src/syscall/exec_linux.go - go - Git at Google

 // 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
 }
	// 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
	}