exec_linux.go

// Copyright 2011 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.

// +build linux

package syscall

import (
	"runtime"
	"unsafe"
)

// SysProcIDMap holds Container ID to Host ID mappings used for User Namespaces in Linux.
// See user_namespaces(7).
type SysProcIDMap struct {
	ContainerID int // Container ID.
	HostID      int // Host ID.
	Size        int // Size.
}

type SysProcAttr struct {
	Chroot       string         // Chroot.
	Credential   *Credential    // Credential.
	Ptrace       bool           // Enable tracing.
	Setsid       bool           // Create session.
	Setpgid      bool           // Set process group ID to Pgid, or, if Pgid == 0, to new pid.
	Setctty      bool           // Set controlling terminal to fd Ctty (only meaningful if Setsid is set)
	Noctty       bool           // Detach fd 0 from controlling terminal
	Ctty         int            // Controlling TTY fd
	Foreground   bool           // Place child's process group in foreground. (Implies Setpgid. Uses Ctty as fd of controlling TTY)
	Pgid         int            // Child's process group ID if Setpgid.
	Pdeathsig    Signal         // Signal that the process will get when its parent dies (Linux only)
	Cloneflags   uintptr        // Flags for clone calls (Linux only)
	Unshareflags uintptr        // Flags for unshare calls (Linux only)
	UidMappings  []SysProcIDMap // User ID mappings for user namespaces.
	GidMappings  []SysProcIDMap // Group ID mappings for user namespaces.
	// GidMappingsEnableSetgroups enabling setgroups syscall.
	// If false, then setgroups syscall will be disabled for the child process.
	// This parameter is no-op if GidMappings == nil. Otherwise for unprivileged
	// users this should be set to false for mappings work.
	GidMappingsEnableSetgroups bool
	AmbientCaps                []uintptr // Ambient capabilities (Linux only)
}

var (
	none  = [...]byte{'n', 'o', 'n', 'e', 0}
	slash = [...]byte{'/', 0}
)

// Implemented in runtime package.
func runtime_BeforeFork()
func runtime_AfterFork()
func runtime_AfterForkInChild()

// Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
// If a dup or exec fails, write the errno error to pipe.
// (Pipe is close-on-exec so if exec succeeds, it will be closed.)
// In the child, this function must not acquire any locks, because
// they might have been locked at the time of the fork. This means
// no rescheduling, no malloc calls, and no new stack segments.
// For the same reason compiler does not race instrument it.
// The calls to RawSyscall are okay because they are assembly
// functions that do not grow the stack.
//go:norace
func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
	// Set up and fork. This returns immediately in the parent or
	// if there's an error.
	r1, err1, p, locked := forkAndExecInChild1(argv0, argv, envv, chroot, dir, attr, sys, pipe)
	if locked {
		runtime_AfterFork()
	}
	if err1 != 0 {
		return 0, err1
	}

	// parent; return PID
	pid = int(r1)

	if sys.UidMappings != nil || sys.GidMappings != nil {
		Close(p[0])
		err := writeUidGidMappings(pid, sys)
		var err2 Errno
		if err != nil {
			err2 = err.(Errno)
		}
		RawSyscall(SYS_WRITE, uintptr(p[1]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
		Close(p[1])
	}

	return pid, 0
}

// forkAndExecInChild1 implements the body of forkAndExecInChild up to
// the parent's post-fork path. This is a separate function so we can
// separate the child's and parent's stack frames if we're using
// vfork.
//
// This is go:noinline because the point is to keep the stack frames
// of this and forkAndExecInChild separate.
//
//go:noinline
//go:norace
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) {
	// Defined in linux/prctl.h starting with Linux 4.3.
	const (
		PR_CAP_AMBIENT       = 0x2f
		PR_CAP_AMBIENT_RAISE = 0x2
	)

	// vfork requires that the child not touch any of the parent's
	// active stack frames. Hence, the child does all post-fork
	// processing in this stack frame and never returns, while the
	// parent returns immediately from this frame and does all
	// post-fork processing in the outer frame.
	// Declare all variables at top in case any
	// declarations require heap allocation (e.g., err1).
	var (
		err2   Errno
		nextfd int
		i      int
	)

	// Record parent PID so child can test if it has died.
	ppid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)

	// Guard against side effects of shuffling fds below.
	// Make sure that nextfd is beyond any currently open files so
	// that we can't run the risk of overwriting any of them.
	fd := make([]int, len(attr.Files))
	nextfd = len(attr.Files)
	for i, ufd := range attr.Files {
		if nextfd < int(ufd) {
			nextfd = int(ufd)
		}
		fd[i] = int(ufd)
	}
	nextfd++

	// Allocate another pipe for parent to child communication for
	// synchronizing writing of User ID/Group ID mappings.
	if sys.UidMappings != nil || sys.GidMappings != nil {
		if err := forkExecPipe(p[:]); err != nil {
			err1 = err.(Errno)
			return
		}
	}

	// About to call fork.
	// No more allocation or calls of non-assembly functions.
	runtime_BeforeFork()
	locked = true
	switch {
	case runtime.GOARCH == "amd64" && sys.Cloneflags&CLONE_NEWUSER == 0:
		r1, err1 = rawVforkSyscall(SYS_CLONE, uintptr(SIGCHLD|CLONE_VFORK|CLONE_VM)|sys.Cloneflags)
	case runtime.GOARCH == "s390x":
		r1, _, err1 = RawSyscall6(SYS_CLONE, 0, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0)
	default:
		r1, _, err1 = RawSyscall6(SYS_CLONE, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0, 0)
	}
	if err1 != 0 || r1 != 0 {
		// If we're in the parent, we must return immediately
		// so we're not in the same stack frame as the child.
		// This can at most use the return PC, which the child
		// will not modify, and the results of
		// rawVforkSyscall, which must have been written after
		// the child was replaced.
		return
	}

	// Fork succeeded, now in child.

	runtime_AfterForkInChild()

	// Enable the "keep capabilities" flag to set ambient capabilities later.
	if len(sys.AmbientCaps) > 0 {
		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_KEEPCAPS, 1, 0, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Wait for User ID/Group ID mappings to be written.
	if sys.UidMappings != nil || sys.GidMappings != nil {
		if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(p[1]), 0, 0); err1 != 0 {
			goto childerror
		}
		r1, _, err1 = RawSyscall(SYS_READ, uintptr(p[0]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
		if err1 != 0 {
			goto childerror
		}
		if r1 != unsafe.Sizeof(err2) {
			err1 = EINVAL
			goto childerror
		}
		if err2 != 0 {
			err1 = err2
			goto childerror
		}
	}

	// Enable tracing if requested.
	if sys.Ptrace {
		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Session ID
	if sys.Setsid {
		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Set process group
	if sys.Setpgid || sys.Foreground {
		// Place child in process group.
		_, _, err1 = RawSyscall(SYS_SETPGID, 0, uintptr(sys.Pgid), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	if sys.Foreground {
		pgrp := int32(sys.Pgid)
		if pgrp == 0 {
			r1, _, err1 = RawSyscall(SYS_GETPID, 0, 0, 0)
			if err1 != 0 {
				goto childerror
			}

			pgrp = int32(r1)
		}

		// Place process group in foreground.
		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
		if err1 != 0 {
			goto childerror
		}
	}

	// Unshare
	if sys.Unshareflags != 0 {
		_, _, err1 = RawSyscall(SYS_UNSHARE, sys.Unshareflags, 0, 0)
		if err1 != 0 {
			goto childerror
		}
		// The unshare system call in Linux doesn't unshare mount points
		// mounted with --shared. Systemd mounts / with --shared. For a
		// long discussion of the pros and cons of this see debian bug 739593.
		// The Go model of unsharing is more like Plan 9, where you ask
		// to unshare and the namespaces are unconditionally unshared.
		// To make this model work we must further mark / as MS_PRIVATE.
		// This is what the standard unshare command does.
		if sys.Unshareflags&CLONE_NEWNS == CLONE_NEWNS {
			_, _, err1 = RawSyscall6(SYS_MOUNT, uintptr(unsafe.Pointer(&none[0])), uintptr(unsafe.Pointer(&slash[0])), 0, MS_REC|MS_PRIVATE, 0, 0)
			if err1 != 0 {
				goto childerror
			}
		}
	}

	// Chroot
	if chroot != nil {
		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// User and groups
	if cred := sys.Credential; cred != nil {
		ngroups := uintptr(len(cred.Groups))
		groups := uintptr(0)
		if ngroups > 0 {
			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
		}
		if !(sys.GidMappings != nil && !sys.GidMappingsEnableSetgroups && ngroups == 0) && !cred.NoSetGroups {
			_, _, err1 = RawSyscall(_SYS_setgroups, ngroups, groups, 0)
			if err1 != 0 {
				goto childerror
			}
		}
		_, _, err1 = RawSyscall(sys_SETGID, uintptr(cred.Gid), 0, 0)
		if err1 != 0 {
			goto childerror
		}
		_, _, err1 = RawSyscall(sys_SETUID, uintptr(cred.Uid), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	for _, c := range sys.AmbientCaps {
		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_CAP_AMBIENT, uintptr(PR_CAP_AMBIENT_RAISE), c, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Chdir
	if dir != nil {
		_, _, err1 = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Parent death signal
	if sys.Pdeathsig != 0 {
		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_PDEATHSIG, uintptr(sys.Pdeathsig), 0, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}

		// Signal self if parent is already dead. This might cause a
		// duplicate signal in rare cases, but it won't matter when
		// using SIGKILL.
		r1, _, _ = RawSyscall(SYS_GETPPID, 0, 0, 0)
		if r1 != ppid {
			pid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
			_, _, err1 := RawSyscall(SYS_KILL, pid, uintptr(sys.Pdeathsig), 0)
			if err1 != 0 {
				goto childerror
			}
		}
	}

	// Pass 1: look for fd[i] < i and move those up above len(fd)
	// so that pass 2 won't stomp on an fd it needs later.
	if pipe < nextfd {
		_, _, err1 = RawSyscall(_SYS_dup, uintptr(pipe), uintptr(nextfd), 0)
		if err1 != 0 {
			goto childerror
		}
		RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
		pipe = nextfd
		nextfd++
	}
	for i = 0; i < len(fd); i++ {
		if fd[i] >= 0 && fd[i] < int(i) {
			if nextfd == pipe { // don't stomp on pipe
				nextfd++
			}
			_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(nextfd), 0)
			if err1 != 0 {
				goto childerror
			}
			RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
			fd[i] = nextfd
			nextfd++
		}
	}

	// Pass 2: dup fd[i] down onto i.
	for i = 0; i < len(fd); i++ {
		if fd[i] == -1 {
			RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
			continue
		}
		if fd[i] == int(i) {
			// dup2(i, i) won't clear close-on-exec flag on Linux,
			// probably not elsewhere either.
			_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(fd[i]), F_SETFD, 0)
			if err1 != 0 {
				goto childerror
			}
			continue
		}
		// The new fd is created NOT close-on-exec,
		// which is exactly what we want.
		_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(i), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// By convention, we don't close-on-exec the fds we are
	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
	// Programs that know they inherit fds >= 3 will need
	// to set them close-on-exec.
	for i = len(fd); i < 3; i++ {
		RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
	}

	// Detach fd 0 from tty
	if sys.Noctty {
		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCNOTTY), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Set the controlling TTY to Ctty
	if sys.Setctty {
		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 1)
		if err1 != 0 {
			goto childerror
		}
	}

	// Time to exec.
	_, _, err1 = RawSyscall(SYS_EXECVE,
		uintptr(unsafe.Pointer(argv0)),
		uintptr(unsafe.Pointer(&argv[0])),
		uintptr(unsafe.Pointer(&envv[0])))

childerror:
	// send error code on pipe
	RawSyscall(SYS_WRITE, uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
	for {
		RawSyscall(SYS_EXIT, 253, 0, 0)
	}
}

// Try to open a pipe with O_CLOEXEC set on both file descriptors.
func forkExecPipe(p []int) (err error) {
	err = Pipe2(p, O_CLOEXEC)
	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
	// might not be implemented.
	if err == ENOSYS {
		if err = Pipe(p); err != nil {
			return
		}
		if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
			return
		}
		_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
	}
	return
}

// writeIDMappings writes the user namespace User ID or Group ID mappings to the specified path.
func writeIDMappings(path string, idMap []SysProcIDMap) error {
	fd, err := Open(path, O_RDWR, 0)
	if err != nil {
		return err
	}

	data := ""
	for _, im := range idMap {
		data = data + itoa(im.ContainerID) + " " + itoa(im.HostID) + " " + itoa(im.Size) + "\n"
	}

	bytes, err := ByteSliceFromString(data)
	if err != nil {
		Close(fd)
		return err
	}

	if _, err := Write(fd, bytes); err != nil {
		Close(fd)
		return err
	}

	if err := Close(fd); err != nil {
		return err
	}

	return nil
}

// writeSetgroups writes to /proc/PID/setgroups "deny" if enable is false
// and "allow" if enable is true.
// This is needed since kernel 3.19, because you can't write gid_map without
// disabling setgroups() system call.
func writeSetgroups(pid int, enable bool) error {
	sgf := "/proc/" + itoa(pid) + "/setgroups"
	fd, err := Open(sgf, O_RDWR, 0)
	if err != nil {
		return err
	}

	var data []byte
	if enable {
		data = []byte("allow")
	} else {
		data = []byte("deny")
	}

	if _, err := Write(fd, data); err != nil {
		Close(fd)
		return err
	}

	return Close(fd)
}

// writeUidGidMappings writes User ID and Group ID mappings for user namespaces
// for a process and it is called from the parent process.
func writeUidGidMappings(pid int, sys *SysProcAttr) error {
	if sys.UidMappings != nil {
		uidf := "/proc/" + itoa(pid) + "/uid_map"
		if err := writeIDMappings(uidf, sys.UidMappings); err != nil {
			return err
		}
	}

	if sys.GidMappings != nil {
		// If the kernel is too old to support /proc/PID/setgroups, writeSetGroups will return ENOENT; this is OK.
		if err := writeSetgroups(pid, sys.GidMappingsEnableSetgroups); err != nil && err != ENOENT {
			return err
		}
		gidf := "/proc/" + itoa(pid) + "/gid_map"
		if err := writeIDMappings(gidf, sys.GidMappings); err != nil {
			return err
		}
	}

	return nil
}