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runtime: make scan of pointer-in-interface same as scan of pointer
The GC program describing a data structure sometimes trusts the pointer base type and other times does not (if not, the garbage collector must fall back on per-allocation type information stored in the heap). Make the scanning of a pointer in an interface do the same. This fixes a crash in a particular use of reflect.SliceHeader. Fixes #8004. LGTM=khr R=golang-codereviews, khr CC=0xe2.0x9a.0x9b, golang-codereviews, iant, r https://golang.org/cl/100470045
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@ -1322,7 +1322,22 @@ dgcsym1(Sym *s, int ot, Type *t, vlong *off, int stack_size)
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// NOTE: Any changes here need to be made to reflect.PtrTo as well.
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if(*off % widthptr != 0)
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fatal("dgcsym1: invalid alignment, %T", t);
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if(!haspointers(t->type) || t->type->etype == TUINT8) {
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// NOTE(rsc): Emitting GC_APTR here for *nonptrtype
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// (pointer to non-pointer-containing type) means that
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// we do not record 'nonptrtype' and instead tell the
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// garbage collector to look up the type of the memory in
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// type information stored in the heap. In effect we are telling
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// the collector "we don't trust our information - use yours".
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// It's not completely clear why we want to do this.
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// It does have the effect that if you have a *SliceHeader and a *[]int
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// pointing at the same actual slice header, *SliceHeader will not be
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// used as an authoritative type for the memory, which is good:
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// if the collector scanned the memory as type *SliceHeader, it would
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// see no pointers inside but mark the block as scanned, preventing
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// the seeing of pointers when we followed the *[]int pointer.
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// Perhaps that kind of situation is the rationale.
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if(!haspointers(t->type)) {
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ot = duintptr(s, ot, GC_APTR);
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ot = duintptr(s, ot, *off);
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} else {
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@ -724,7 +724,7 @@ scanblock(Workbuf *wbuf, bool keepworking)
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uintptr *pc, precise_type, nominal_size;
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uintptr *chan_ret, chancap;
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void *obj;
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Type *t;
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Type *t, *et;
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Slice *sliceptr;
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String *stringptr;
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Frame *stack_ptr, stack_top, stack[GC_STACK_CAPACITY+4];
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@ -941,8 +941,14 @@ scanblock(Workbuf *wbuf, bool keepworking)
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continue;
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obj = eface->data;
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if((t->kind & ~KindNoPointers) == KindPtr)
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objti = (uintptr)((PtrType*)t)->elem->gc;
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if((t->kind & ~KindNoPointers) == KindPtr) {
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// Only use type information if it is a pointer-containing type.
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// This matches the GC programs written by cmd/gc/reflect.c's
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// dgcsym1 in case TPTR32/case TPTR64. See rationale there.
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et = ((PtrType*)t)->elem;
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if(!(et->kind & KindNoPointers))
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objti = (uintptr)((PtrType*)t)->elem->gc;
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}
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} else {
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obj = eface->data;
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objti = (uintptr)t->gc;
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@ -973,8 +979,14 @@ scanblock(Workbuf *wbuf, bool keepworking)
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continue;
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obj = iface->data;
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if((t->kind & ~KindNoPointers) == KindPtr)
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objti = (uintptr)((PtrType*)t)->elem->gc;
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if((t->kind & ~KindNoPointers) == KindPtr) {
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// Only use type information if it is a pointer-containing type.
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// This matches the GC programs written by cmd/gc/reflect.c's
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// dgcsym1 in case TPTR32/case TPTR64. See rationale there.
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et = ((PtrType*)t)->elem;
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if(!(et->kind & KindNoPointers))
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objti = (uintptr)((PtrType*)t)->elem->gc;
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}
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} else {
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obj = iface->data;
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objti = (uintptr)t->gc;
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59
test/fixedbugs/issue8004.go
Normal file
59
test/fixedbugs/issue8004.go
Normal file
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@ -0,0 +1,59 @@
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// run
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import (
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"reflect"
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"runtime"
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"unsafe"
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)
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func main() {
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test1()
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test2()
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}
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func test1() {
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var all []interface{}
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for i := 0; i < 100; i++ {
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p := new([]int)
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*p = append(*p, 1, 2, 3, 4)
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h := (*reflect.SliceHeader)(unsafe.Pointer(p))
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all = append(all, h, p)
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}
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runtime.GC()
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for i := 0; i < 100; i++ {
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p := *all[2*i+1].(*[]int)
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if p[0] != 1 || p[1] != 2 || p[2] != 3 || p[3] != 4 {
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println("BUG test1: bad slice at index", i, p[0], p[1], p[2], p[3])
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return
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}
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}
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}
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type T struct {
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H *reflect.SliceHeader
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P *[]int
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}
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func test2() {
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var all []T
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for i := 0; i < 100; i++ {
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p := new([]int)
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*p = append(*p, 1, 2, 3, 4)
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h := (*reflect.SliceHeader)(unsafe.Pointer(p))
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all = append(all, T{H: h}, T{P: p})
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}
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runtime.GC()
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for i := 0; i < 100; i++ {
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p := *all[2*i+1].P
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if p[0] != 1 || p[1] != 2 || p[2] != 3 || p[3] != 4 {
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println("BUG test2: bad slice at index", i, p[0], p[1], p[2], p[3])
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return
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}
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}
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}
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