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// run
//go:build linux || darwin
// 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.
// Test that if a slice access causes a fault, a deferred func
// sees the most recent value of the variables it accesses.
// This is true today; the role of the test is to ensure it stays true.
//
// In the test, memcopy is the function that will fault, during dst[i] = src[i].
// The deferred func recovers from the error and returns, making memcopy
// return the current value of n. If n is not being flushed to memory
// after each modification, the result will be a stale value of n.
//
// The test is set up by mmapping a 64 kB block of memory and then
// unmapping a 16 kB hole in the middle of it. Running memcopy
// on the resulting slice will fault when it reaches the hole.
package main
import (
"log"
"runtime/debug"
"syscall"
)
func memcopy(dst, src []byte) (n int, err error) {
defer func() {
if r, ok := recover().(error); ok {
err = r
}
}()
for i := 0; i < len(dst) && i < len(src); i++ {
dst[i] = src[i]
n++
}
return
}
func main() {
// Turn the eventual fault into a panic, not a program crash,
// so that memcopy can recover.
debug.SetPanicOnFault(true)
size := syscall.Getpagesize()
// Map 16 pages of data with a 4-page hole in the middle.
data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
if err != nil {
log.Fatalf("mmap: %v", err)
}
// Create a hole in the mapping that's PROT_NONE.
// Note that we can't use munmap here because the Go runtime
// could create a mapping that ends up in this hole otherwise,
// invalidating the test.
hole := data[len(data)/2 : 3*(len(data)/4)]
if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
log.Fatalf("mprotect: %v", err)
}
// Check that memcopy returns the actual amount copied
// before the fault.
const offset = 5
n, err := memcopy(data[offset:], make([]byte, len(data)))
if err == nil {
log.Fatal("no error from memcopy across memory hole")
}
if expect := len(data)/2 - offset; n != expect {
log.Fatalf("memcopy returned %d, want %d", n, expect)
}
}