1// run
2
3//go:build linux || darwin
4
5// Copyright 2020 The Go Authors. All rights reserved.
6// Use of this source code is governed by a BSD-style
7// license that can be found in the LICENSE file.
8
9// This is an optimization check. We want to make sure that we compare
10// string lengths, and other scalar fields, before checking string
11// contents.  There's no way to verify this in the language, and
12// codegen tests in test/codegen can't really detect ordering
13// optimizations like this. Instead, we generate invalid strings with
14// bad backing store pointers but nonzero length, so we can check that
15// the backing store never gets compared.
16//
17// We use two different bad strings so that pointer comparisons of
18// backing store pointers fail.
19
20package main
21
22import (
23	"fmt"
24	"reflect"
25	"syscall"
26	"unsafe"
27)
28
29type SI struct {
30	s string
31	i int
32}
33
34type SS struct {
35	s string
36	t string
37}
38
39func main() {
40	bad1 := "foo"
41	bad2 := "foo"
42
43	p := syscall.Getpagesize()
44	b, err := syscall.Mmap(-1, 0, p, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
45	if err != nil {
46		panic(err)
47	}
48	err = syscall.Mprotect(b, syscall.PROT_NONE)
49	if err != nil {
50		panic(err)
51	}
52	// write inaccessible pointers as the data fields of bad1 and bad2.
53	(*reflect.StringHeader)(unsafe.Pointer(&bad1)).Data = uintptr(unsafe.Pointer(&b[0]))
54	(*reflect.StringHeader)(unsafe.Pointer(&bad2)).Data = uintptr(unsafe.Pointer(&b[1]))
55
56	for _, test := range []struct {
57		a, b interface{}
58	}{
59		{SI{s: bad1, i: 1}, SI{s: bad2, i: 2}},
60		{SS{s: bad1, t: "a"}, SS{s: bad2, t: "aa"}},
61		{SS{s: "a", t: bad1}, SS{s: "b", t: bad2}},
62		// This one would panic because the length of both strings match, and we check
63		// the body of the bad strings before the body of the good strings.
64		//{SS{s: bad1, t: "a"}, SS{s: bad2, t: "b"}},
65	} {
66		if test.a == test.b {
67			panic(fmt.Sprintf("values %#v and %#v should not be equal", test.a, test.b))
68		}
69	}
70
71}
72