exec_linux.go

Documentation: syscall

     1  // Copyright 2011 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 syscall
     8  
     9  import (
    10  	"runtime"
    11  	"unsafe"
    12  )
    13  
    14  // SysProcIDMap holds Container ID to Host ID mappings used for User Namespaces in Linux.
    15  // See user_namespaces(7).
    16  type SysProcIDMap struct {
    17  	ContainerID int // Container ID.
    18  	HostID      int // Host ID.
    19  	Size        int // Size.
    20  }
    21  
    22  type SysProcAttr struct {
    23  	Chroot     string      // Chroot.
    24  	Credential *Credential // Credential.
    25  	// Ptrace tells the child to call ptrace(PTRACE_TRACEME).
    26  	// Call runtime.LockOSThread before starting a process with this set,
    27  	// and don't call UnlockOSThread until done with PtraceSyscall calls.
    28  	Ptrace       bool
    29  	Setsid       bool           // Create session.
    30  	Setpgid      bool           // Set process group ID to Pgid, or, if Pgid == 0, to new pid.
    31  	Setctty      bool           // Set controlling terminal to fd Ctty (only meaningful if Setsid is set)
    32  	Noctty       bool           // Detach fd 0 from controlling terminal
    33  	Ctty         int            // Controlling TTY fd
    34  	Foreground   bool           // Place child's process group in foreground. (Implies Setpgid. Uses Ctty as fd of controlling TTY)
    35  	Pgid         int            // Child's process group ID if Setpgid.
    36  	Pdeathsig    Signal         // Signal that the process will get when its parent dies (Linux only)
    37  	Cloneflags   uintptr        // Flags for clone calls (Linux only)
    38  	Unshareflags uintptr        // Flags for unshare calls (Linux only)
    39  	UidMappings  []SysProcIDMap // User ID mappings for user namespaces.
    40  	GidMappings  []SysProcIDMap // Group ID mappings for user namespaces.
    41  	// GidMappingsEnableSetgroups enabling setgroups syscall.
    42  	// If false, then setgroups syscall will be disabled for the child process.
    43  	// This parameter is no-op if GidMappings == nil. Otherwise for unprivileged
    44  	// users this should be set to false for mappings work.
    45  	GidMappingsEnableSetgroups bool
    46  	AmbientCaps                []uintptr // Ambient capabilities (Linux only)
    47  }
    48  
    49  var (
    50  	none  = [...]byte{'n', 'o', 'n', 'e', 0}
    51  	slash = [...]byte{'/', 0}
    52  )
    53  
    54  // Implemented in runtime package.
    55  func runtime_BeforeFork()
    56  func runtime_AfterFork()
    57  func runtime_AfterForkInChild()
    58  
    59  // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
    60  // If a dup or exec fails, write the errno error to pipe.
    61  // (Pipe is close-on-exec so if exec succeeds, it will be closed.)
    62  // In the child, this function must not acquire any locks, because
    63  // they might have been locked at the time of the fork. This means
    64  // no rescheduling, no malloc calls, and no new stack segments.
    65  // For the same reason compiler does not race instrument it.
    66  // The calls to RawSyscall are okay because they are assembly
    67  // functions that do not grow the stack.
    68  //go:norace
    69  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
    70  	// Set up and fork. This returns immediately in the parent or
    71  	// if there's an error.
    72  	r1, err1, p, locked := forkAndExecInChild1(argv0, argv, envv, chroot, dir, attr, sys, pipe)
    73  	if locked {
    74  		runtime_AfterFork()
    75  	}
    76  	if err1 != 0 {
    77  		return 0, err1
    78  	}
    79  
    80  	// parent; return PID
    81  	pid = int(r1)
    82  
    83  	if sys.UidMappings != nil || sys.GidMappings != nil {
    84  		Close(p[0])
    85  		err := writeUidGidMappings(pid, sys)
    86  		var err2 Errno
    87  		if err != nil {
    88  			err2 = err.(Errno)
    89  		}
    90  		RawSyscall(SYS_WRITE, uintptr(p[1]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
    91  		Close(p[1])
    92  	}
    93  
    94  	return pid, 0
    95  }
    96  
    97  // forkAndExecInChild1 implements the body of forkAndExecInChild up to
    98  // the parent's post-fork path. This is a separate function so we can
    99  // separate the child's and parent's stack frames if we're using
   100  // vfork.
   101  //
   102  // This is go:noinline because the point is to keep the stack frames
   103  // of this and forkAndExecInChild separate.
   104  //
   105  //go:noinline
   106  //go:norace
   107  func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (r1 uintptr, err1 Errno, p [2]int, locked bool) {
   108  	// Defined in linux/prctl.h starting with Linux 4.3.
   109  	const (
   110  		PR_CAP_AMBIENT       = 0x2f
   111  		PR_CAP_AMBIENT_RAISE = 0x2
   112  	)
   113  
   114  	// vfork requires that the child not touch any of the parent's
   115  	// active stack frames. Hence, the child does all post-fork
   116  	// processing in this stack frame and never returns, while the
   117  	// parent returns immediately from this frame and does all
   118  	// post-fork processing in the outer frame.
   119  	// Declare all variables at top in case any
   120  	// declarations require heap allocation (e.g., err1).
   121  	var (
   122  		err2   Errno
   123  		nextfd int
   124  		i      int
   125  	)
   126  
   127  	// Record parent PID so child can test if it has died.
   128  	ppid, _ := rawSyscallNoError(SYS_GETPID, 0, 0, 0)
   129  
   130  	// Guard against side effects of shuffling fds below.
   131  	// Make sure that nextfd is beyond any currently open files so
   132  	// that we can't run the risk of overwriting any of them.
   133  	fd := make([]int, len(attr.Files))
   134  	nextfd = len(attr.Files)
   135  	for i, ufd := range attr.Files {
   136  		if nextfd < int(ufd) {
   137  			nextfd = int(ufd)
   138  		}
   139  		fd[i] = int(ufd)
   140  	}
   141  	nextfd++
   142  
   143  	// Allocate another pipe for parent to child communication for
   144  	// synchronizing writing of User ID/Group ID mappings.
   145  	if sys.UidMappings != nil || sys.GidMappings != nil {
   146  		if err := forkExecPipe(p[:]); err != nil {
   147  			err1 = err.(Errno)
   148  			return
   149  		}
   150  	}
   151  
   152  	// About to call fork.
   153  	// No more allocation or calls of non-assembly functions.
   154  	runtime_BeforeFork()
   155  	locked = true
   156  	switch {
   157  	case runtime.GOARCH == "amd64" && sys.Cloneflags&CLONE_NEWUSER == 0:
   158  		r1, err1 = rawVforkSyscall(SYS_CLONE, uintptr(SIGCHLD|CLONE_VFORK|CLONE_VM)|sys.Cloneflags)
   159  	case runtime.GOARCH == "s390x":
   160  		r1, _, err1 = RawSyscall6(SYS_CLONE, 0, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0)
   161  	default:
   162  		r1, _, err1 = RawSyscall6(SYS_CLONE, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0, 0)
   163  	}
   164  	if err1 != 0 || r1 != 0 {
   165  		// If we're in the parent, we must return immediately
   166  		// so we're not in the same stack frame as the child.
   167  		// This can at most use the return PC, which the child
   168  		// will not modify, and the results of
   169  		// rawVforkSyscall, which must have been written after
   170  		// the child was replaced.
   171  		return
   172  	}
   173  
   174  	// Fork succeeded, now in child.
   175  
   176  	runtime_AfterForkInChild()
   177  
   178  	// Enable the "keep capabilities" flag to set ambient capabilities later.
   179  	if len(sys.AmbientCaps) > 0 {
   180  		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_KEEPCAPS, 1, 0, 0, 0, 0)
   181  		if err1 != 0 {
   182  			goto childerror
   183  		}
   184  	}
   185  
   186  	// Wait for User ID/Group ID mappings to be written.
   187  	if sys.UidMappings != nil || sys.GidMappings != nil {
   188  		if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(p[1]), 0, 0); err1 != 0 {
   189  			goto childerror
   190  		}
   191  		r1, _, err1 = RawSyscall(SYS_READ, uintptr(p[0]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
   192  		if err1 != 0 {
   193  			goto childerror
   194  		}
   195  		if r1 != unsafe.Sizeof(err2) {
   196  			err1 = EINVAL
   197  			goto childerror
   198  		}
   199  		if err2 != 0 {
   200  			err1 = err2
   201  			goto childerror
   202  		}
   203  	}
   204  
   205  	// Session ID
   206  	if sys.Setsid {
   207  		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
   208  		if err1 != 0 {
   209  			goto childerror
   210  		}
   211  	}
   212  
   213  	// Set process group
   214  	if sys.Setpgid || sys.Foreground {
   215  		// Place child in process group.
   216  		_, _, err1 = RawSyscall(SYS_SETPGID, 0, uintptr(sys.Pgid), 0)
   217  		if err1 != 0 {
   218  			goto childerror
   219  		}
   220  	}
   221  
   222  	if sys.Foreground {
   223  		pgrp := int32(sys.Pgid)
   224  		if pgrp == 0 {
   225  			r1, _ = rawSyscallNoError(SYS_GETPID, 0, 0, 0)
   226  
   227  			pgrp = int32(r1)
   228  		}
   229  
   230  		// Place process group in foreground.
   231  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   232  		if err1 != 0 {
   233  			goto childerror
   234  		}
   235  	}
   236  
   237  	// Unshare
   238  	if sys.Unshareflags != 0 {
   239  		_, _, err1 = RawSyscall(SYS_UNSHARE, sys.Unshareflags, 0, 0)
   240  		if err1 != 0 {
   241  			goto childerror
   242  		}
   243  		// The unshare system call in Linux doesn't unshare mount points
   244  		// mounted with --shared. Systemd mounts / with --shared. For a
   245  		// long discussion of the pros and cons of this see debian bug 739593.
   246  		// The Go model of unsharing is more like Plan 9, where you ask
   247  		// to unshare and the namespaces are unconditionally unshared.
   248  		// To make this model work we must further mark / as MS_PRIVATE.
   249  		// This is what the standard unshare command does.
   250  		if sys.Unshareflags&CLONE_NEWNS == CLONE_NEWNS {
   251  			_, _, err1 = RawSyscall6(SYS_MOUNT, uintptr(unsafe.Pointer(&none[0])), uintptr(unsafe.Pointer(&slash[0])), 0, MS_REC|MS_PRIVATE, 0, 0)
   252  			if err1 != 0 {
   253  				goto childerror
   254  			}
   255  		}
   256  	}
   257  
   258  	// Chroot
   259  	if chroot != nil {
   260  		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
   261  		if err1 != 0 {
   262  			goto childerror
   263  		}
   264  	}
   265  
   266  	// User and groups
   267  	if cred := sys.Credential; cred != nil {
   268  		ngroups := uintptr(len(cred.Groups))
   269  		groups := uintptr(0)
   270  		if ngroups > 0 {
   271  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   272  		}
   273  		if !(sys.GidMappings != nil && !sys.GidMappingsEnableSetgroups && ngroups == 0) && !cred.NoSetGroups {
   274  			_, _, err1 = RawSyscall(_SYS_setgroups, ngroups, groups, 0)
   275  			if err1 != 0 {
   276  				goto childerror
   277  			}
   278  		}
   279  		_, _, err1 = RawSyscall(sys_SETGID, uintptr(cred.Gid), 0, 0)
   280  		if err1 != 0 {
   281  			goto childerror
   282  		}
   283  		_, _, err1 = RawSyscall(sys_SETUID, uintptr(cred.Uid), 0, 0)
   284  		if err1 != 0 {
   285  			goto childerror
   286  		}
   287  	}
   288  
   289  	for _, c := range sys.AmbientCaps {
   290  		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_CAP_AMBIENT, uintptr(PR_CAP_AMBIENT_RAISE), c, 0, 0, 0)
   291  		if err1 != 0 {
   292  			goto childerror
   293  		}
   294  	}
   295  
   296  	// Chdir
   297  	if dir != nil {
   298  		_, _, err1 = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
   299  		if err1 != 0 {
   300  			goto childerror
   301  		}
   302  	}
   303  
   304  	// Parent death signal
   305  	if sys.Pdeathsig != 0 {
   306  		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_PDEATHSIG, uintptr(sys.Pdeathsig), 0, 0, 0, 0)
   307  		if err1 != 0 {
   308  			goto childerror
   309  		}
   310  
   311  		// Signal self if parent is already dead. This might cause a
   312  		// duplicate signal in rare cases, but it won't matter when
   313  		// using SIGKILL.
   314  		r1, _ = rawSyscallNoError(SYS_GETPPID, 0, 0, 0)
   315  		if r1 != ppid {
   316  			pid, _ := rawSyscallNoError(SYS_GETPID, 0, 0, 0)
   317  			_, _, err1 := RawSyscall(SYS_KILL, pid, uintptr(sys.Pdeathsig), 0)
   318  			if err1 != 0 {
   319  				goto childerror
   320  			}
   321  		}
   322  	}
   323  
   324  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   325  	// so that pass 2 won't stomp on an fd it needs later.
   326  	if pipe < nextfd {
   327  		_, _, err1 = RawSyscall(_SYS_dup, uintptr(pipe), uintptr(nextfd), 0)
   328  		if err1 != 0 {
   329  			goto childerror
   330  		}
   331  		RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   332  		pipe = nextfd
   333  		nextfd++
   334  	}
   335  	for i = 0; i < len(fd); i++ {
   336  		if fd[i] >= 0 && fd[i] < int(i) {
   337  			if nextfd == pipe { // don't stomp on pipe
   338  				nextfd++
   339  			}
   340  			_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(nextfd), 0)
   341  			if err1 != 0 {
   342  				goto childerror
   343  			}
   344  			RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   345  			fd[i] = nextfd
   346  			nextfd++
   347  		}
   348  	}
   349  
   350  	// Pass 2: dup fd[i] down onto i.
   351  	for i = 0; i < len(fd); i++ {
   352  		if fd[i] == -1 {
   353  			RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   354  			continue
   355  		}
   356  		if fd[i] == int(i) {
   357  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   358  			// probably not elsewhere either.
   359  			_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(fd[i]), F_SETFD, 0)
   360  			if err1 != 0 {
   361  				goto childerror
   362  			}
   363  			continue
   364  		}
   365  		// The new fd is created NOT close-on-exec,
   366  		// which is exactly what we want.
   367  		_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(i), 0)
   368  		if err1 != 0 {
   369  			goto childerror
   370  		}
   371  	}
   372  
   373  	// By convention, we don't close-on-exec the fds we are
   374  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   375  	// Programs that know they inherit fds >= 3 will need
   376  	// to set them close-on-exec.
   377  	for i = len(fd); i < 3; i++ {
   378  		RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   379  	}
   380  
   381  	// Detach fd 0 from tty
   382  	if sys.Noctty {
   383  		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCNOTTY), 0)
   384  		if err1 != 0 {
   385  			goto childerror
   386  		}
   387  	}
   388  
   389  	// Set the controlling TTY to Ctty
   390  	if sys.Setctty {
   391  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 1)
   392  		if err1 != 0 {
   393  			goto childerror
   394  		}
   395  	}
   396  
   397  	// Enable tracing if requested.
   398  	// Do this right before exec so that we don't unnecessarily trace the runtime
   399  	// setting up after the fork. See issue #21428.
   400  	if sys.Ptrace {
   401  		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
   402  		if err1 != 0 {
   403  			goto childerror
   404  		}
   405  	}
   406  
   407  	// Time to exec.
   408  	_, _, err1 = RawSyscall(SYS_EXECVE,
   409  		uintptr(unsafe.Pointer(argv0)),
   410  		uintptr(unsafe.Pointer(&argv[0])),
   411  		uintptr(unsafe.Pointer(&envv[0])))
   412  
   413  childerror:
   414  	// send error code on pipe
   415  	RawSyscall(SYS_WRITE, uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   416  	for {
   417  		RawSyscall(SYS_EXIT, 253, 0, 0)
   418  	}
   419  }
   420  
   421  // Try to open a pipe with O_CLOEXEC set on both file descriptors.
   422  func forkExecPipe(p []int) (err error) {
   423  	err = Pipe2(p, O_CLOEXEC)
   424  	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
   425  	// might not be implemented.
   426  	if err == ENOSYS {
   427  		if err = Pipe(p); err != nil {
   428  			return
   429  		}
   430  		if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
   431  			return
   432  		}
   433  		_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
   434  	}
   435  	return
   436  }
   437  
   438  // writeIDMappings writes the user namespace User ID or Group ID mappings to the specified path.
   439  func writeIDMappings(path string, idMap []SysProcIDMap) error {
   440  	fd, err := Open(path, O_RDWR, 0)
   441  	if err != nil {
   442  		return err
   443  	}
   444  
   445  	data := ""
   446  	for _, im := range idMap {
   447  		data = data + itoa(im.ContainerID) + " " + itoa(im.HostID) + " " + itoa(im.Size) + "\n"
   448  	}
   449  
   450  	bytes, err := ByteSliceFromString(data)
   451  	if err != nil {
   452  		Close(fd)
   453  		return err
   454  	}
   455  
   456  	if _, err := Write(fd, bytes); err != nil {
   457  		Close(fd)
   458  		return err
   459  	}
   460  
   461  	if err := Close(fd); err != nil {
   462  		return err
   463  	}
   464  
   465  	return nil
   466  }
   467  
   468  // writeSetgroups writes to /proc/PID/setgroups "deny" if enable is false
   469  // and "allow" if enable is true.
   470  // This is needed since kernel 3.19, because you can't write gid_map without
   471  // disabling setgroups() system call.
   472  func writeSetgroups(pid int, enable bool) error {
   473  	sgf := "/proc/" + itoa(pid) + "/setgroups"
   474  	fd, err := Open(sgf, O_RDWR, 0)
   475  	if err != nil {
   476  		return err
   477  	}
   478  
   479  	var data []byte
   480  	if enable {
   481  		data = []byte("allow")
   482  	} else {
   483  		data = []byte("deny")
   484  	}
   485  
   486  	if _, err := Write(fd, data); err != nil {
   487  		Close(fd)
   488  		return err
   489  	}
   490  
   491  	return Close(fd)
   492  }
   493  
   494  // writeUidGidMappings writes User ID and Group ID mappings for user namespaces
   495  // for a process and it is called from the parent process.
   496  func writeUidGidMappings(pid int, sys *SysProcAttr) error {
   497  	if sys.UidMappings != nil {
   498  		uidf := "/proc/" + itoa(pid) + "/uid_map"
   499  		if err := writeIDMappings(uidf, sys.UidMappings); err != nil {
   500  			return err
   501  		}
   502  	}
   503  
   504  	if sys.GidMappings != nil {
   505  		// If the kernel is too old to support /proc/PID/setgroups, writeSetGroups will return ENOENT; this is OK.
   506  		if err := writeSetgroups(pid, sys.GidMappingsEnableSetgroups); err != nil && err != ENOENT {
   507  			return err
   508  		}
   509  		gidf := "/proc/" + itoa(pid) + "/gid_map"
   510  		if err := writeIDMappings(gidf, sys.GidMappings); err != nil {
   511  			return err
   512  		}
   513  	}
   514  
   515  	return nil
   516  }
   517
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