cmd/cgo: enable #cgo noescape/nocallback

In Go 1.22 we added code to the go/build package to ignore #cgo noescape
and nocallback directives. That permits us to enable these directives in Go 1.24.

Also, this fixed a Bug in CL 497837:
After retiring _Cgo_use for parameters, the compiler will treat the
parameters, start from the second, as non-alive. Then, they will be marked
as scalar in stackmap, which means the pointer won't be copied correctly
in copystack.

Fixes #56378.
Fixes #63739.

Change-Id: I46e773240f8a467c3c4ba201dc5b4ee473cf6e3e
GitHub-Last-Rev: 42fcc50
GitHub-Pull-Request: #66879
Reviewed-on: https://go-review.googlesource.com/c/go/+/579955
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/cmd/cgo/doc.go

@@ -425,6 +425,30 @@ passing uninitialized C memory to Go code if the Go code is going to
 store pointer values in it. Zero out the memory in C before passing it
 to Go.
 
+# Optimizing calls of C code
+
+When passing a Go pointer to a C function the compiler normally ensures
+that the Go object lives on the heap. If the C function does not keep
+a copy of the Go pointer, and never passes the Go pointer back to Go code,
+then this is unnecessary. The #cgo noescape directive may be used to tell
+the compiler that no Go pointers escape via the named C function.
+If the noescape directive is used and the C function does not handle the
+pointer safely, the program may crash or see memory corruption.
+
+For example:
+
+	// #cgo noescape cFunctionName
+
+When a Go function calls a C function, it prepares for the C function to
+call back to a Go function. The #cgo nocallback directive may be used to
+tell the compiler that these preparations are not necessary.
+If the nocallback directive is used and the C function does call back into
+Go code, the program will panic.
+
+For example:
+
+	// #cgo nocallback cFunctionName
+
 # Special cases
 
 A few special C types which would normally be represented by a pointer

src/cmd/cgo/gcc.go

@@ -94,10 +94,8 @@ func (f *File) ProcessCgoDirectives() {
 				directive := fields[1]
 				funcName := fields[2]
 				if directive == "nocallback" {
-					fatalf("#cgo nocallback disabled until Go 1.23")
 					f.NoCallbacks[funcName] = true
 				} else if directive == "noescape" {
-					fatalf("#cgo noescape disabled until Go 1.23")
 					f.NoEscapes[funcName] = true
 				}
 			}

src/cmd/cgo/internal/test/test.go

@@ -117,8 +117,8 @@ int add(int x, int y) {
 
 // escape vs noescape
 
-// TODO(#56378): enable in Go 1.23:
-// #cgo noescape handleGoStringPointerNoescape
+#cgo noescape handleGoStringPointerNoescape
+#cgo nocallback handleGoStringPointerNoescape
 void handleGoStringPointerNoescape(void *s) {}
 
 void handleGoStringPointerEscape(void *s) {}

src/cmd/cgo/internal/testerrors/testdata/notmatchedcfunction.go

@@ -5,8 +5,7 @@
 package main
 
 /*
-// TODO(#56378): change back to "#cgo noescape noMatchedCFunction: no matched C function" in Go 1.23
-// ERROR MESSAGE: #cgo noescape disabled until Go 1.23
+// ERROR MESSAGE: #cgo noescape noMatchedCFunction: no matched C function
 #cgo noescape noMatchedCFunction
 */
 import "C"

src/cmd/cgo/out.go

@@ -104,6 +104,9 @@ func (p *Package) writeDefs() {
 		fmt.Fprintf(fgo2, "var _Cgo_always_false bool\n")
 		fmt.Fprintf(fgo2, "//go:linkname _Cgo_use runtime.cgoUse\n")
 		fmt.Fprintf(fgo2, "func _Cgo_use(interface{})\n")
+		fmt.Fprintf(fgo2, "//go:linkname _Cgo_keepalive runtime.cgoKeepAlive\n")
+		fmt.Fprintf(fgo2, "//go:noescape\n")
+		fmt.Fprintf(fgo2, "func _Cgo_keepalive(interface{})\n")
 	}
 	fmt.Fprintf(fgo2, "//go:linkname _Cgo_no_callback runtime.cgoNoCallback\n")
 	fmt.Fprintf(fgo2, "func _Cgo_no_callback(bool)\n")
@@ -639,17 +642,20 @@ func (p *Package) writeDefsFunc(fgo2 io.Writer, n *Name, callsMalloc *bool) {
 		fmt.Fprintf(fgo2, "\t_Cgo_no_callback(false)\n")
 	}
 
-	// skip _Cgo_use when noescape exist,
+	// Use _Cgo_keepalive instead of _Cgo_use when noescape & nocallback exist,
 	// so that the compiler won't force to escape them to heap.
-	if !p.noEscapes[n.C] {
-		fmt.Fprintf(fgo2, "\tif _Cgo_always_false {\n")
-		if d.Type.Params != nil {
-			for i := range d.Type.Params.List {
-				fmt.Fprintf(fgo2, "\t\t_Cgo_use(p%d)\n", i)
-			}
+	// Instead, make the compiler keep them alive by using _Cgo_keepalive.
+	touchFunc := "_Cgo_use"
+	if p.noEscapes[n.C] && p.noCallbacks[n.C] {
+		touchFunc = "_Cgo_keepalive"
+	}
+	fmt.Fprintf(fgo2, "\tif _Cgo_always_false {\n")
+	if d.Type.Params != nil {
+		for _, name := range paramnames {
+			fmt.Fprintf(fgo2, "\t\t%s(%s)\n", touchFunc, name)
 		}
-		fmt.Fprintf(fgo2, "\t}\n")
 	}
+	fmt.Fprintf(fgo2, "\t}\n")
 	fmt.Fprintf(fgo2, "\treturn\n")
 	fmt.Fprintf(fgo2, "}\n")
 }

src/runtime/cgo.go

@@ -72,11 +72,20 @@ var cgoHasExtraM bool
 // cgoUse should not actually be called (see cgoAlwaysFalse).
 func cgoUse(any) { throw("cgoUse should not be called") }
 
+// cgoKeepAlive is called by cgo-generated code (using go:linkname to get at
+// an unexported name). This call keeps its argument alive until the call site;
+// cgo emits the call after the last possible use of the argument by C code.
+// cgoKeepAlive is marked in the cgo-generated code as //go:noescape, so
+// unlike cgoUse it does not force the argument to escape to the heap.
+// This is used to implement the #cgo noescape directive.
+func cgoKeepAlive(any) { throw("cgoKeepAlive should not be called") }
+
 // cgoAlwaysFalse is a boolean value that is always false.
-// The cgo-generated code says if cgoAlwaysFalse { cgoUse(p) }.
+// The cgo-generated code says if cgoAlwaysFalse { cgoUse(p) },
+// or if cgoAlwaysFalse { cgoKeepAlive(p) }.
 // The compiler cannot see that cgoAlwaysFalse is always false,
 // so it emits the test and keeps the call, giving the desired
-// escape analysis result. The test is cheaper than the call.
+// escape/alive analysis result. The test is cheaper than the call.
 var cgoAlwaysFalse bool
 
 var cgo_yield = &_cgo_yield

src/runtime/crash_cgo_test.go

@@ -750,7 +750,6 @@ func TestNeedmDeadlock(t *testing.T) {
 }
 
 func TestCgoNoCallback(t *testing.T) {
-	t.Skip("TODO(#56378): enable in Go 1.23")
 	got := runTestProg(t, "testprogcgo", "CgoNoCallback")
 	want := "function marked with #cgo nocallback called back into Go"
 	if !strings.Contains(got, want) {
@@ -759,14 +758,22 @@ func TestCgoNoCallback(t *testing.T) {
 }
 
 func TestCgoNoEscape(t *testing.T) {
-	t.Skip("TODO(#56378): enable in Go 1.23")
 	got := runTestProg(t, "testprogcgo", "CgoNoEscape")
 	want := "OK\n"
 	if got != want {
 		t.Fatalf("want %s, got %s\n", want, got)
 	}
 }
 
+// Issue #63739.
+func TestCgoEscapeWithMultiplePointers(t *testing.T) {
+	got := runTestProg(t, "testprogcgo", "CgoEscapeWithMultiplePointers")
+	want := "OK\n"
+	if got != want {
+		t.Fatalf("output is %s; want %s", got, want)
+	}
+}
+
 func TestCgoTracebackGoroutineProfile(t *testing.T) {
 	output := runTestProg(t, "testprogcgo", "GoroutineProfile")
 	want := "OK\n"

src/runtime/linkname.go

@@ -13,6 +13,7 @@ import _ "unsafe"
 //go:linkname _cgo_panic_internal
 //go:linkname cgoAlwaysFalse
 //go:linkname cgoUse
+//go:linkname cgoKeepAlive
 //go:linkname cgoCheckPointer
 //go:linkname cgoCheckResult
 //go:linkname cgoNoCallback

src/runtime/testdata/testprogcgo/cgonocallback.go

@@ -8,7 +8,8 @@ package main
 // But it do callback to go in this test, Go should crash here.
 
 /*
-// TODO(#56378): #cgo nocallback runCShouldNotCallback
+#cgo nocallback runCShouldNotCallback
+
 extern void runCShouldNotCallback();
 */
 import "C"

src/runtime/testdata/testprogcgo/cgonoescape.go

@@ -13,7 +13,8 @@ package main
 // 2. less than 100 new allocated heap objects after invoking withoutNoEscape 100 times.
 
 /*
-// TODO(#56378): #cgo noescape runCWithNoEscape
+#cgo noescape runCWithNoEscape
+#cgo nocallback runCWithNoEscape
 
 void runCWithNoEscape(void *p) {
 }

src/runtime/testdata/testprogcgo/issue63739.go

@@ -0,0 +1,59 @@
+// 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.
+
+package main
+
+// This is for issue #63739.
+// Ensure that parameters are kept alive until the end of the C call. If not,
+// then a stack copy at just the right time while calling into C might think
+// that any stack pointers are not alive and fail to update them, causing the C
+// function to see the old, no longer correct, pointer values.
+
+/*
+int add_from_multiple_pointers(int *a, int *b, int *c) {
+	*a = *a + 1;
+	*b = *b + 1;
+	*c = *c + 1;
+	return *a + *b + *c;
+}
+#cgo noescape add_from_multiple_pointers
+#cgo nocallback add_from_multiple_pointers
+*/
+import "C"
+
+import (
+	"fmt"
+)
+
+const (
+	maxStack = 1024
+)
+
+func init() {
+	register("CgoEscapeWithMultiplePointers", CgoEscapeWithMultiplePointers)
+}
+
+func CgoEscapeWithMultiplePointers() {
+	stackGrow(maxStack)
+	fmt.Println("OK")
+}
+
+//go:noinline
+func testCWithMultiplePointers() {
+	var a C.int = 1
+	var b C.int = 2
+	var c C.int = 3
+	v := C.add_from_multiple_pointers(&a, &b, &c)
+	if v != 9 || a != 2 || b != 3 || c != 4 {
+		fmt.Printf("%d + %d + %d != %d\n", a, b, c, v)
+	}
+}
+
+func stackGrow(n int) {
+	if n == 0 {
+		return
+	}
+	testCWithMultiplePointers()
+	stackGrow(n - 1)
+}