go/test/fixedbugs/issue8606b.go

// run
// +build linux darwin

// Copyright 2020 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.

// This is an optimization check. We want to make sure that we compare
// string lengths, and other scalar fields, before checking string
// contents.  There's no way to verify this in the language, and
// codegen tests in test/codegen can't really detect ordering
// optimizations like this. Instead, we generate invalid strings with
// bad backing store pointers but nonzero length, so we can check that
// the backing store never gets compared.
//
// We use two different bad strings so that pointer comparisons of
// backing store pointers fail.

package main

import (
	"fmt"
	"reflect"
	"syscall"
	"unsafe"
)

type SI struct {
	s string
	i int
}

type SS struct {
	s string
	t string
}

func main() {
	bad1 := "foo"
	bad2 := "foo"

	p := syscall.Getpagesize()
	b, err := syscall.Mmap(-1, 0, p, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
	if err != nil {
		panic(err)
	}
	err = syscall.Mprotect(b, syscall.PROT_NONE)
	if err != nil {
		panic(err)
	}
	// write inaccessible pointers as the data fields of bad1 and bad2.
	(*reflect.StringHeader)(unsafe.Pointer(&bad1)).Data = uintptr(unsafe.Pointer(&b[0]))
	(*reflect.StringHeader)(unsafe.Pointer(&bad2)).Data = uintptr(unsafe.Pointer(&b[1]))

	for _, test := range []struct {
		a, b interface{}
	}{
		{SI{s: bad1, i: 1}, SI{s: bad2, i: 2}},
		{SS{s: bad1, t: "a"}, SS{s: bad2, t: "aa"}},
		{SS{s: "a", t: bad1}, SS{s: "b", t: bad2}},
		// This one would panic because the length of both strings match, and we check
		// the body of the bad strings before the body of the good strings.
		//{SS{s: bad1, t: "a"}, SS{s: bad2, t: "b"}},
	} {
		if test.a == test.b {
			panic(fmt.Sprintf("values %#v and %#v should not be equal", test.a, test.b))
		}
	}

}