github.com/Andyfoo/golang/x/sys@v0.0.0-20190901054642-57c1bf301704/unix/syscall_linux_test.go (about)

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // +build linux
     6  
     7  package unix_test
     8  
     9  import (
    10  	"bufio"
    11  	"bytes"
    12  	"errors"
    13  	"fmt"
    14  	"io/ioutil"
    15  	"os"
    16  	"runtime"
    17  	"runtime/debug"
    18  	"strconv"
    19  	"strings"
    20  	"testing"
    21  	"time"
    22  
    23  	"github.com/Andyfoo/golang/x/sys/unix"
    24  )
    25  
    26  func TestIoctlGetInt(t *testing.T) {
    27  	f, err := os.Open("/dev/random")
    28  	if err != nil {
    29  		t.Fatalf("failed to open device: %v", err)
    30  	}
    31  	defer f.Close()
    32  
    33  	v, err := unix.IoctlGetInt(int(f.Fd()), unix.RNDGETENTCNT)
    34  	if err != nil {
    35  		t.Fatalf("failed to perform ioctl: %v", err)
    36  	}
    37  
    38  	t.Logf("%d bits of entropy available", v)
    39  }
    40  
    41  func TestIoctlGetRTCTime(t *testing.T) {
    42  	f, err := os.Open("/dev/rtc0")
    43  	if err != nil {
    44  		t.Skipf("skipping test, %v", err)
    45  	}
    46  	defer f.Close()
    47  
    48  	v, err := unix.IoctlGetRTCTime(int(f.Fd()))
    49  	if err != nil {
    50  		t.Fatalf("failed to perform ioctl: %v", err)
    51  	}
    52  
    53  	t.Logf("RTC time: %04d-%02d-%02d %02d:%02d:%02d", v.Year+1900, v.Mon+1, v.Mday, v.Hour, v.Min, v.Sec)
    54  }
    55  
    56  func TestPpoll(t *testing.T) {
    57  	if runtime.GOOS == "android" {
    58  		t.Skip("mkfifo syscall is not available on android, skipping test")
    59  	}
    60  
    61  	defer chtmpdir(t)()
    62  	f, cleanup := mktmpfifo(t)
    63  	defer cleanup()
    64  
    65  	const timeout = 100 * time.Millisecond
    66  
    67  	ok := make(chan bool, 1)
    68  	go func() {
    69  		select {
    70  		case <-time.After(10 * timeout):
    71  			t.Errorf("Ppoll: failed to timeout after %d", 10*timeout)
    72  		case <-ok:
    73  		}
    74  	}()
    75  
    76  	fds := []unix.PollFd{{Fd: int32(f.Fd()), Events: unix.POLLIN}}
    77  	timeoutTs := unix.NsecToTimespec(int64(timeout))
    78  	n, err := unix.Ppoll(fds, &timeoutTs, nil)
    79  	ok <- true
    80  	if err != nil {
    81  		t.Errorf("Ppoll: unexpected error: %v", err)
    82  		return
    83  	}
    84  	if n != 0 {
    85  		t.Errorf("Ppoll: wrong number of events: got %v, expected %v", n, 0)
    86  		return
    87  	}
    88  }
    89  
    90  func TestTime(t *testing.T) {
    91  	var ut unix.Time_t
    92  	ut2, err := unix.Time(&ut)
    93  	if err != nil {
    94  		t.Fatalf("Time: %v", err)
    95  	}
    96  	if ut != ut2 {
    97  		t.Errorf("Time: return value %v should be equal to argument %v", ut2, ut)
    98  	}
    99  
   100  	var now time.Time
   101  
   102  	for i := 0; i < 10; i++ {
   103  		ut, err = unix.Time(nil)
   104  		if err != nil {
   105  			t.Fatalf("Time: %v", err)
   106  		}
   107  
   108  		now = time.Now()
   109  
   110  		if int64(ut) == now.Unix() {
   111  			return
   112  		}
   113  	}
   114  
   115  	t.Errorf("Time: return value %v should be nearly equal to time.Now().Unix() %v", ut, now.Unix())
   116  }
   117  
   118  func TestUtime(t *testing.T) {
   119  	defer chtmpdir(t)()
   120  
   121  	touch(t, "file1")
   122  
   123  	buf := &unix.Utimbuf{
   124  		Modtime: 12345,
   125  	}
   126  
   127  	err := unix.Utime("file1", buf)
   128  	if err != nil {
   129  		t.Fatalf("Utime: %v", err)
   130  	}
   131  
   132  	fi, err := os.Stat("file1")
   133  	if err != nil {
   134  		t.Fatal(err)
   135  	}
   136  
   137  	if fi.ModTime().Unix() != 12345 {
   138  		t.Errorf("Utime: failed to change modtime: expected %v, got %v", 12345, fi.ModTime().Unix())
   139  	}
   140  }
   141  
   142  func TestUtimesNanoAt(t *testing.T) {
   143  	defer chtmpdir(t)()
   144  
   145  	symlink := "symlink1"
   146  	os.Remove(symlink)
   147  	err := os.Symlink("nonexisting", symlink)
   148  	if err != nil {
   149  		t.Fatal(err)
   150  	}
   151  
   152  	ts := []unix.Timespec{
   153  		{Sec: 1111, Nsec: 2222},
   154  		{Sec: 3333, Nsec: 4444},
   155  	}
   156  	err = unix.UtimesNanoAt(unix.AT_FDCWD, symlink, ts, unix.AT_SYMLINK_NOFOLLOW)
   157  	if err != nil {
   158  		t.Fatalf("UtimesNanoAt: %v", err)
   159  	}
   160  
   161  	var st unix.Stat_t
   162  	err = unix.Lstat(symlink, &st)
   163  	if err != nil {
   164  		t.Fatalf("Lstat: %v", err)
   165  	}
   166  
   167  	// Only check Mtim, Atim might not be supported by the underlying filesystem
   168  	expected := ts[1]
   169  	if st.Mtim.Nsec == 0 {
   170  		// Some filesystems only support 1-second time stamp resolution
   171  		// and will always set Nsec to 0.
   172  		expected.Nsec = 0
   173  	}
   174  	if st.Mtim != expected {
   175  		t.Errorf("UtimesNanoAt: wrong mtime: expected %v, got %v", expected, st.Mtim)
   176  	}
   177  }
   178  
   179  func TestRlimitAs(t *testing.T) {
   180  	// disable GC during to avoid flaky test
   181  	defer debug.SetGCPercent(debug.SetGCPercent(-1))
   182  
   183  	var rlim unix.Rlimit
   184  	err := unix.Getrlimit(unix.RLIMIT_AS, &rlim)
   185  	if err != nil {
   186  		t.Fatalf("Getrlimit: %v", err)
   187  	}
   188  	var zero unix.Rlimit
   189  	if zero == rlim {
   190  		t.Fatalf("Getrlimit: got zero value %#v", rlim)
   191  	}
   192  	set := rlim
   193  	set.Cur = uint64(unix.Getpagesize())
   194  	err = unix.Setrlimit(unix.RLIMIT_AS, &set)
   195  	if err != nil {
   196  		t.Fatalf("Setrlimit: set failed: %#v %v", set, err)
   197  	}
   198  
   199  	// RLIMIT_AS was set to the page size, so mmap()'ing twice the page size
   200  	// should fail. See 'man 2 getrlimit'.
   201  	_, err = unix.Mmap(-1, 0, 2*unix.Getpagesize(), unix.PROT_NONE, unix.MAP_ANON|unix.MAP_PRIVATE)
   202  	if err == nil {
   203  		t.Fatal("Mmap: unexpectedly succeeded after setting RLIMIT_AS")
   204  	}
   205  
   206  	err = unix.Setrlimit(unix.RLIMIT_AS, &rlim)
   207  	if err != nil {
   208  		t.Fatalf("Setrlimit: restore failed: %#v %v", rlim, err)
   209  	}
   210  
   211  	b, err := unix.Mmap(-1, 0, 2*unix.Getpagesize(), unix.PROT_NONE, unix.MAP_ANON|unix.MAP_PRIVATE)
   212  	if err != nil {
   213  		t.Fatalf("Mmap: %v", err)
   214  	}
   215  	err = unix.Munmap(b)
   216  	if err != nil {
   217  		t.Fatalf("Munmap: %v", err)
   218  	}
   219  }
   220  
   221  func TestSelect(t *testing.T) {
   222  	_, err := unix.Select(0, nil, nil, nil, &unix.Timeval{Sec: 0, Usec: 0})
   223  	if err != nil {
   224  		t.Fatalf("Select: %v", err)
   225  	}
   226  
   227  	dur := 150 * time.Millisecond
   228  	tv := unix.NsecToTimeval(int64(dur))
   229  	start := time.Now()
   230  	_, err = unix.Select(0, nil, nil, nil, &tv)
   231  	took := time.Since(start)
   232  	if err != nil {
   233  		t.Fatalf("Select: %v", err)
   234  	}
   235  
   236  	if took < dur {
   237  		t.Errorf("Select: timeout should have been at least %v, got %v", dur, took)
   238  	}
   239  }
   240  
   241  func TestPselect(t *testing.T) {
   242  	_, err := unix.Pselect(0, nil, nil, nil, &unix.Timespec{Sec: 0, Nsec: 0}, nil)
   243  	if err != nil {
   244  		t.Fatalf("Pselect: %v", err)
   245  	}
   246  
   247  	dur := 2500 * time.Microsecond
   248  	ts := unix.NsecToTimespec(int64(dur))
   249  	start := time.Now()
   250  	_, err = unix.Pselect(0, nil, nil, nil, &ts, nil)
   251  	took := time.Since(start)
   252  	if err != nil {
   253  		t.Fatalf("Pselect: %v", err)
   254  	}
   255  
   256  	if took < dur {
   257  		t.Errorf("Pselect: timeout should have been at least %v, got %v", dur, took)
   258  	}
   259  }
   260  
   261  func TestSchedSetaffinity(t *testing.T) {
   262  	runtime.LockOSThread()
   263  	defer runtime.UnlockOSThread()
   264  
   265  	var oldMask unix.CPUSet
   266  	err := unix.SchedGetaffinity(0, &oldMask)
   267  	if err != nil {
   268  		t.Fatalf("SchedGetaffinity: %v", err)
   269  	}
   270  
   271  	var newMask unix.CPUSet
   272  	newMask.Zero()
   273  	if newMask.Count() != 0 {
   274  		t.Errorf("CpuZero: didn't zero CPU set: %v", newMask)
   275  	}
   276  	cpu := 1
   277  	newMask.Set(cpu)
   278  	if newMask.Count() != 1 || !newMask.IsSet(cpu) {
   279  		t.Errorf("CpuSet: didn't set CPU %d in set: %v", cpu, newMask)
   280  	}
   281  	cpu = 5
   282  	newMask.Set(cpu)
   283  	if newMask.Count() != 2 || !newMask.IsSet(cpu) {
   284  		t.Errorf("CpuSet: didn't set CPU %d in set: %v", cpu, newMask)
   285  	}
   286  	newMask.Clear(cpu)
   287  	if newMask.Count() != 1 || newMask.IsSet(cpu) {
   288  		t.Errorf("CpuClr: didn't clear CPU %d in set: %v", cpu, newMask)
   289  	}
   290  
   291  	if runtime.NumCPU() < 2 {
   292  		t.Skip("skipping setaffinity tests on single CPU system")
   293  	}
   294  	if runtime.GOOS == "android" {
   295  		t.Skip("skipping setaffinity tests on android")
   296  	}
   297  
   298  	// On a system like ppc64x where some cores can be disabled using ppc64_cpu,
   299  	// setaffinity should only be called with enabled cores. The valid cores
   300  	// are found from the oldMask, but if none are found then the setaffinity
   301  	// tests are skipped. Issue #27875.
   302  	if !oldMask.IsSet(cpu) {
   303  		newMask.Zero()
   304  		for i := 0; i < len(oldMask); i++ {
   305  			if oldMask.IsSet(i) {
   306  				newMask.Set(i)
   307  				break
   308  			}
   309  		}
   310  		if newMask.Count() == 0 {
   311  			t.Skip("skipping setaffinity tests if CPU not available")
   312  		}
   313  	}
   314  
   315  	err = unix.SchedSetaffinity(0, &newMask)
   316  	if err != nil {
   317  		t.Fatalf("SchedSetaffinity: %v", err)
   318  	}
   319  
   320  	var gotMask unix.CPUSet
   321  	err = unix.SchedGetaffinity(0, &gotMask)
   322  	if err != nil {
   323  		t.Fatalf("SchedGetaffinity: %v", err)
   324  	}
   325  
   326  	if gotMask != newMask {
   327  		t.Errorf("SchedSetaffinity: returned affinity mask does not match set affinity mask")
   328  	}
   329  
   330  	// Restore old mask so it doesn't affect successive tests
   331  	err = unix.SchedSetaffinity(0, &oldMask)
   332  	if err != nil {
   333  		t.Fatalf("SchedSetaffinity: %v", err)
   334  	}
   335  }
   336  
   337  func TestStatx(t *testing.T) {
   338  	var stx unix.Statx_t
   339  	err := unix.Statx(unix.AT_FDCWD, ".", 0, 0, &stx)
   340  	if err == unix.ENOSYS || err == unix.EPERM {
   341  		t.Skip("statx syscall is not available, skipping test")
   342  	} else if err != nil {
   343  		t.Fatalf("Statx: %v", err)
   344  	}
   345  
   346  	defer chtmpdir(t)()
   347  	touch(t, "file1")
   348  
   349  	var st unix.Stat_t
   350  	err = unix.Stat("file1", &st)
   351  	if err != nil {
   352  		t.Fatalf("Stat: %v", err)
   353  	}
   354  
   355  	flags := unix.AT_STATX_SYNC_AS_STAT
   356  	err = unix.Statx(unix.AT_FDCWD, "file1", flags, unix.STATX_ALL, &stx)
   357  	if err != nil {
   358  		t.Fatalf("Statx: %v", err)
   359  	}
   360  
   361  	if uint32(stx.Mode) != st.Mode {
   362  		t.Errorf("Statx: returned stat mode does not match Stat")
   363  	}
   364  
   365  	ctime := unix.StatxTimestamp{Sec: int64(st.Ctim.Sec), Nsec: uint32(st.Ctim.Nsec)}
   366  	mtime := unix.StatxTimestamp{Sec: int64(st.Mtim.Sec), Nsec: uint32(st.Mtim.Nsec)}
   367  
   368  	if stx.Ctime != ctime {
   369  		t.Errorf("Statx: returned stat ctime does not match Stat")
   370  	}
   371  	if stx.Mtime != mtime {
   372  		t.Errorf("Statx: returned stat mtime does not match Stat")
   373  	}
   374  
   375  	err = os.Symlink("file1", "symlink1")
   376  	if err != nil {
   377  		t.Fatal(err)
   378  	}
   379  
   380  	err = unix.Lstat("symlink1", &st)
   381  	if err != nil {
   382  		t.Fatalf("Lstat: %v", err)
   383  	}
   384  
   385  	err = unix.Statx(unix.AT_FDCWD, "symlink1", flags, unix.STATX_BASIC_STATS, &stx)
   386  	if err != nil {
   387  		t.Fatalf("Statx: %v", err)
   388  	}
   389  
   390  	// follow symlink, expect a regulat file
   391  	if stx.Mode&unix.S_IFREG == 0 {
   392  		t.Errorf("Statx: didn't follow symlink")
   393  	}
   394  
   395  	err = unix.Statx(unix.AT_FDCWD, "symlink1", flags|unix.AT_SYMLINK_NOFOLLOW, unix.STATX_ALL, &stx)
   396  	if err != nil {
   397  		t.Fatalf("Statx: %v", err)
   398  	}
   399  
   400  	// follow symlink, expect a symlink
   401  	if stx.Mode&unix.S_IFLNK == 0 {
   402  		t.Errorf("Statx: unexpectedly followed symlink")
   403  	}
   404  	if uint32(stx.Mode) != st.Mode {
   405  		t.Errorf("Statx: returned stat mode does not match Lstat")
   406  	}
   407  
   408  	ctime = unix.StatxTimestamp{Sec: int64(st.Ctim.Sec), Nsec: uint32(st.Ctim.Nsec)}
   409  	mtime = unix.StatxTimestamp{Sec: int64(st.Mtim.Sec), Nsec: uint32(st.Mtim.Nsec)}
   410  
   411  	if stx.Ctime != ctime {
   412  		t.Errorf("Statx: returned stat ctime does not match Lstat")
   413  	}
   414  	if stx.Mtime != mtime {
   415  		t.Errorf("Statx: returned stat mtime does not match Lstat")
   416  	}
   417  }
   418  
   419  // stringsFromByteSlice converts a sequence of attributes to a []string.
   420  // On Linux, each entry is a NULL-terminated string.
   421  func stringsFromByteSlice(buf []byte) []string {
   422  	var result []string
   423  	off := 0
   424  	for i, b := range buf {
   425  		if b == 0 {
   426  			result = append(result, string(buf[off:i]))
   427  			off = i + 1
   428  		}
   429  	}
   430  	return result
   431  }
   432  
   433  func TestFaccessat(t *testing.T) {
   434  	defer chtmpdir(t)()
   435  	touch(t, "file1")
   436  
   437  	err := unix.Faccessat(unix.AT_FDCWD, "file1", unix.R_OK, 0)
   438  	if err != nil {
   439  		t.Errorf("Faccessat: unexpected error: %v", err)
   440  	}
   441  
   442  	err = unix.Faccessat(unix.AT_FDCWD, "file1", unix.R_OK, 2)
   443  	if err != unix.EINVAL {
   444  		t.Errorf("Faccessat: unexpected error: %v, want EINVAL", err)
   445  	}
   446  
   447  	err = unix.Faccessat(unix.AT_FDCWD, "file1", unix.R_OK, unix.AT_EACCESS)
   448  	if err != nil {
   449  		t.Errorf("Faccessat: unexpected error: %v", err)
   450  	}
   451  
   452  	err = os.Symlink("file1", "symlink1")
   453  	if err != nil {
   454  		t.Fatal(err)
   455  	}
   456  
   457  	err = unix.Faccessat(unix.AT_FDCWD, "symlink1", unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
   458  	if err != nil {
   459  		t.Errorf("Faccessat SYMLINK_NOFOLLOW: unexpected error %v", err)
   460  	}
   461  
   462  	// We can't really test AT_SYMLINK_NOFOLLOW, because there
   463  	// doesn't seem to be any way to change the mode of a symlink.
   464  	// We don't test AT_EACCESS because such tests are only
   465  	// meaningful if run as root.
   466  
   467  	err = unix.Fchmodat(unix.AT_FDCWD, "file1", 0, 0)
   468  	if err != nil {
   469  		t.Errorf("Fchmodat: unexpected error %v", err)
   470  	}
   471  
   472  	err = unix.Faccessat(unix.AT_FDCWD, "file1", unix.F_OK, unix.AT_SYMLINK_NOFOLLOW)
   473  	if err != nil {
   474  		t.Errorf("Faccessat: unexpected error: %v", err)
   475  	}
   476  
   477  	err = unix.Faccessat(unix.AT_FDCWD, "file1", unix.R_OK, unix.AT_SYMLINK_NOFOLLOW)
   478  	if err != unix.EACCES {
   479  		if unix.Getuid() != 0 {
   480  			t.Errorf("Faccessat: unexpected error: %v, want EACCES", err)
   481  		}
   482  	}
   483  }
   484  
   485  func TestSyncFileRange(t *testing.T) {
   486  	file, err := ioutil.TempFile("", "TestSyncFileRange")
   487  	if err != nil {
   488  		t.Fatal(err)
   489  	}
   490  	defer os.Remove(file.Name())
   491  	defer file.Close()
   492  
   493  	err = unix.SyncFileRange(int(file.Fd()), 0, 0, 0)
   494  	if err == unix.ENOSYS || err == unix.EPERM {
   495  		t.Skip("sync_file_range syscall is not available, skipping test")
   496  	} else if err != nil {
   497  		t.Fatalf("SyncFileRange: %v", err)
   498  	}
   499  
   500  	// invalid flags
   501  	flags := 0xf00
   502  	err = unix.SyncFileRange(int(file.Fd()), 0, 0, flags)
   503  	if err != unix.EINVAL {
   504  		t.Fatalf("SyncFileRange: unexpected error: %v, want EINVAL", err)
   505  	}
   506  }
   507  
   508  func TestClockNanosleep(t *testing.T) {
   509  	delay := 100 * time.Millisecond
   510  
   511  	// Relative timespec.
   512  	start := time.Now()
   513  	rel := unix.NsecToTimespec(delay.Nanoseconds())
   514  	err := unix.ClockNanosleep(unix.CLOCK_MONOTONIC, 0, &rel, nil)
   515  	if err == unix.ENOSYS || err == unix.EPERM {
   516  		t.Skip("clock_nanosleep syscall is not available, skipping test")
   517  	} else if err != nil {
   518  		t.Errorf("ClockNanosleep(CLOCK_MONOTONIC, 0, %#v, nil) = %v", &rel, err)
   519  	} else if slept := time.Since(start); slept < delay {
   520  		t.Errorf("ClockNanosleep(CLOCK_MONOTONIC, 0, %#v, nil) slept only %v", &rel, slept)
   521  	}
   522  
   523  	// Absolute timespec.
   524  	start = time.Now()
   525  	until := start.Add(delay)
   526  	abs := unix.NsecToTimespec(until.UnixNano())
   527  	err = unix.ClockNanosleep(unix.CLOCK_REALTIME, unix.TIMER_ABSTIME, &abs, nil)
   528  	if err != nil {
   529  		t.Errorf("ClockNanosleep(CLOCK_REALTIME, TIMER_ABSTIME, %#v (=%v), nil) = %v", &abs, until, err)
   530  	} else if slept := time.Since(start); slept < delay {
   531  		t.Errorf("ClockNanosleep(CLOCK_REALTIME, TIMER_ABSTIME, %#v (=%v), nil) slept only %v", &abs, until, slept)
   532  	}
   533  
   534  	// Invalid clock. clock_nanosleep(2) says EINVAL, but it’s actually EOPNOTSUPP.
   535  	err = unix.ClockNanosleep(unix.CLOCK_THREAD_CPUTIME_ID, 0, &rel, nil)
   536  	if err != unix.EINVAL && err != unix.EOPNOTSUPP {
   537  		t.Errorf("ClockNanosleep(CLOCK_THREAD_CPUTIME_ID, 0, %#v, nil) = %v, want EINVAL or EOPNOTSUPP", &rel, err)
   538  	}
   539  }
   540  
   541  func TestOpenByHandleAt(t *testing.T) {
   542  	skipIfNotSupported := func(t *testing.T, name string, err error) {
   543  		if err == unix.EPERM {
   544  			t.Skipf("skipping %s test without CAP_DAC_READ_SEARCH", name)
   545  		}
   546  		if err == unix.ENOSYS {
   547  			t.Skipf("%s system call not available", name)
   548  		}
   549  		if err == unix.EOPNOTSUPP {
   550  			t.Skipf("%s not supported on this filesystem", name)
   551  		}
   552  	}
   553  
   554  	h, mountID, err := unix.NameToHandleAt(unix.AT_FDCWD, "syscall_linux_test.go", 0)
   555  	if err != nil {
   556  		skipIfNotSupported(t, "name_to_handle_at", err)
   557  		t.Fatalf("NameToHandleAt: %v", err)
   558  	}
   559  	t.Logf("mountID: %v, handle: size=%d, type=%d, bytes=%q", mountID,
   560  		h.Size(), h.Type(), h.Bytes())
   561  	mount, err := openMountByID(mountID)
   562  	if err != nil {
   563  		t.Fatalf("openMountByID: %v", err)
   564  	}
   565  	defer mount.Close()
   566  
   567  	for _, clone := range []bool{false, true} {
   568  		t.Run("clone="+strconv.FormatBool(clone), func(t *testing.T) {
   569  			if clone {
   570  				h = unix.NewFileHandle(h.Type(), h.Bytes())
   571  			}
   572  			fd, err := unix.OpenByHandleAt(int(mount.Fd()), h, unix.O_RDONLY)
   573  			skipIfNotSupported(t, "open_by_handle_at", err)
   574  			if err != nil {
   575  				t.Fatalf("OpenByHandleAt: %v", err)
   576  			}
   577  			defer unix.Close(fd)
   578  
   579  			t.Logf("opened fd %v", fd)
   580  			f := os.NewFile(uintptr(fd), "")
   581  			slurp, err := ioutil.ReadAll(f)
   582  			if err != nil {
   583  				t.Fatal(err)
   584  			}
   585  			const substr = "Some substring for a test."
   586  			if !strings.Contains(string(slurp), substr) {
   587  				t.Errorf("didn't find substring %q in opened file; read %d bytes", substr, len(slurp))
   588  			}
   589  		})
   590  	}
   591  }
   592  
   593  func openMountByID(mountID int) (f *os.File, err error) {
   594  	mi, err := os.Open("/proc/self/mountinfo")
   595  	if err != nil {
   596  		return nil, err
   597  	}
   598  	defer mi.Close()
   599  	bs := bufio.NewScanner(mi)
   600  	wantPrefix := []byte(fmt.Sprintf("%v ", mountID))
   601  	for bs.Scan() {
   602  		if !bytes.HasPrefix(bs.Bytes(), wantPrefix) {
   603  			continue
   604  		}
   605  		fields := strings.Fields(bs.Text())
   606  		dev := fields[4]
   607  		return os.Open(dev)
   608  	}
   609  	if err := bs.Err(); err != nil {
   610  		return nil, err
   611  	}
   612  	return nil, errors.New("mountID not found")
   613  }