cmd/compile: add missing WBs for reflect.{Slice,String}Header.Data

Fixes #19168.

Change-Id: I3f4fcc0b189c53819ac29ef8de86fdad76a17488
Reviewed-on: https://go-review.googlesource.com/37663
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -3454,6 +3454,11 @@ func (s *state) insertWBstore(t *Type, left, right *ssa.Value, skip skipMask) {
 	if !s.WBPos.IsKnown() {
 		s.WBPos = left.Pos
 	}
+	if t == Types[TUINTPTR] {
+		// Stores to reflect.{Slice,String}Header.Data.
+		s.vars[&memVar] = s.newValue3I(ssa.OpStoreWB, ssa.TypeMem, s.config.PtrSize, left, right, s.mem())
+		return
+	}
 	s.storeTypeScalars(t, left, right, skip)
 	s.storeTypePtrsWB(t, left, right)
 }

src/cmd/compile/internal/gc/walk.go

@@ -2006,6 +2006,29 @@ func isstack(n *Node) bool {
 	return false
 }
 
+// isReflectHeaderDataField reports whether l is an expression p.Data
+// where p has type reflect.SliceHeader or reflect.StringHeader.
+func isReflectHeaderDataField(l *Node) bool {
+	if l.Type != Types[TUINTPTR] {
+		return false
+	}
+
+	var tsym *Sym
+	switch l.Op {
+	case ODOT:
+		tsym = l.Left.Type.Sym
+	case ODOTPTR:
+		tsym = l.Left.Type.Elem().Sym
+	default:
+		return false
+	}
+
+	if tsym == nil || l.Sym.Name != "Data" || tsym.Pkg.Path != "reflect" {
+		return false
+	}
+	return tsym.Name == "SliceHeader" || tsym.Name == "StringHeader"
+}
+
 // Do we need a write barrier for assigning to l?
 func needwritebarrier(l *Node) bool {
 	if !use_writebarrier {
@@ -2016,15 +2039,21 @@ func needwritebarrier(l *Node) bool {
 		return false
 	}
 
-	// No write barrier for write of non-pointers.
-	dowidth(l.Type)
-
-	if !haspointers(l.Type) {
+	// No write barrier for write to stack.
+	if isstack(l) {
 		return false
 	}
 
-	// No write barrier for write to stack.
-	if isstack(l) {
+	// Package unsafe's documentation says storing pointers into
+	// reflect.SliceHeader and reflect.StringHeader's Data fields
+	// is valid, even though they have type uintptr (#19168).
+	if isReflectHeaderDataField(l) {
+		return true
+	}
+
+	// No write barrier for write of non-pointers.
+	dowidth(l.Type)
+	if !haspointers(l.Type) {
 		return false
 	}
 

test/fixedbugs/issue19168.go

@@ -0,0 +1,58 @@
+// errorcheck -0 -l -d=wb
+
+// Copyright 2017 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 p
+
+import (
+	"reflect"
+	"unsafe"
+
+	reflect2 "reflect"
+)
+
+func sink(e interface{})
+
+func a(hdr *reflect.SliceHeader, p *byte) {
+	hdr.Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+}
+
+func b(hdr *reflect.StringHeader, p *byte) {
+	hdr.Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+}
+
+func c(hdrs *[1]reflect.SliceHeader, p *byte) {
+	hdrs[0].Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+}
+
+func d(hdr *struct{ s reflect.StringHeader }, p *byte) {
+	hdr.s.Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+}
+
+func e(p *byte) (resHeap, resStack string) {
+	sink(&resHeap)
+
+	hdr := (*reflect.StringHeader)(unsafe.Pointer(&resHeap))
+	hdr.Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+
+	// No write barrier for non-escaping stack vars.
+	hdr = (*reflect.StringHeader)(unsafe.Pointer(&resStack))
+	hdr.Data = uintptr(unsafe.Pointer(p))
+
+	return
+}
+
+func f(hdr *reflect2.SliceHeader, p *byte) {
+	hdr.Data = uintptr(unsafe.Pointer(p)) // ERROR "write barrier"
+}
+
+type SliceHeader struct {
+	Data uintptr
+}
+
+func g(hdr *SliceHeader, p *byte) {
+	// No write barrier for lookalike SliceHeader.
+	hdr.Data = uintptr(unsafe.Pointer(p))
+}