cmd/link: invalidate kernel cache on darwin

Apparently, the darwin kernel may cache the code signature at
mmap. When we mmap the output buffer, it doesn't have a code
signature (as we haven't generated one). Invalidate the kernel
cache after writing the file.

See #42684 (comment)
for more information.

Updates #38485.
Fixes #42684.

Change-Id: Iac2ef756ca1454c856944423e5040b8e17a6b420
Reviewed-on: https://go-review.googlesource.com/c/go/+/272258
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/link/internal/ld/outbuf.go

@@ -113,6 +113,7 @@ func (out *OutBuf) Close() error {
 	}
 	if out.isMmapped() {
 		out.copyHeap()
+		out.purgeSignatureCache()
 		out.munmap()
 	}
 	if out.f == nil {

src/cmd/link/internal/ld/outbuf_darwin.go

@@ -36,3 +36,12 @@ func (out *OutBuf) fallocate(size uint64) error {
 
 	return nil
 }
+
+func (out *OutBuf) purgeSignatureCache() {
+	// Apparently, the Darwin kernel may cache the code signature at mmap.
+	// When we mmap the output buffer, it doesn't have a code signature
+	// (as we haven't generated one). Invalidate the kernel cache now that
+	// we have generated the signature. See issue #42684.
+	syscall.Syscall(syscall.SYS_MSYNC, uintptr(unsafe.Pointer(&out.buf[0])), uintptr(len(out.buf)), syscall.MS_INVALIDATE)
+	// Best effort. Ignore error.
+}

src/cmd/link/internal/ld/outbuf_notdarwin.go

@@ -0,0 +1,9 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !darwin
+
+package ld
+
+func (out *OutBuf) purgeSignatureCache() {}