cmd/cgo: add #cgo noescape/nocallback annotations

When passing pointers of Go objects from Go to C, the cgo command generate _Cgo_use(pN) for the unsafe.Pointer type arguments, so that the Go compiler will escape these object to heap.

Since the C function may callback to Go, then the Go stack might grow/shrink, that means the pointers that the C function have will be invalid.

After adding the #cgo noescape annotation for a C function, the cgo command won't generate _Cgo_use(pN), and the Go compiler won't force the object escape to heap.

After adding the #cgo nocallback annotation for a C function, which means the C function won't callback to Go, if it do callback to Go, the Go process will crash.

Fixes #56378

Change-Id: Ifdca070584e0d349c7b12276270e50089e481f7a
GitHub-Last-Rev: f1a17b08b0
GitHub-Pull-Request: golang/go#60399
Reviewed-on: https://go-review.googlesource.com/c/go/+/497837
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/cgo/doc.go

@@ -420,6 +420,30 @@ 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

@@ -73,18 +73,32 @@ func cname(s string) string {
 	return s
 }
 
-// DiscardCgoDirectives processes the import C preamble, and discards
-// all #cgo CFLAGS and LDFLAGS directives, so they don't make their
-// way into _cgo_export.h.
-func (f *File) DiscardCgoDirectives() {
+// ProcessCgoDirectives processes the import C preamble:
+//  1. discards all #cgo CFLAGS, LDFLAGS, nocallback and noescape directives,
+//     so they don't make their way into _cgo_export.h.
+//  2. parse the nocallback and noescape directives.
+func (f *File) ProcessCgoDirectives() {
 	linesIn := strings.Split(f.Preamble, "\n")
 	linesOut := make([]string, 0, len(linesIn))
+	f.NoCallbacks = make(map[string]bool)
+	f.NoEscapes = make(map[string]bool)
 	for _, line := range linesIn {
 		l := strings.TrimSpace(line)
 		if len(l) < 5 || l[:4] != "#cgo" || !unicode.IsSpace(rune(l[4])) {
 			linesOut = append(linesOut, line)
 		} else {
 			linesOut = append(linesOut, "")
+
+			// #cgo (nocallback|noescape) <function name>
+			if fields := strings.Fields(l); len(fields) == 3 {
+				directive := fields[1]
+				funcName := fields[2]
+				if directive == "nocallback" {
+					f.NoCallbacks[funcName] = true
+				} else if directive == "noescape" {
+					f.NoEscapes[funcName] = true
+				}
+			}
 		}
 	}
 	f.Preamble = strings.Join(linesOut, "\n")

src/cmd/cgo/internal/testerrors/errors_test.go

@@ -39,16 +39,23 @@ func check(t *testing.T, file string) {
 				continue
 			}
 
-			_, frag, ok := bytes.Cut(line, []byte("ERROR HERE: "))
-			if !ok {
-				continue
+			if _, frag, ok := bytes.Cut(line, []byte("ERROR HERE: ")); ok {
+				re, err := regexp.Compile(fmt.Sprintf(":%d:.*%s", i+1, frag))
+				if err != nil {
+					t.Errorf("Invalid regexp after `ERROR HERE: `: %#q", frag)
+					continue
+				}
+				errors = append(errors, re)
 			}
-			re, err := regexp.Compile(fmt.Sprintf(":%d:.*%s", i+1, frag))
-			if err != nil {
-				t.Errorf("Invalid regexp after `ERROR HERE: `: %#q", frag)
-				continue
+
+			if _, frag, ok := bytes.Cut(line, []byte("ERROR MESSAGE: ")); ok {
+				re, err := regexp.Compile(string(frag))
+				if err != nil {
+					t.Errorf("Invalid regexp after `ERROR MESSAGE: `: %#q", frag)
+					continue
+				}
+				errors = append(errors, re)
 			}
-			errors = append(errors, re)
 		}
 		if len(errors) == 0 {
 			t.Fatalf("cannot find ERROR HERE")
@@ -165,3 +172,8 @@ func TestMallocCrashesOnNil(t *testing.T) {
 		t.Fatalf("succeeded unexpectedly")
 	}
 }
+
+func TestNotMatchedCFunction(t *testing.T) {
+	file := "notmatchedcfunction.go"
+	check(t, file)
+}

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

@@ -0,0 +1,14 @@
+// Copyright 2023 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
+
+/*
+// ERROR MESSAGE: #cgo noescape noMatchedCFunction: no matched C function
+#cgo noescape noMatchedCFunction
+*/
+import "C"
+
+func main() {
+}

src/cmd/cgo/main.go

@@ -48,6 +48,8 @@ type Package struct {
 	Preamble    string          // collected preamble for _cgo_export.h
 	typedefs    map[string]bool // type names that appear in the types of the objects we're interested in
 	typedefList []typedefInfo
+	noCallbacks map[string]bool // C function names with #cgo nocallback directive
+	noEscapes   map[string]bool // C function names with #cgo noescape directive
 }
 
 // A typedefInfo is an element on Package.typedefList: a typedef name
@@ -59,16 +61,18 @@ type typedefInfo struct {
 
 // A File collects information about a single Go input file.
 type File struct {
-	AST      *ast.File           // parsed AST
-	Comments []*ast.CommentGroup // comments from file
-	Package  string              // Package name
-	Preamble string              // C preamble (doc comment on import "C")
-	Ref      []*Ref              // all references to C.xxx in AST
-	Calls    []*Call             // all calls to C.xxx in AST
-	ExpFunc  []*ExpFunc          // exported functions for this file
-	Name     map[string]*Name    // map from Go name to Name
-	NamePos  map[*Name]token.Pos // map from Name to position of the first reference
-	Edit     *edit.Buffer
+	AST         *ast.File           // parsed AST
+	Comments    []*ast.CommentGroup // comments from file
+	Package     string              // Package name
+	Preamble    string              // C preamble (doc comment on import "C")
+	Ref         []*Ref              // all references to C.xxx in AST
+	Calls       []*Call             // all calls to C.xxx in AST
+	ExpFunc     []*ExpFunc          // exported functions for this file
+	Name        map[string]*Name    // map from Go name to Name
+	NamePos     map[*Name]token.Pos // map from Name to position of the first reference
+	NoCallbacks map[string]bool     // C function names that with #cgo nocallback directive
+	NoEscapes   map[string]bool     // C function names that with #cgo noescape directive
+	Edit        *edit.Buffer
 }
 
 func (f *File) offset(p token.Pos) int {
@@ -374,7 +378,7 @@ func main() {
 		f := new(File)
 		f.Edit = edit.NewBuffer(b)
 		f.ParseGo(input, b)
-		f.DiscardCgoDirectives()
+		f.ProcessCgoDirectives()
 		fs[i] = f
 	}
 
@@ -413,6 +417,25 @@ func main() {
 			p.writeOutput(f, input)
 		}
 	}
+	cFunctions := make(map[string]bool)
+	for _, key := range nameKeys(p.Name) {
+		n := p.Name[key]
+		if n.FuncType != nil {
+			cFunctions[n.C] = true
+		}
+	}
+
+	for funcName := range p.noEscapes {
+		if _, found := cFunctions[funcName]; !found {
+			error_(token.NoPos, "#cgo noescape %s: no matched C function", funcName)
+		}
+	}
+
+	for funcName := range p.noCallbacks {
+		if _, found := cFunctions[funcName]; !found {
+			error_(token.NoPos, "#cgo nocallback %s: no matched C function", funcName)
+		}
+	}
 
 	if !*godefs {
 		p.writeDefs()
@@ -450,10 +473,12 @@ func newPackage(args []string) *Package {
 	os.Setenv("LC_ALL", "C")
 
 	p := &Package{
-		PtrSize:  ptrSize,
-		IntSize:  intSize,
-		CgoFlags: make(map[string][]string),
-		Written:  make(map[string]bool),
+		PtrSize:     ptrSize,
+		IntSize:     intSize,
+		CgoFlags:    make(map[string][]string),
+		Written:     make(map[string]bool),
+		noCallbacks: make(map[string]bool),
+		noEscapes:   make(map[string]bool),
 	}
 	p.addToFlag("CFLAGS", args)
 	return p
@@ -487,6 +512,14 @@ func (p *Package) Record(f *File) {
 		}
 	}
 
+	// merge nocallback & noescape
+	for k, v := range f.NoCallbacks {
+		p.noCallbacks[k] = v
+	}
+	for k, v := range f.NoEscapes {
+		p.noEscapes[k] = v
+	}
+
 	if f.ExpFunc != nil {
 		p.ExpFunc = append(p.ExpFunc, f.ExpFunc...)
 		p.Preamble += "\n" + f.Preamble

src/cmd/cgo/out.go

@@ -106,6 +106,8 @@ func (p *Package) writeDefs() {
 		fmt.Fprintf(fgo2, "//go:linkname _Cgo_use runtime.cgoUse\n")
 		fmt.Fprintf(fgo2, "func _Cgo_use(interface{})\n")
 	}
+	fmt.Fprintf(fgo2, "//go:linkname _Cgo_no_callback runtime.cgoNoCallback\n")
+	fmt.Fprintf(fgo2, "func _Cgo_no_callback(bool)\n")
 
 	typedefNames := make([]string, 0, len(typedef))
 	for name := range typedef {
@@ -612,6 +614,12 @@ func (p *Package) writeDefsFunc(fgo2 io.Writer, n *Name, callsMalloc *bool) {
 		arg = "uintptr(unsafe.Pointer(&r1))"
 	}
 
+	noCallback := p.noCallbacks[n.C]
+	if noCallback {
+		// disable cgocallback, will check it in runtime.
+		fmt.Fprintf(fgo2, "\t_Cgo_no_callback(true)\n")
+	}
+
 	prefix := ""
 	if n.AddError {
 		prefix = "errno := "
@@ -620,13 +628,21 @@ func (p *Package) writeDefsFunc(fgo2 io.Writer, n *Name, callsMalloc *bool) {
 	if n.AddError {
 		fmt.Fprintf(fgo2, "\tif errno != 0 { r2 = syscall.Errno(errno) }\n")
 	}
-	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)
-		}
+	if noCallback {
+		fmt.Fprintf(fgo2, "\t_Cgo_no_callback(false)\n")
+	}
+
+	// skip _Cgo_use when noescape 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)
+			}
+		}
+		fmt.Fprintf(fgo2, "\t}\n")
 	}
-	fmt.Fprintf(fgo2, "\t}\n")
 	fmt.Fprintf(fgo2, "\treturn\n")
 	fmt.Fprintf(fgo2, "}\n")
 }
@@ -1612,9 +1628,11 @@ const goProlog = `
 func _cgo_runtime_cgocall(unsafe.Pointer, uintptr) int32
 
 //go:linkname _cgoCheckPointer runtime.cgoCheckPointer
+//go:noescape
 func _cgoCheckPointer(interface{}, interface{})
 
 //go:linkname _cgoCheckResult runtime.cgoCheckResult
+//go:noescape
 func _cgoCheckResult(interface{})
 `
 

src/cmd/go/internal/modindex/build.go

@@ -622,6 +622,11 @@ func (ctxt *Context) saveCgo(filename string, di *build.Package, text string) er
 			continue
 		}
 
+		// #cgo (nocallback|noescape) <function name>
+		if fields := strings.Fields(line); len(fields) == 3 && (fields[1] == "nocallback" || fields[1] == "noescape") {
+			continue
+		}
+
 		// Split at colon.
 		line, argstr, ok := strings.Cut(strings.TrimSpace(line[4:]), ":")
 		if !ok {

src/go/build/build.go

@@ -1687,6 +1687,11 @@ func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup)
 			continue
 		}
 
+		// #cgo (nocallback|noescape) <function name>
+		if fields := strings.Fields(line); len(fields) == 3 && (fields[1] == "nocallback" || fields[1] == "noescape") {
+			continue
+		}
+
 		// Split at colon.
 		line, argstr, ok := strings.Cut(strings.TrimSpace(line[4:]), ":")
 		if !ok {

src/runtime/cgo.go

@@ -61,3 +61,11 @@ func cgoUse(any) { throw("cgoUse should not be called") }
 var cgoAlwaysFalse bool
 
 var cgo_yield = &_cgo_yield
+
+func cgoNoCallback(v bool) {
+	g := getg()
+	if g.nocgocallback && v {
+		panic("runtime: unexpected setting cgoNoCallback")
+	}
+	g.nocgocallback = v
+}

src/runtime/cgocall.go

@@ -242,6 +242,10 @@ func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 
 	osPreemptExtExit(gp.m)
 
+	if gp.nocgocallback {
+		panic("runtime: function marked with #cgo nocallback called back into Go")
+	}
+
 	cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread
 
 	// At this point unlockOSThread has been called.

src/runtime/crash_cgo_test.go

@@ -753,6 +753,22 @@ func TestNeedmDeadlock(t *testing.T) {
 	}
 }
 
+func TestCgoNoCallback(t *testing.T) {
+	got := runTestProg(t, "testprogcgo", "CgoNoCallback")
+	want := "function marked with #cgo nocallback called back into Go"
+	if !strings.Contains(got, want) {
+		t.Fatalf("did not see %q in output:\n%s", want, got)
+	}
+}
+
+func TestCgoNoEscape(t *testing.T) {
+	got := runTestProg(t, "testprogcgo", "CgoNoEscape")
+	want := "OK\n"
+	if got != want {
+		t.Fatalf("want %s, got %s\n", want, got)
+	}
+}
+
 func TestCgoTracebackGoroutineProfile(t *testing.T) {
 	output := runTestProg(t, "testprogcgo", "GoroutineProfile")
 	want := "OK\n"

src/runtime/runtime2.go

@@ -481,6 +481,7 @@ type g struct {
 	parkingOnChan atomic.Bool
 
 	raceignore    int8  // ignore race detection events
+	nocgocallback bool  // whether disable callback from C
 	tracking      bool  // whether we're tracking this G for sched latency statistics
 	trackingSeq   uint8 // used to decide whether to track this G
 	trackingStamp int64 // timestamp of when the G last started being tracked

src/runtime/testdata/testprogcgo/cgonocallback.c

@@ -0,0 +1,9 @@
+// Copyright 2023 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.
+
+#include "_cgo_export.h"
+
+void runCShouldNotCallback() {
+	CallbackToGo();
+}

src/runtime/testdata/testprogcgo/cgonocallback.go

@@ -0,0 +1,32 @@
+// Copyright 2023 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
+
+// #cgo nocallback annotations for a C function means it should not callback to Go.
+// But it do callback to go in this test, Go should crash here.
+
+/*
+#cgo nocallback runCShouldNotCallback
+
+extern void runCShouldNotCallback();
+*/
+import "C"
+
+import (
+	"fmt"
+)
+
+func init() {
+	register("CgoNoCallback", CgoNoCallback)
+}
+
+//export CallbackToGo
+func CallbackToGo() {
+}
+
+func CgoNoCallback() {
+	C.runCShouldNotCallback()
+	fmt.Println("OK")
+}

src/runtime/testdata/testprogcgo/cgonoescape.go

@@ -0,0 +1,84 @@
+// Copyright 2023 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
+
+// #cgo noescape annotations for a C function means its arguments won't escape to heap.
+
+// We assume that there won't be 100 new allocated heap objects in other places,
+// i.e. runtime.ReadMemStats or other runtime background works.
+// So, the tests are:
+// 1. at least 100 new allocated heap objects after invoking withoutNoEscape 100 times.
+// 2. less than 100 new allocated heap objects after invoking withoutNoEscape 100 times.
+
+/*
+#cgo noescape runCWithNoEscape
+
+void runCWithNoEscape(void *p) {
+}
+void runCWithoutNoEscape(void *p) {
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+	"runtime/debug"
+	"unsafe"
+)
+
+const num = 100
+
+func init() {
+	register("CgoNoEscape", CgoNoEscape)
+}
+
+//go:noinline
+func withNoEscape() {
+	var str string
+	C.runCWithNoEscape(unsafe.Pointer(&str))
+}
+
+//go:noinline
+func withoutNoEscape() {
+	var str string
+	C.runCWithoutNoEscape(unsafe.Pointer(&str))
+}
+
+func CgoNoEscape() {
+	// make GC stop to see the heap objects allocated
+	debug.SetGCPercent(-1)
+
+	var stats runtime.MemStats
+	runtime.ReadMemStats(&stats)
+	preHeapObjects := stats.HeapObjects
+
+	for i := 0; i < num; i++ {
+		withNoEscape()
+	}
+
+	runtime.ReadMemStats(&stats)
+	nowHeapObjects := stats.HeapObjects
+
+	if nowHeapObjects-preHeapObjects >= num {
+		fmt.Printf("too many heap objects allocated, pre: %v, now: %v\n", preHeapObjects, nowHeapObjects)
+	}
+
+	runtime.ReadMemStats(&stats)
+	preHeapObjects = stats.HeapObjects
+
+	for i := 0; i < num; i++ {
+		withoutNoEscape()
+	}
+
+	runtime.ReadMemStats(&stats)
+	nowHeapObjects = stats.HeapObjects
+
+	if nowHeapObjects-preHeapObjects < num {
+		fmt.Printf("too few heap objects allocated, pre: %v, now: %v\n", preHeapObjects, nowHeapObjects)
+	}
+
+	fmt.Println("OK")
+}