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      1 // +build linux darwin
      2 // run
      3 
      4 // Copyright 2015 The Go Authors. All rights reserved.
      5 // Use of this source code is governed by a BSD-style
      6 // license that can be found in the LICENSE file.
      7 
      8 // Test that if a slice access causes a fault, a deferred func
      9 // sees the most recent value of the variables it accesses.
     10 // This is true today; the role of the test is to ensure it stays true.
     11 //
     12 // In the test, memcopy is the function that will fault, during dst[i] = src[i].
     13 // The deferred func recovers from the error and returns, making memcopy
     14 // return the current value of n. If n is not being flushed to memory
     15 // after each modification, the result will be a stale value of n.
     16 //
     17 // The test is set up by mmapping a 64 kB block of memory and then
     18 // unmapping a 16 kB hole in the middle of it. Running memcopy
     19 // on the resulting slice will fault when it reaches the hole.
     20 
     21 package main
     22 
     23 import (
     24 	"log"
     25 	"runtime/debug"
     26 	"syscall"
     27 	"unsafe"
     28 )
     29 
     30 func memcopy(dst, src []byte) (n int, err error) {
     31 	defer func() {
     32 		err = recover().(error)
     33 	}()
     34 
     35 	for i := 0; i < len(dst) && i < len(src); i++ {
     36 		dst[i] = src[i]
     37 		n++
     38 	}
     39 	return
     40 }
     41 
     42 func main() {
     43 	// Turn the eventual fault into a panic, not a program crash,
     44 	// so that memcopy can recover.
     45 	debug.SetPanicOnFault(true)
     46 
     47 	size := syscall.Getpagesize()
     48 
     49 	// Map 16 pages of data with a 4-page hole in the middle.
     50 	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
     51 	if err != nil {
     52 		log.Fatalf("mmap: %v", err)
     53 	}
     54 
     55 	other := make([]byte, 16*size)
     56 
     57 	// Note: Cannot call syscall.Munmap, because Munmap checks
     58 	// that you are unmapping a whole region returned by Mmap.
     59 	// We are trying to unmap just a hole in the middle.
     60 	if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 {
     61 		log.Fatalf("munmap: %v", err)
     62 	}
     63 
     64 	// Check that memcopy returns the actual amount copied
     65 	// before the fault (8*size - 5, the offset we skip in the argument).
     66 	n, err := memcopy(data[5:], other)
     67 	if err == nil {
     68 		log.Fatal("no error from memcopy across memory hole")
     69 	}
     70 	if n != 8*size-5 {
     71 		log.Fatal("memcopy returned %d, want %d", n, 8*size-5)
     72 	}
     73 }
     74