runtime: when using explicit argmap, also use arglen

When we set an explicit argmap, we may want only a prefix of that
argmap.  Argmap is set when the function is reflect.makeFuncStub or
reflect.methodValueCall. In this case, arglen specifies how much of
the args section is actually live. (It could be either all the args +
results, or just the args.)

Fixes #28750

Change-Id: Idf060607f15a298ac591016994e58e22f7f92d83
Reviewed-on: https://go-review.googlesource.com/c/149217
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/stack.go

@@ -1254,7 +1254,14 @@ func getStackMap(frame *stkframe, cache *pcvalueCache, debug bool) (locals, args
 	// Arguments.
 	if frame.arglen > 0 {
 		if frame.argmap != nil {
+			// argmap is set when the function is reflect.makeFuncStub or reflect.methodValueCall.
+			// In this case, arglen specifies how much of the args section is actually live.
+			// (It could be either all the args + results, or just the args.)
 			args = *frame.argmap
+			n := int32(frame.arglen / sys.PtrSize)
+			if n < args.n {
+				args.n = n // Don't use more of the arguments than arglen.
+			}
 		} else {
 			stackmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))
 			if stackmap == nil || stackmap.n <= 0 {

test/fixedbugs/issue27695c.go

+// run
+
+// Copyright 2018 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.
+
+// Make sure return values aren't scanned until they
+// are initialized, when calling functions and methods
+// via reflect.
+
+package main
+
+import (
+	"io"
+	"reflect"
+	"runtime"
+	"unsafe"
+)
+
+var badPtr uintptr
+
+var sink []byte
+
+func init() {
+	// Allocate large enough to use largeAlloc.
+	b := make([]byte, 1<<16-1)
+	sink = b // force heap allocation
+	//  Any space between the object and the end of page is invalid to point to.
+	badPtr = uintptr(unsafe.Pointer(&b[len(b)-1])) + 1
+}
+
+func f(d func(error) error) error {
+	// Initialize callee args section with a bad pointer.
+	g(badPtr, badPtr, badPtr, badPtr)
+
+	// Then call a function which returns a pointer.
+	// That return slot starts out holding a bad pointer.
+	return d(io.EOF)
+}
+
+//go:noinline
+func g(x, y, z, w uintptr) {
+}
+
+type T struct {
+}
+
+func (t *T) Foo(e error) error {
+	runtime.GC()
+	return e
+}
+
+func main() {
+	// Functions
+	d := reflect.MakeFunc(reflect.TypeOf(func(e error) error { return e }),
+		func(args []reflect.Value) []reflect.Value {
+			runtime.GC()
+			return args
+		}).Interface().(func(error) error)
+	f(d)
+
+	// Methods
+	x := reflect.ValueOf(&T{}).Method(0).Interface().(func(error) error)
+	f(x)
+}