internal/asan: add new package

The internal/asan package contains helper functions for manually
instrumenting code for the address sanitizer. It reexports the asan
routines in runtime unconditionally, making the functions a no-op if the
build flag "asan" is not present.

For #64611

Change-Id: Ie79e698aea7a6d969afd2a5f008c084c9545b1a5
GitHub-Last-Rev: e658670c14
GitHub-Pull-Request: golang/go#64635
Reviewed-on: https://go-review.googlesource.com/c/go/+/548695
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/go/build/deps_test.go

@@ -75,6 +75,7 @@ var depsRules = `
 	< runtime
 	< sync/atomic
 	< internal/race
+	< internal/asan
 	< sync
 	< internal/bisect
 	< internal/godebug

src/internal/asan/asan.go

@@ -0,0 +1,19 @@
+// Copyright 2024 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.
+
+//go:build asan
+
+package asan
+
+import (
+	"unsafe"
+)
+
+const Enabled = true
+
+//go:linkname Read runtime.asanread
+func Read(addr unsafe.Pointer, len int)
+
+//go:linkname Write runtime.asanwrite
+func Write(addr unsafe.Pointer, len int)

src/internal/asan/doc.go

@@ -0,0 +1,10 @@
+// Copyright 2024 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.
+
+// Package asan contains helper functions for manually instrumenting
+// code for the address sanitizer.
+// The runtime package intentionally exports these functions only in the
+// asan build; this package exports them unconditionally but without the
+// "asan" build tag they are no-ops.
+package asan

src/internal/asan/noasan.go

@@ -0,0 +1,19 @@
+// Copyright 2024 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.
+
+//go:build !asan
+
+package asan
+
+import (
+	"unsafe"
+)
+
+const Enabled = false
+
+func Read(addr unsafe.Pointer, len int) {
+}
+
+func Write(addr unsafe.Pointer, len int) {
+}

src/runtime/asan.go

@@ -29,6 +29,7 @@ const asanenabled = true
 // asan{read,write} are nosplit because they may be called between
 // fork and exec, when the stack must not grow. See issue #50391.
 
+//go:linkname asanread
 //go:nosplit
 func asanread(addr unsafe.Pointer, sz uintptr) {
 	sp := getcallersp()
@@ -36,6 +37,7 @@ func asanread(addr unsafe.Pointer, sz uintptr) {
 	doasanread(addr, sz, sp, pc)
 }
 
+//go:linkname asanwrite
 //go:nosplit
 func asanwrite(addr unsafe.Pointer, sz uintptr) {
 	sp := getcallersp()