test/nilptr.go

// run

// Copyright 2011 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.

// Test that the implementation catches nil ptr indirection
// in a large address space.

package main

import "unsafe"

// Having a big address space means that indexing
// at a 256 MB offset from a nil pointer might not
// cause a memory access fault. This test checks
// that Go is doing the correct explicit checks to catch
// these nil pointer accesses, not just relying on the hardware.
var dummy [256 << 20]byte // give us a big address space

func main() {
	// the test only tests what we intend to test
	// if dummy starts in the first 256 MB of memory.
	// otherwise there might not be anything mapped
	// at the address that might be accidentally
	// dereferenced below.
	if uintptr(unsafe.Pointer(&dummy)) > 256<<20 {
		panic("dummy too far out")
	}

	shouldPanic(p1)
	shouldPanic(p2)
	shouldPanic(p3)
	shouldPanic(p4)
	shouldPanic(p5)
	shouldPanic(p6)
	shouldPanic(p7)
	shouldPanic(p8)
	shouldPanic(p9)
	shouldPanic(p10)
}

func shouldPanic(f func()) {
	defer func() {
		if recover() == nil {
			panic("memory reference did not panic")
		}
	}()
	f()
}

func p1() {
	// Array index.
	var p *[1 << 30]byte = nil
	println(p[256<<20]) // very likely to be inside dummy, but should panic
}

var xb byte

func p2() {
	var p *[1 << 30]byte = nil
	xb = 123

	// Array index.
	println(p[uintptr(unsafe.Pointer(&xb))]) // should panic
}

func p3() {
	// Array to slice.
	var p *[1 << 30]byte = nil
	var x []byte = p[0:] // should panic
	_ = x
}

var q *[1 << 30]byte

func p4() {
	// Array to slice.
	var x []byte
	var y = &x
	*y = q[0:] // should crash (uses arraytoslice runtime routine)
}

func fb([]byte) {
	panic("unreachable")
}

func p5() {
	// Array to slice.
	var p *[1 << 30]byte = nil
	fb(p[0:]) // should crash
}

func p6() {
	// Array to slice.
	var p *[1 << 30]byte = nil
	var _ []byte = p[10 : len(p)-10] // should crash
}

type T struct {
	x [256 << 20]byte
	i int
}

func f() *T {
	return nil
}

var y *T
var x = &y

func p7() {
	// Struct field access with large offset.
	println(f().i) // should crash
}

func p8() {
	// Struct field access with large offset.
	println((*x).i) // should crash
}

func p9() {
	// Struct field access with large offset.
	var t *T
	println(&t.i) // should crash
}

func p10() {
	// Struct field access with large offset.
	var t *T
	println(t.i) // should crash
}