Source file test/recover4.go

     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  )
    28  
    29  func memcopy(dst, src []byte) (n int, err error) {
    30  	defer func() {
    31  		if r, ok := recover().(error); ok {
    32  			err = r
    33  		}
    34  	}()
    35  
    36  	for i := 0; i < len(dst) && i < len(src); i++ {
    37  		dst[i] = src[i]
    38  		n++
    39  	}
    40  	return
    41  }
    42  
    43  func main() {
    44  	// Turn the eventual fault into a panic, not a program crash,
    45  	// so that memcopy can recover.
    46  	debug.SetPanicOnFault(true)
    47  
    48  	size := syscall.Getpagesize()
    49  
    50  	// Map 16 pages of data with a 4-page hole in the middle.
    51  	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
    52  	if err != nil {
    53  		log.Fatalf("mmap: %v", err)
    54  	}
    55  
    56  	// Create a hole in the mapping that's PROT_NONE.
    57  	// Note that we can't use munmap here because the Go runtime
    58  	// could create a mapping that ends up in this hole otherwise,
    59  	// invalidating the test.
    60  	hole := data[len(data)/2 : 3*(len(data)/4)]
    61  	if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
    62  		log.Fatalf("mprotect: %v", err)
    63  	}
    64  
    65  	// Check that memcopy returns the actual amount copied
    66  	// before the fault.
    67  	const offset = 5
    68  	n, err := memcopy(data[offset:], make([]byte, len(data)))
    69  	if err == nil {
    70  		log.Fatal("no error from memcopy across memory hole")
    71  	}
    72  	if expect := len(data)/2 - offset; n != expect {
    73  		log.Fatalf("memcopy returned %d, want %d", n, expect)
    74  	}
    75  }
    76  

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