runtime: skip writes to persistent memory in cgo checker

Fixes #23899
Fixes #28458

Change-Id: Ie177f2d4c399445d8d5e1a327f2419c7866cb45e
Reviewed-on: https://go-review.googlesource.com/c/155697
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

misc/cgo/errors/ptr_test.go

@@ -406,6 +406,24 @@ var ptrTests = []ptrTest{
 		body:    `var b bytes.Buffer; b.WriteString("a"); C.f(unsafe.Pointer(&b.Bytes()[0]))`,
 		fail:    false,
 	},
+	{
+		// Test that bgsweep releasing a finalizer is OK.
+		name:    "finalizer",
+		c:       `// Nothing to declare.`,
+		imports: []string{"os"},
+		support: `func open() { os.Open(os.Args[0]) }; var G [][]byte`,
+		body:    `for i := 0; i < 10000; i++ { G = append(G, make([]byte, 4096)); if i % 100 == 0 { G = nil; open() } }`,
+		fail:    false,
+	},
+	{
+		// Test that converting generated struct to interface is OK.
+		name:    "structof",
+		c:       `// Nothing to declare.`,
+		imports: []string{"reflect"},
+		support: `type MyInt int; func (i MyInt) Get() int { return int(i) }; type Getter interface { Get() int }`,
+		body:    `t := reflect.StructOf([]reflect.StructField{{Name: "MyInt", Type: reflect.TypeOf(MyInt(0)), Anonymous: true}}); v := reflect.New(t).Elem(); v.Interface().(Getter).Get()`,
+		fail:    false,
+	},
 }
 
 func TestPointerChecks(t *testing.T) {
@@ -478,7 +496,7 @@ func testOne(t *testing.T, pt ptrTest) {
 
 	cmd := exec.Command("go", "build")
 	cmd.Dir = src
-	cmd.Env = addEnv("GOPATH", gopath)
+	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
 	buf, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Logf("%#q:\n%s", args(cmd), buf)
@@ -550,16 +568,5 @@ func testOne(t *testing.T, pt ptrTest) {
 }
 
 func cgocheckEnv(val string) []string {
-	return addEnv("GODEBUG", "cgocheck="+val)
-}
-
-func addEnv(key, val string) []string {
-	env := []string{key + "=" + val}
-	look := key + "="
-	for _, e := range os.Environ() {
-		if !strings.HasPrefix(e, look) {
-			env = append(env, e)
-		}
-	}
-	return env
+	return append(os.Environ(), "GODEBUG=cgocheck="+val)
 }

src/runtime/cgocheck.go

@@ -43,6 +43,13 @@ func cgoCheckWriteBarrier(dst *uintptr, src uintptr) {
 		return
 	}
 
+	// It's OK if writing to memory allocated by persistentalloc.
+	// Do this check last because it is more expensive and rarely true.
+	// If it is false the expense doesn't matter since we are crashing.
+	if inPersistentAlloc(uintptr(unsafe.Pointer(dst))) {
+		return
+	}
+
 	systemstack(func() {
 		println("write of Go pointer", hex(src), "to non-Go memory", hex(uintptr(unsafe.Pointer(dst))))
 		throw(cgoWriteBarrierFail)

src/runtime/malloc.go

@@ -1167,6 +1167,15 @@ var globalAlloc struct {
 	persistentAlloc
 }
 
+// persistentChunkSize is the number of bytes we allocate when we grow
+// a persistentAlloc.
+const persistentChunkSize = 256 << 10
+
+// persistentChunks is a list of all the persistent chunks we have
+// allocated. The list is maintained through the first word in the
+// persistent chunk. This is updated atomically.
+var persistentChunks *notInHeap
+
 // Wrapper around sysAlloc that can allocate small chunks.
 // There is no associated free operation.
 // Intended for things like function/type/debug-related persistent data.
@@ -1187,7 +1196,6 @@ func persistentalloc(size, align uintptr, sysStat *uint64) unsafe.Pointer {
 //go:systemstack
 func persistentalloc1(size, align uintptr, sysStat *uint64) *notInHeap {
 	const (
-		chunk    = 256 << 10
 		maxBlock = 64 << 10 // VM reservation granularity is 64K on windows
 	)
 
@@ -1218,15 +1226,24 @@ func persistentalloc1(size, align uintptr, sysStat *uint64) *notInHeap {
 		persistent = &globalAlloc.persistentAlloc
 	}
 	persistent.off = round(persistent.off, align)
-	if persistent.off+size > chunk || persistent.base == nil {
-		persistent.base = (*notInHeap)(sysAlloc(chunk, &memstats.other_sys))
+	if persistent.off+size > persistentChunkSize || persistent.base == nil {
+		persistent.base = (*notInHeap)(sysAlloc(persistentChunkSize, &memstats.other_sys))
 		if persistent.base == nil {
 			if persistent == &globalAlloc.persistentAlloc {
 				unlock(&globalAlloc.mutex)
 			}
 			throw("runtime: cannot allocate memory")
 		}
-		persistent.off = 0
+
+		// Add the new chunk to the persistentChunks list.
+		for {
+			chunks := uintptr(unsafe.Pointer(persistentChunks))
+			*(*uintptr)(unsafe.Pointer(persistent.base)) = chunks
+			if atomic.Casuintptr((*uintptr)(unsafe.Pointer(&persistentChunks)), chunks, uintptr(unsafe.Pointer(persistent.base))) {
+				break
+			}
+		}
+		persistent.off = sys.PtrSize
 	}
 	p := persistent.base.add(persistent.off)
 	persistent.off += size
@@ -1242,6 +1259,21 @@ func persistentalloc1(size, align uintptr, sysStat *uint64) *notInHeap {
 	return p
 }
 
+// inPersistentAlloc reports whether p points to memory allocated by
+// persistentalloc. This must be nosplit because it is called by the
+// cgo checker code, which is called by the write barrier code.
+//go:nosplit
+func inPersistentAlloc(p uintptr) bool {
+	chunk := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&persistentChunks)))
+	for chunk != 0 {
+		if p >= chunk && p < chunk+persistentChunkSize {
+			return true
+		}
+		chunk = *(*uintptr)(unsafe.Pointer(chunk))
+	}
+	return false
+}
+
 // linearAlloc is a simple linear allocator that pre-reserves a region
 // of memory and then maps that region as needed. The caller is
 // responsible for locking.