runtime: don't return a slice with nil ptr but non-zero len from growslice

Fixes #10135.

Change-Id: Ic4c5ab15bcb7b9c3fcc685a788d3b59c60c26e1e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7400
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 7a2eb624b78b7a8f006f4ae3328787c245382b9a..ae46d9c1aca431a4b5bf62ed547e7ebc68a7df69 100644 (file)
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -50,7 +50,9 @@ func growslice(t *slicetype, old sliceStruct, n int) sliceStruct {
 
        et := t.elem
        if et.size == 0 {
-               return sliceStruct{old.array, old.len, cap}
+               // append should not create a slice with nil pointer but non-zero len.
+               // We assume that append doesn't need to preserve old.array in this case.
+               return sliceStruct{unsafe.Pointer(&zerobase), old.len, cap}
        }
 
        newcap := old.cap

diff --git a/test/fixedbugs/issue10135.go b/test/fixedbugs/issue10135.go
new file mode 100644 (file)
index 0000000..9985e5a
--- /dev/null
+++ b/test/fixedbugs/issue10135.go
@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2015 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 10135: append a slice with zero-sized element used
+// to always return a slice with the same data pointer as the
+// old slice, even if it's nil, so this program used to panic
+// with nil pointer dereference because after append, s is a
+// slice with nil data pointer but non-zero len and cap.
+
+package main
+
+type empty struct{}
+
+func main() {
+       var s []empty
+
+       s = append(s, empty{})
+
+       for _, v := range s {
+               _ = v
+       }
+}