cmd/gc: fix Offsetof computation.

The offset of an embedded field s.X must be relative to s
and not to the implicit s.Field of which X is a direct field.
Moreover, no indirections may happen on the path.

Fixes #4909.

R=nigeltao, ality, daniel.morsing, iant, gri, r
CC=golang-dev
https://golang.org/cl/8287043

src/cmd/gc/unsafe.c

@@ -16,7 +16,7 @@
 Node*
 unsafenmagic(Node *nn)
 {
-	Node *r, *n;
+	Node *r, *n, *base, *r1;
 	Sym *s;
 	Type *t, *tr;
 	long v;
@@ -49,11 +49,43 @@ unsafenmagic(Node *nn)
 		goto yes;
 	}
 	if(strcmp(s->name, "Offsetof") == 0) {
-		typecheck(&r, Erv);
-		if(r->op != ODOT && r->op != ODOTPTR)
+		// must be a selector.
+		if(r->op != OXDOT)
 			goto bad;
+		// Remember base of selector to find it back after dot insertion.
+		// Since r->left may be mutated by typechecking, check it explicitly
+		// first to track it correctly.
+		typecheck(&r->left, Erv);
+		base = r->left;
 		typecheck(&r, Erv);
-		v = r->xoffset;
+		switch(r->op) {
+		case ODOT:
+		case ODOTPTR:
+			break;
+		case OCALLPART:
+			yyerror("invalid expression %N: argument is a method value", nn);
+			v = 0;
+			goto ret;
+		default:
+			goto bad;
+		}
+		v = 0;
+		// add offsets for inserted dots.
+		for(r1=r; r1->left!=base; r1=r1->left) {
+			switch(r1->op) {
+			case ODOT:
+				v += r1->xoffset;
+				break;
+			case ODOTPTR:
+				yyerror("invalid expression %N: selector implies indirection of embedded %N", nn, r1->left);
+				goto ret;
+			default:
+				dump("unsafenmagic", r);
+				fatal("impossible %#O node after dot insertion", r1->op);
+				goto bad;
+			}
+		}
+		v += r1->xoffset;
 		goto yes;
 	}
 	if(strcmp(s->name, "Alignof") == 0) {

test/fixedbugs/issue4909a.go

@@ -0,0 +1,35 @@
+// errorcheck
+
+// Copyright 2013 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.
+
+// Issue 4909: compiler incorrectly accepts unsafe.Offsetof(t.x)
+// where x is a field of an embedded pointer field.
+
+package p
+
+import (
+	"unsafe"
+)
+
+type T struct {
+	A int
+	*B
+}
+
+func (t T) Method() {}
+
+type B struct {
+	X, Y int
+}
+
+var t T
+var p *T
+
+const N1 = unsafe.Offsetof(t.X)      // ERROR "indirection"
+const N2 = unsafe.Offsetof(p.X)      // ERROR "indirection"
+const N3 = unsafe.Offsetof(t.B.X)    // valid
+const N4 = unsafe.Offsetof(p.B.X)    // valid
+const N5 = unsafe.Offsetof(t.Method) // ERROR "method value"
+const N6 = unsafe.Offsetof(p.Method) // ERROR "method value"

test/fixedbugs/issue4909b.go

@@ -0,0 +1,80 @@
+// errorcheckoutput
+
+package main
+
+import "fmt"
+
+// We are going to define 256 types T(n),
+// such that T(n) embeds T(2n) and *T(2n+1).
+
+func main() {
+	fmt.Printf("// errorcheck\n\n")
+	fmt.Printf("package p\n\n")
+	fmt.Println(`import "unsafe"`)
+
+	// Dump types.
+	for n := 1; n < 256; n++ {
+		writeStruct(n)
+	}
+	// Dump leaves
+	for n := 256; n < 512; n++ {
+		fmt.Printf("type T%d int\n", n)
+	}
+
+	fmt.Printf("var t T1\n")
+	fmt.Printf("var p *T1\n")
+
+	// Simple selectors
+	for n := 2; n < 256; n++ {
+		writeDot(n)
+	}
+
+	// Double selectors
+	for n := 128; n < 256; n++ {
+		writeDot(n/16, n)
+	}
+
+	// Triple selectors
+	for n := 128; n < 256; n++ {
+		writeDot(n/64, n/8, n)
+	}
+}
+
+const structTpl = `
+type T%d struct {
+	A%d int
+	T%d
+	*T%d
+}
+`
+
+func writeStruct(n int) {
+	fmt.Printf(structTpl, n, n, 2*n, 2*n+1)
+}
+
+func writeDot(ns ...int) {
+	for _, root := range []string{"t", "p"} {
+		fmt.Printf("const _ = unsafe.Offsetof(%s", root)
+		for _, n := range ns {
+			fmt.Printf(".T%d", n)
+		}
+		// Does it involve an indirection?
+		nlast := ns[len(ns)-1]
+		nprev := 1
+		if len(ns) > 1 {
+			nprev = ns[len(ns)-2]
+		}
+		isIndirect := false
+		for n := nlast / 2; n > nprev; n /= 2 {
+			if n%2 == 1 {
+				isIndirect = true
+				break
+			}
+		}
+		fmt.Print(")")
+		if isIndirect {
+			fmt.Print(` // ERROR "indirection"`)
+		}
+		fmt.Print("\n")
+	}
+}

test/sizeof.go

@@ -4,8 +4,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Test unsafe.Sizeof, unsafe.Alignof, and unsafe.Offsetof all return uintptr.
-
 package main
 
 import "unsafe"
@@ -18,8 +16,143 @@ var t T
 
 func isUintptr(uintptr) {}
 
+type T2 struct {
+	A int32
+	U2
+}
+
+type U2 struct {
+	B int32
+	C int32
+}
+
+var t2 T2
+var p2 *T2
+
 func main() {
+	// Test unsafe.Sizeof, unsafe.Alignof, and unsafe.Offsetof all return uintptr.
 	isUintptr(unsafe.Sizeof(t))
 	isUintptr(unsafe.Alignof(t))
 	isUintptr(unsafe.Offsetof(t.X))
+
+	// Test correctness of Offsetof with respect to embedded fields (issue 4909).
+	if unsafe.Offsetof(t2.C) != 8 {
+		println(unsafe.Offsetof(t2.C), "!= 8")
+		panic("unsafe.Offsetof(t2.C) != 8")
+	}
+	if unsafe.Offsetof(p2.C) != 8 {
+		println(unsafe.Offsetof(p2.C), "!= 8")
+		panic("unsafe.Offsetof(p2.C) != 8")
+	}
+	if unsafe.Offsetof(t2.U2.C) != 4 {
+		println(unsafe.Offsetof(t2.U2.C), "!= 4")
+		panic("unsafe.Offsetof(t2.U2.C) != 4")
+	}
+	if unsafe.Offsetof(p2.U2.C) != 4 {
+		println(unsafe.Offsetof(p2.U2.C), "!= 4")
+		panic("unsafe.Offsetof(p2.U2.C) != 4")
+	}
+	testDeep()
+	testNotEmbedded()
+}
+
+type (
+	S1 struct {
+		A int32
+		S2
+	}
+	S2 struct {
+		B int32
+		S3
+	}
+	S3 struct {
+		C int32
+		S4
+	}
+	S4 struct {
+		D int32
+		S5
+	}
+	S5 struct {
+		E int32
+		S6
+	}
+	S6 struct {
+		F int32
+		S7
+	}
+	S7 struct {
+		G int32
+		S8
+	}
+	S8 struct {
+		H int32
+		*S1
+	}
+)
+
+func testDeep() {
+	var s1 S1
+	switch {
+	case unsafe.Offsetof(s1.A) != 0:
+		panic("unsafe.Offsetof(s1.A) != 0")
+	case unsafe.Offsetof(s1.B) != 4:
+		panic("unsafe.Offsetof(s1.B) != 4")
+	case unsafe.Offsetof(s1.C) != 8:
+		panic("unsafe.Offsetof(s1.C) != 8")
+	case unsafe.Offsetof(s1.D) != 12:
+		panic("unsafe.Offsetof(s1.D) != 12")
+	case unsafe.Offsetof(s1.E) != 16:
+		panic("unsafe.Offsetof(s1.E) != 16")
+	case unsafe.Offsetof(s1.F) != 20:
+		panic("unsafe.Offsetof(s1.F) != 20")
+	case unsafe.Offsetof(s1.G) != 24:
+		panic("unsafe.Offsetof(s1.G) != 24")
+	case unsafe.Offsetof(s1.H) != 28:
+		panic("unsafe.Offsetof(s1.H) != 28")
+	case unsafe.Offsetof(s1.S1) != 32:
+		panic("unsafe.Offsetof(s1.S1) != 32")
+	case unsafe.Offsetof(s1.S1.S2.S3.S4.S5.S6.S7.S8.S1.S2) != 4:
+		panic("unsafe.Offsetof(s1.S1.S2.S3.S4.S5.S6.S7.S8.S1.S2) != 4")
+	}
+}
+
+func testNotEmbedded() {
+	type T2 struct {
+		B int32
+		C int32
+	}
+	type T1 struct {
+		A int32
+		T2
+	}
+	type T struct {
+		Dummy int32
+		F     T1
+		P     *T1
+	}
+
+	var t T
+	var p *T
+	switch {
+	case unsafe.Offsetof(t.F.B) != 4:
+		panic("unsafe.Offsetof(t.F.B) != 4")
+	case unsafe.Offsetof(t.F.C) != 8:
+		panic("unsafe.Offsetof(t.F.C) != 8")
+
+	case unsafe.Offsetof(t.P.B) != 4:
+		panic("unsafe.Offsetof(t.P.B) != 4")
+	case unsafe.Offsetof(t.P.C) != 8:
+		panic("unsafe.Offsetof(t.P.C) != 8")
+
+	case unsafe.Offsetof(p.F.B) != 4:
+		panic("unsafe.Offsetof(p.F.B) != 4")
+	case unsafe.Offsetof(p.F.C) != 8:
+		panic("unsafe.Offsetof(p.F.C) != 8")
+
+	case unsafe.Offsetof(p.P.B) != 4:
+		panic("unsafe.Offsetof(p.P.B) != 4")
+	case unsafe.Offsetof(p.P.C) != 8:
+		panic("unsafe.Offsetof(p.P.C) != 8")
+	}
 }