're doing the copy ourselves, so we don't need msan to do it also.

More importantly, msan doing the copy clobbers the target before
we issue the write barrier, which causes pointers to get lost.

Fixes #76138

Change-Id: I17aea739f9444de21fac2bbfd81e48534a39481d
Reviewed-on: https://go-review.googlesource.com/c/go/+/719020
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: t hepudds <thepudds1460@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Radu Berinde <radu@cockroachlabs.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/msan/msan.go

@@ -13,8 +13,6 @@ package msan
 #include <stdint.h>
 #include <sanitizer/msan_interface.h>
 
-extern void __msan_memmove(void*, const void*, uintptr_t);
-
 void __msan_read_go(void *addr, uintptr_t sz) {
 	__msan_check_mem_is_initialized(addr, sz);
 }
@@ -32,7 +30,11 @@ void __msan_free_go(void *addr, uintptr_t sz) {
 }
 
 void __msan_memmove_go(void *to, const void *from, uintptr_t sz) {
-	__msan_memmove(to, from, sz);
+	// Note: don't use msan_memmove, as it actually does
+	// the move. We do the move ourselves, so it isn't necessary.
+	// Also, it clobbers the target before we issue the write
+	// barrier, which causes pointers to get lost. See issue 76138.
+	__msan_copy_shadow(to, from, sz);
 }
 */
 import "C"