Split remaining gohacks functions into separate files

Functions using linkname are checked via checklinkname. Noescape has no
automatic check anymore, but if a future version of Go invalidates the
implementation, the result will be values escaping to the heap (performance
loss), rather than memory corruption, so I think having no check is OK.

PiperOrigin-RevId: 505173611

nogo.yaml

@@ -162,7 +162,7 @@ analyzers:
       exclude:
         - ".*_test.go"                                             # Exclude tests.
         - "pkg/flipcall/.*_unsafe.go"                              # Special case.
-        - pkg/gohacks/gohacks_unsafe.go                            # Special case.
+        - pkg/gohacks/noescape_unsafe.go                           # Special case.
         - pkg/ring0/pagetables/allocator_unsafe.go                 # Special case.
         - pkg/sentry/fsutil/host_file_mapper_unsafe.go             # Special case.
         - pkg/sentry/platform/kvm/bluepill_unsafe.go               # Special case.
@@ -207,7 +207,7 @@ analyzers:
   SA4016: # Useless bitwise operations.
     internal:
       exclude:
-        - pkg/gohacks/gohacks_unsafe.go # x ^ 0 always equals x.
+        - pkg/gohacks/noescape_unsafe.go # x ^ 0 always equals x.
   ST1019: # Multiple imports of the same package.
     generated:
       exclude:

pkg/gohacks/BUILD

@@ -5,7 +5,8 @@ package(licenses = ["notice"])
 go_library(
     name = "gohacks",
     srcs = [
-        "gohacks_unsafe.go",
+        "linkname_go113_unsafe.go",
+        "noescape_unsafe.go",
         "slice_go113_unsafe.go",
         "slice_go120_unsafe.go",
         "string_go113_unsafe.go",

pkg/gohacks/linkname_go113_unsafe.go

@@ -1,4 +1,4 @@
-// Copyright 2020 The gVisor Authors.
+// Copyright 2023 The gVisor Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,17 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build go1.13 && !go1.21
-// +build go1.13,!go1.21
+//go:build go1.13
 
 // //go:linkname directives type-checked by checklinkname. Any other
 // non-linkname assumptions outside the Go 1 compatibility guarantee should
 // have an accompanied vet check or version guard build tag.
 
-// Check type signatures and Noescape when updating Go version.
-//
-// TODO(b/165820485): add these checks to checklinkname.
-
 // Package gohacks contains utilities for subverting the Go compiler.
 package gohacks
 
@@ -30,19 +25,6 @@ import (
 	"unsafe"
 )
 
-// Noescape hides a pointer from escape analysis. Noescape is the identity
-// function but escape analysis doesn't think the output depends on the input.
-// Noescape is inlined and currently compiles down to zero instructions.
-// USE CAREFULLY!
-//
-// (Noescape is copy/pasted from Go's runtime/stubs.go:noescape().)
-//
-//go:nosplit
-func Noescape(p unsafe.Pointer) unsafe.Pointer {
-	x := uintptr(p)
-	return unsafe.Pointer(x ^ 0)
-}
-
 // Note that go:linkname silently doesn't work if the local name is exported,
 // necessitating an indirection for exported functions.
 

pkg/gohacks/noescape_unsafe.go

@@ -0,0 +1,34 @@
+// Copyright 2023 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gohacks
+
+import (
+	"unsafe"
+)
+
+// Noescape hides a pointer from escape analysis. Noescape is the identity
+// function but escape analysis doesn't think the output depends on the input.
+// Noescape is inlined and currently compiles down to zero instructions.
+// USE CAREFULLY!
+//
+// Noescape is copy/pasted from Go's runtime/stubs.go:noescape(), and is valid
+// as of Go 1.20. It is possible that this approach stops working in future
+// versions of the toolchain, at which point `p` may still escape.
+//
+//go:nosplit
+func Noescape(p unsafe.Pointer) unsafe.Pointer {
+	x := uintptr(p)
+	return unsafe.Pointer(x ^ 0)
+}