src/syscall/exec_plan9.go - go - Git at Google

 // Copyright 2009 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.

 // Fork, exec, wait, etc.

 package syscall

 import (
 	"runtime"
 	"sync"
 	"unsafe"
 )

 // Lock synchronizing creation of new file descriptors with fork.
 //
 // We want the child in a fork/exec sequence to inherit only the
 // file descriptors we intend.  To do that, we mark all file
 // descriptors close-on-exec and then, in the child, explicitly
 // unmark the ones we want the exec'ed program to keep.
 // Unix doesn't make this easy: there is, in general, no way to
 // allocate a new file descriptor close-on-exec.  Instead you
 // have to allocate the descriptor and then mark it close-on-exec.
 // If a fork happens between those two events, the child's exec
 // will inherit an unwanted file descriptor.
 //
 // This lock solves that race: the create new fd/mark close-on-exec
 // operation is done holding ForkLock for reading, and the fork itself
 // is done holding ForkLock for writing.  At least, that's the idea.
 // There are some complications.
 //
 // Some system calls that create new file descriptors can block
 // for arbitrarily long times: open on a hung NFS server or named
 // pipe, accept on a socket, and so on.  We can't reasonably grab
 // the lock across those operations.
 //
 // It is worse to inherit some file descriptors than others.
 // If a non-malicious child accidentally inherits an open ordinary file,
 // that's not a big deal.  On the other hand, if a long-lived child
 // accidentally inherits the write end of a pipe, then the reader
 // of that pipe will not see EOF until that child exits, potentially
 // causing the parent program to hang.  This is a common problem
 // in threaded C programs that use popen.
 //
 // Luckily, the file descriptors that are most important not to
 // inherit are not the ones that can take an arbitrarily long time
 // to create: pipe returns instantly, and the net package uses
 // non-blocking I/O to accept on a listening socket.
 // The rules for which file descriptor-creating operations use the
 // ForkLock are as follows:
 //
 // 1) Pipe.    Does not block.  Use the ForkLock.
 // 2) Socket.  Does not block.  Use the ForkLock.
 // 3) Accept.  If using non-blocking mode, use the ForkLock.
 //             Otherwise, live with the race.
 // 4) Open.    Can block.  Use O_CLOEXEC if available (Linux).
 //             Otherwise, live with the race.
 // 5) Dup.     Does not block.  Use the ForkLock.
 //             On Linux, could use fcntl F_DUPFD_CLOEXEC
 //             instead of the ForkLock, but only for dup(fd, -1).

 var ForkLock sync.RWMutex

 // StringSlicePtr is deprecated. Use SlicePtrFromStrings instead.
 // If any string contains a NUL byte this function panics instead
 // of returning an error.
 func StringSlicePtr(ss []string) []*byte {
 	bb := make([]*byte, len(ss)+1)
 	for i := 0; i < len(ss); i++ {
 		bb[i] = StringBytePtr(ss[i])
 	}
 	bb[len(ss)] = nil
 	return bb
 }

 // SlicePtrFromStrings converts a slice of strings to a slice of
 // pointers to NUL-terminated byte slices. If any string contains
 // a NUL byte, it returns (nil, EINVAL).
 func SlicePtrFromStrings(ss []string) ([]*byte, error) {
 	var err error
 	bb := make([]*byte, len(ss)+1)
 	for i := 0; i < len(ss); i++ {
 		bb[i], err = BytePtrFromString(ss[i])
 		if err != nil {
 			return nil, err
 		}
 	}
 	bb[len(ss)] = nil
 	return bb, nil
 }

 // readdirnames returns the names of files inside the directory represented by dirfd.
 func readdirnames(dirfd int) (names []string, err error) {
 	names = make([]string, 0, 100)
 	var buf [STATMAX]byte

 	for {
 		n, e := Read(dirfd, buf[:])
 		if e != nil {
 			return nil, e
 		}
 		if n == 0 {
 			break
 		}
 		for i := 0; i < n; {
 			m, _ := gbit16(buf[i:])
 			m += 2

 			if m < STATFIXLEN {
 				return nil, ErrBadStat
 			}

 			s, _, ok := gstring(buf[i+41:])
 			if !ok {
 				return nil, ErrBadStat
 			}
 			names = append(names, s)
 			i += int(m)
 		}
 	}
 	return
 }

 // readdupdevice returns a list of currently opened fds (excluding stdin, stdout, stderr) from the dup device #d.
 // ForkLock should be write locked before calling, so that no new fds would be created while the fd list is being read.
 func readdupdevice() (fds []int, err error) {
 	dupdevfd, err := Open("#d", O_RDONLY)
 	if err != nil {
 		return
 	}
 	defer Close(dupdevfd)

 	names, err := readdirnames(dupdevfd)
 	if err != nil {
 		return
 	}

 	fds = make([]int, 0, len(names)/2)
 	for _, name := range names {
 		if n := len(name); n > 3 && name[n-3:n] == "ctl" {
 			continue
 		}
 		fd := int(atoi([]byte(name)))
 		switch fd {
 		case 0, 1, 2, dupdevfd:
 			continue
 		}
 		fds = append(fds, fd)
 	}
 	return
 }

 var startupFds []int

 // Plan 9 does not allow clearing the OCEXEC flag
 // from the underlying channel backing an open file descriptor,
 // therefore we store a list of already opened file descriptors
 // inside startupFds and skip them when manually closing descriptors
 // not meant to be passed to a child exec.
 func init() {
 	startupFds, _ = readdupdevice()
 }

 // forkAndExecInChild forks the process, calling dup onto 0..len(fd)
 // and finally invoking exec(argv0, argvv, envv) in the child.
 // If a dup or exec fails, it writes the error string to pipe.
 // (The pipe write end 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.
 // The calls to RawSyscall are okay because they are assembly
 // functions that do not grow the stack.
 func forkAndExecInChild(argv0 *byte, argv []*byte, envv []envItem, dir *byte, attr *ProcAttr, fdsToClose []int, pipe int, rflag int) (pid int, err error) {
 	// Declare all variables at top in case any
 	// declarations require heap allocation (e.g., errbuf).
 	var (
 		r1       uintptr
 		nextfd   int
 		i        int
 		clearenv int
 		envfd    int
 		errbuf   [ERRMAX]byte
 	)

 	// 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++

 	if envv != nil {
 		clearenv = RFCENVG
 	}

 	// About to call fork.
 	// No more allocation or calls of non-assembly functions.
 	r1, _, _ = RawSyscall(SYS_RFORK, uintptr(RFPROC|RFFDG|RFREND|clearenv|rflag), 0, 0)

 	if r1 != 0 {
 		if int32(r1) == -1 {
 			return 0, NewError(errstr())
 		}
 		// parent; return PID
 		return int(r1), nil
 	}

 	// Fork succeeded, now in child.

 	// Close fds we don't need.
 	for i = 0; i < len(fdsToClose); i++ {
 		r1, _, _ = RawSyscall(SYS_CLOSE, uintptr(fdsToClose[i]), 0, 0)
 		if int32(r1) == -1 {
 			goto childerror
 		}
 	}

 	if envv != nil {
 		// Write new environment variables.
 		for i = 0; i < len(envv); i++ {
 			r1, _, _ = RawSyscall(SYS_CREATE, uintptr(unsafe.Pointer(envv[i].name)), uintptr(O_WRONLY), uintptr(0666))

 			if int32(r1) == -1 {
 				goto childerror
 			}

 			envfd = int(r1)

 			r1, _, _ = RawSyscall6(SYS_PWRITE, uintptr(envfd), uintptr(unsafe.Pointer(envv[i].value)), uintptr(envv[i].nvalue),
 				^uintptr(0), ^uintptr(0), 0)

 			if int32(r1) == -1 || int(r1) != envv[i].nvalue {
 				goto childerror
 			}

 			r1, _, _ = RawSyscall(SYS_CLOSE, uintptr(envfd), 0, 0)

 			if int32(r1) == -1 {
 				goto childerror
 			}
 		}
 	}

 	// Chdir
 	if dir != nil {
 		r1, _, _ = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
 		if int32(r1) == -1 {
 			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 {
 		r1, _, _ = RawSyscall(SYS_DUP, uintptr(pipe), uintptr(nextfd), 0)
 		if int32(r1) == -1 {
 			goto childerror
 		}
 		pipe = nextfd
 		nextfd++
 	}
 	for i = 0; i < len(fd); i++ {
 		if fd[i] >= 0 && fd[i] < int(i) {
 			r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(nextfd), 0)
 			if int32(r1) == -1 {
 				goto childerror
 			}

 			fd[i] = nextfd
 			nextfd++
 			if nextfd == pipe { // don't stomp on pipe
 				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) {
 			continue
 		}
 		r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(i), 0)
 		if int32(r1) == -1 {
 			goto childerror
 		}
 	}

 	// Pass 3: close fd[i] if it was moved in the previous pass.
 	for i = 0; i < len(fd); i++ {
 		if fd[i] >= 0 && fd[i] != int(i) {
 			RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
 		}
 	}

 	// Time to exec.
 	r1, _, _ = RawSyscall(SYS_EXEC,
 		uintptr(unsafe.Pointer(argv0)),
 		uintptr(unsafe.Pointer(&argv[0])), 0)

 childerror:
 	// send error string on pipe
 	RawSyscall(SYS_ERRSTR, uintptr(unsafe.Pointer(&errbuf[0])), uintptr(len(errbuf)), 0)
 	errbuf[len(errbuf)-1] = 0
 	i = 0
 	for i < len(errbuf) && errbuf[i] != 0 {
 		i++
 	}

 	RawSyscall6(SYS_PWRITE, uintptr(pipe), uintptr(unsafe.Pointer(&errbuf[0])), uintptr(i),
 		^uintptr(0), ^uintptr(0), 0)

 	for {
 		RawSyscall(SYS_EXITS, 0, 0, 0)
 	}

 	// Calling panic is not actually safe,
 	// but the for loop above won't break
 	// and this shuts up the compiler.
 	panic("unreached")
 }

 func cexecPipe(p []int) error {
 	e := Pipe(p)
 	if e != nil {
 		return e
 	}

 	fd, e := Open("#d/"+itoa(p[1]), O_CLOEXEC)
 	if e != nil {
 		Close(p[0])
 		Close(p[1])
 		return e
 	}

 	Close(fd)
 	return nil
 }

 type envItem struct {
 	name   *byte
 	value  *byte
 	nvalue int
 }

 type ProcAttr struct {
 	Dir   string    // Current working directory.
 	Env   []string  // Environment.
 	Files []uintptr // File descriptors.
 	Sys   *SysProcAttr
 }

 type SysProcAttr struct {
 	Rfork int // additional flags to pass to rfork
 }

 var zeroProcAttr ProcAttr
 var zeroSysProcAttr SysProcAttr

 func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
 	var (
 		p      [2]int
 		n      int
 		errbuf [ERRMAX]byte
 		wmsg   Waitmsg
 	)

 	if attr == nil {
 		attr = &zeroProcAttr
 	}
 	sys := attr.Sys
 	if sys == nil {
 		sys = &zeroSysProcAttr
 	}

 	p[0] = -1
 	p[1] = -1

 	// Convert args to C form.
 	argv0p, err := BytePtrFromString(argv0)
 	if err != nil {
 		return 0, err
 	}
 	argvp, err := SlicePtrFromStrings(argv)
 	if err != nil {
 		return 0, err
 	}

 	var dir *byte
 	if attr.Dir != "" {
 		dir, err = BytePtrFromString(attr.Dir)
 		if err != nil {
 			return 0, err
 		}
 	}
 	var envvParsed []envItem
 	if attr.Env != nil {
 		envvParsed = make([]envItem, 0, len(attr.Env))
 		for _, v := range attr.Env {
 			i := 0
 			for i < len(v) && v[i] != '=' {
 				i++
 			}

 			envname, err := BytePtrFromString("/env/" + v[:i])
 			if err != nil {
 				return 0, err
 			}
 			envvalue := make([]byte, len(v)-i)
 			copy(envvalue, v[i+1:])
 			envvParsed = append(envvParsed, envItem{envname, &envvalue[0], len(v) - i})
 		}
 	}

 	// Acquire the fork lock to prevent other threads from creating new fds before we fork.
 	ForkLock.Lock()

 	// get a list of open fds, excluding stdin,stdout and stderr that need to be closed in the child.
 	// no new fds can be created while we hold the ForkLock for writing.
 	openFds, e := readdupdevice()
 	if e != nil {
 		ForkLock.Unlock()
 		return 0, e
 	}

 	fdsToClose := make([]int, 0, len(openFds))
 	for _, fd := range openFds {
 		doClose := true

 		// exclude files opened at startup.
 		for _, sfd := range startupFds {
 			if fd == sfd {
 				doClose = false
 				break
 			}
 		}

 		// exclude files explicitly requested by the caller.
 		for _, rfd := range attr.Files {
 			if fd == int(rfd) {
 				doClose = false
 				break
 			}
 		}

 		if doClose {
 			fdsToClose = append(fdsToClose, fd)
 		}
 	}

 	// Allocate child status pipe close on exec.
 	e = cexecPipe(p[:])

 	if e != nil {
 		return 0, e
 	}
 	fdsToClose = append(fdsToClose, p[0])

 	// Kick off child.
 	pid, err = forkAndExecInChild(argv0p, argvp, envvParsed, dir, attr, fdsToClose, p[1], sys.Rfork)

 	if err != nil {
 		if p[0] >= 0 {
 			Close(p[0])
 			Close(p[1])
 		}
 		ForkLock.Unlock()
 		return 0, err
 	}
 	ForkLock.Unlock()

 	// Read child error status from pipe.
 	Close(p[1])
 	n, err = Read(p[0], errbuf[:])
 	Close(p[0])

 	if err != nil || n != 0 {
 		if n != 0 {
 			err = NewError(string(errbuf[:n]))
 		}

 		// Child failed; wait for it to exit, to make sure
 		// the zombies don't accumulate.
 		for wmsg.Pid != pid {
 			Await(&wmsg)
 		}
 		return 0, err
 	}

 	// Read got EOF, so pipe closed on exec, so exec succeeded.
 	return pid, nil
 }

 type waitErr struct {
 	Waitmsg
 	err error
 }

 var procs struct {
 	sync.Mutex
 	waits map[int]chan *waitErr
 }

 // startProcess starts a new goroutine, tied to the OS
 // thread, which runs the process and subsequently waits
 // for it to finish, communicating the process stats back
 // to any goroutines that may have been waiting on it.
 //
 // Such a dedicated goroutine is needed because on
 // Plan 9, only the parent thread can wait for a child,
 // whereas goroutines tend to jump OS threads (e.g.,
 // between starting a process and running Wait(), the
 // goroutine may have been rescheduled).
 func startProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
 	type forkRet struct {
 		pid int
 		err error
 	}

 	forkc := make(chan forkRet, 1)
 	go func() {
 		runtime.LockOSThread()
 		var ret forkRet

 		ret.pid, ret.err = forkExec(argv0, argv, attr)
 		// If fork fails there is nothing to wait for.
 		if ret.err != nil || ret.pid == 0 {
 			forkc <- ret
 			return
 		}

 		waitc := make(chan *waitErr, 1)

 		// Mark that the process is running.
 		procs.Lock()
 		if procs.waits == nil {
 			procs.waits = make(map[int]chan *waitErr)
 		}
 		procs.waits[ret.pid] = waitc
 		procs.Unlock()

 		forkc <- ret

 		var w waitErr
 		for w.err == nil && w.Pid != ret.pid {
 			w.err = Await(&w.Waitmsg)
 		}
 		waitc <- &w
 		close(waitc)
 	}()
 	ret := <-forkc
 	return ret.pid, ret.err
 }

 // Combination of fork and exec, careful to be thread safe.
 func ForkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
 	return startProcess(argv0, argv, attr)
 }

 // StartProcess wraps ForkExec for package os.
 func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle uintptr, err error) {
 	pid, err = startProcess(argv0, argv, attr)
 	return pid, 0, err
 }

 // Ordinary exec.
 func Exec(argv0 string, argv []string, envv []string) (err error) {
 	if envv != nil {
 		r1, _, _ := RawSyscall(SYS_RFORK, RFCENVG, 0, 0)
 		if int32(r1) == -1 {
 			return NewError(errstr())
 		}

 		for _, v := range envv {
 			i := 0
 			for i < len(v) && v[i] != '=' {
 				i++
 			}

 			fd, e := Create("/env/"+v[:i], O_WRONLY, 0666)
 			if e != nil {
 				return e
 			}

 			_, e = Write(fd, []byte(v[i+1:]))
 			if e != nil {
 				Close(fd)
 				return e
 			}
 			Close(fd)
 		}
 	}

 	argv0p, err := BytePtrFromString(argv0)
 	if err != nil {
 		return err
 	}
 	argvp, err := SlicePtrFromStrings(argv)
 	if err != nil {
 		return err
 	}
 	_, _, e1 := Syscall(SYS_EXEC,
 		uintptr(unsafe.Pointer(argv0p)),
 		uintptr(unsafe.Pointer(&argvp[0])),
 		0)

 	return e1
 }

 // WaitProcess waits until the pid of a
 // running process is found in the queue of
 // wait messages. It is used in conjunction
 // with ForkExec/StartProcess to wait for a
 // running process to exit.
 func WaitProcess(pid int, w *Waitmsg) (err error) {
 	procs.Lock()
 	ch := procs.waits[pid]
 	procs.Unlock()

 	var wmsg *waitErr
 	if ch != nil {
 		wmsg = <-ch
 		procs.Lock()
 		if procs.waits[pid] == ch {
 			delete(procs.waits, pid)
 		}
 		procs.Unlock()
 	}
 	if wmsg == nil {
 		// ch was missing or ch is closed
 		return NewError("process not found")
 	}
 	if wmsg.err != nil {
 		return wmsg.err
 	}
 	if w != nil {
 		*w = wmsg.Waitmsg
 	}
 	return nil
 }
	// Copyright 2009 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.

	// Fork, exec, wait, etc.

	package syscall

	import (
	"runtime"
	"sync"
	"unsafe"
	)

	// Lock synchronizing creation of new file descriptors with fork.
	//
	// We want the child in a fork/exec sequence to inherit only the
	// file descriptors we intend. To do that, we mark all file
	// descriptors close-on-exec and then, in the child, explicitly
	// unmark the ones we want the exec'ed program to keep.
	// Unix doesn't make this easy: there is, in general, no way to
	// allocate a new file descriptor close-on-exec. Instead you
	// have to allocate the descriptor and then mark it close-on-exec.
	// If a fork happens between those two events, the child's exec
	// will inherit an unwanted file descriptor.
	//
	// This lock solves that race: the create new fd/mark close-on-exec
	// operation is done holding ForkLock for reading, and the fork itself
	// is done holding ForkLock for writing. At least, that's the idea.
	// There are some complications.
	//
	// Some system calls that create new file descriptors can block
	// for arbitrarily long times: open on a hung NFS server or named
	// pipe, accept on a socket, and so on. We can't reasonably grab
	// the lock across those operations.
	//
	// It is worse to inherit some file descriptors than others.
	// If a non-malicious child accidentally inherits an open ordinary file,
	// that's not a big deal. On the other hand, if a long-lived child
	// accidentally inherits the write end of a pipe, then the reader
	// of that pipe will not see EOF until that child exits, potentially
	// causing the parent program to hang. This is a common problem
	// in threaded C programs that use popen.
	//
	// Luckily, the file descriptors that are most important not to
	// inherit are not the ones that can take an arbitrarily long time
	// to create: pipe returns instantly, and the net package uses
	// non-blocking I/O to accept on a listening socket.
	// The rules for which file descriptor-creating operations use the
	// ForkLock are as follows:
	//
	// 1) Pipe. Does not block. Use the ForkLock.
	// 2) Socket. Does not block. Use the ForkLock.
	// 3) Accept. If using non-blocking mode, use the ForkLock.
	// Otherwise, live with the race.
	// 4) Open. Can block. Use O_CLOEXEC if available (Linux).
	// Otherwise, live with the race.
	// 5) Dup. Does not block. Use the ForkLock.
	// On Linux, could use fcntl F_DUPFD_CLOEXEC
	// instead of the ForkLock, but only for dup(fd, -1).

	var ForkLock sync.RWMutex

	// StringSlicePtr is deprecated. Use SlicePtrFromStrings instead.
	// If any string contains a NUL byte this function panics instead
	// of returning an error.
	func StringSlicePtr(ss []string) []*byte {
	bb := make([]*byte, len(ss)+1)
	for i := 0; i < len(ss); i++ {
	bb[i] = StringBytePtr(ss[i])
	}
	bb[len(ss)] = nil
	return bb
	}

	// SlicePtrFromStrings converts a slice of strings to a slice of
	// pointers to NUL-terminated byte slices. If any string contains
	// a NUL byte, it returns (nil, EINVAL).
	func SlicePtrFromStrings(ss []string) ([]*byte, error) {
	var err error
	bb := make([]*byte, len(ss)+1)
	for i := 0; i < len(ss); i++ {
	bb[i], err = BytePtrFromString(ss[i])
	if err != nil {
	return nil, err
	}
	}
	bb[len(ss)] = nil
	return bb, nil
	}

	// readdirnames returns the names of files inside the directory represented by dirfd.
	func readdirnames(dirfd int) (names []string, err error) {
	names = make([]string, 0, 100)
	var buf [STATMAX]byte

	for {
	n, e := Read(dirfd, buf[:])
	if e != nil {
	return nil, e
	}
	if n == 0 {
	break
	}
	for i := 0; i < n; {
	m, _ := gbit16(buf[i:])
	m += 2

	if m < STATFIXLEN {
	return nil, ErrBadStat
	}

	s, _, ok := gstring(buf[i+41:])
	if !ok {
	return nil, ErrBadStat
	}
	names = append(names, s)
	i += int(m)
	}
	}
	return
	}

	// readdupdevice returns a list of currently opened fds (excluding stdin, stdout, stderr) from the dup device #d.
	// ForkLock should be write locked before calling, so that no new fds would be created while the fd list is being read.
	func readdupdevice() (fds []int, err error) {
	dupdevfd, err := Open("#d", O_RDONLY)
	if err != nil {
	return
	}
	defer Close(dupdevfd)

	names, err := readdirnames(dupdevfd)
	if err != nil {
	return
	}

	fds = make([]int, 0, len(names)/2)
	for _, name := range names {
	if n := len(name); n > 3 && name[n-3:n] == "ctl" {
	continue
	}
	fd := int(atoi([]byte(name)))
	switch fd {
	case 0, 1, 2, dupdevfd:
	continue
	}
	fds = append(fds, fd)
	}
	return
	}

	var startupFds []int

	// Plan 9 does not allow clearing the OCEXEC flag
	// from the underlying channel backing an open file descriptor,
	// therefore we store a list of already opened file descriptors
	// inside startupFds and skip them when manually closing descriptors
	// not meant to be passed to a child exec.
	func init() {
	startupFds, _ = readdupdevice()
	}

	// forkAndExecInChild forks the process, calling dup onto 0..len(fd)
	// and finally invoking exec(argv0, argvv, envv) in the child.
	// If a dup or exec fails, it writes the error string to pipe.
	// (The pipe write end 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.
	// The calls to RawSyscall are okay because they are assembly
	// functions that do not grow the stack.
	func forkAndExecInChild(argv0 byte, argv []byte, envv []envItem, dir byte, attr ProcAttr, fdsToClose []int, pipe int, rflag int) (pid int, err error) {
	// Declare all variables at top in case any
	// declarations require heap allocation (e.g., errbuf).
	var (
	r1 uintptr
	nextfd int
	i int
	clearenv int
	envfd int
	errbuf [ERRMAX]byte
	)

	// 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++

	if envv != nil {
	clearenv = RFCENVG
	}

	// About to call fork.
	// No more allocation or calls of non-assembly functions.
	r1, _, _ = RawSyscall(SYS_RFORK, uintptr(RFPROC\|RFFDG\|RFREND\|clearenv\|rflag), 0, 0)

	if r1 != 0 {
	if int32(r1) == -1 {
	return 0, NewError(errstr())
	}
	// parent; return PID
	return int(r1), nil
	}

	// Fork succeeded, now in child.

	// Close fds we don't need.
	for i = 0; i < len(fdsToClose); i++ {
	r1, _, _ = RawSyscall(SYS_CLOSE, uintptr(fdsToClose[i]), 0, 0)
	if int32(r1) == -1 {
	goto childerror
	}
	}

	if envv != nil {
	// Write new environment variables.
	for i = 0; i < len(envv); i++ {
	r1, _, _ = RawSyscall(SYS_CREATE, uintptr(unsafe.Pointer(envv[i].name)), uintptr(O_WRONLY), uintptr(0666))

	if int32(r1) == -1 {
	goto childerror
	}

	envfd = int(r1)

	r1, _, _ = RawSyscall6(SYS_PWRITE, uintptr(envfd), uintptr(unsafe.Pointer(envv[i].value)), uintptr(envv[i].nvalue),
	^uintptr(0), ^uintptr(0), 0)

	if int32(r1) == -1 \|\| int(r1) != envv[i].nvalue {
	goto childerror
	}

	r1, _, _ = RawSyscall(SYS_CLOSE, uintptr(envfd), 0, 0)

	if int32(r1) == -1 {
	goto childerror
	}
	}
	}

	// Chdir
	if dir != nil {
	r1, _, _ = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
	if int32(r1) == -1 {
	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 {
	r1, _, _ = RawSyscall(SYS_DUP, uintptr(pipe), uintptr(nextfd), 0)
	if int32(r1) == -1 {
	goto childerror
	}
	pipe = nextfd
	nextfd++
	}
	for i = 0; i < len(fd); i++ {
	if fd[i] >= 0 && fd[i] < int(i) {
	r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(nextfd), 0)
	if int32(r1) == -1 {
	goto childerror
	}

	fd[i] = nextfd
	nextfd++
	if nextfd == pipe { // don't stomp on pipe
	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) {
	continue
	}
	r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(i), 0)
	if int32(r1) == -1 {
	goto childerror
	}
	}

	// Pass 3: close fd[i] if it was moved in the previous pass.
	for i = 0; i < len(fd); i++ {
	if fd[i] >= 0 && fd[i] != int(i) {
	RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
	}
	}

	// Time to exec.
	r1, _, _ = RawSyscall(SYS_EXEC,
	uintptr(unsafe.Pointer(argv0)),
	uintptr(unsafe.Pointer(&argv[0])), 0)

	childerror:
	// send error string on pipe
	RawSyscall(SYS_ERRSTR, uintptr(unsafe.Pointer(&errbuf[0])), uintptr(len(errbuf)), 0)
	errbuf[len(errbuf)-1] = 0
	i = 0
	for i < len(errbuf) && errbuf[i] != 0 {
	i++
	}

	RawSyscall6(SYS_PWRITE, uintptr(pipe), uintptr(unsafe.Pointer(&errbuf[0])), uintptr(i),
	^uintptr(0), ^uintptr(0), 0)

	for {
	RawSyscall(SYS_EXITS, 0, 0, 0)
	}

	// Calling panic is not actually safe,
	// but the for loop above won't break
	// and this shuts up the compiler.
	panic("unreached")
	}

	func cexecPipe(p []int) error {
	e := Pipe(p)
	if e != nil {
	return e
	}

	fd, e := Open("#d/"+itoa(p[1]), O_CLOEXEC)
	if e != nil {
	Close(p[0])
	Close(p[1])
	return e
	}

	Close(fd)
	return nil
	}

	type envItem struct {
	name *byte
	value *byte
	nvalue int
	}

	type ProcAttr struct {
	Dir string // Current working directory.
	Env []string // Environment.
	Files []uintptr // File descriptors.
	Sys *SysProcAttr
	}

	type SysProcAttr struct {
	Rfork int // additional flags to pass to rfork
	}

	var zeroProcAttr ProcAttr
	var zeroSysProcAttr SysProcAttr

	func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
	var (
	p [2]int
	n int
	errbuf [ERRMAX]byte
	wmsg Waitmsg
	)

	if attr == nil {
	attr = &zeroProcAttr
	}
	sys := attr.Sys
	if sys == nil {
	sys = &zeroSysProcAttr
	}

	p[0] = -1
	p[1] = -1

	// Convert args to C form.
	argv0p, err := BytePtrFromString(argv0)
	if err != nil {
	return 0, err
	}
	argvp, err := SlicePtrFromStrings(argv)
	if err != nil {
	return 0, err
	}

	var dir *byte
	if attr.Dir != "" {
	dir, err = BytePtrFromString(attr.Dir)
	if err != nil {
	return 0, err
	}
	}
	var envvParsed []envItem
	if attr.Env != nil {
	envvParsed = make([]envItem, 0, len(attr.Env))
	for _, v := range attr.Env {
	i := 0
	for i < len(v) && v[i] != '=' {
	i++
	}

	envname, err := BytePtrFromString("/env/" + v[:i])
	if err != nil {
	return 0, err
	}
	envvalue := make([]byte, len(v)-i)
	copy(envvalue, v[i+1:])
	envvParsed = append(envvParsed, envItem{envname, &envvalue[0], len(v) - i})
	}
	}

	// Acquire the fork lock to prevent other threads from creating new fds before we fork.
	ForkLock.Lock()

	// get a list of open fds, excluding stdin,stdout and stderr that need to be closed in the child.
	// no new fds can be created while we hold the ForkLock for writing.
	openFds, e := readdupdevice()
	if e != nil {
	ForkLock.Unlock()
	return 0, e
	}

	fdsToClose := make([]int, 0, len(openFds))
	for _, fd := range openFds {
	doClose := true

	// exclude files opened at startup.
	for _, sfd := range startupFds {
	if fd == sfd {
	doClose = false
	break
	}
	}

	// exclude files explicitly requested by the caller.
	for _, rfd := range attr.Files {
	if fd == int(rfd) {
	doClose = false
	break
	}
	}

	if doClose {
	fdsToClose = append(fdsToClose, fd)
	}
	}

	// Allocate child status pipe close on exec.
	e = cexecPipe(p[:])

	if e != nil {
	return 0, e
	}
	fdsToClose = append(fdsToClose, p[0])

	// Kick off child.
	pid, err = forkAndExecInChild(argv0p, argvp, envvParsed, dir, attr, fdsToClose, p[1], sys.Rfork)

	if err != nil {
	if p[0] >= 0 {
	Close(p[0])
	Close(p[1])
	}
	ForkLock.Unlock()
	return 0, err
	}
	ForkLock.Unlock()

	// Read child error status from pipe.
	Close(p[1])
	n, err = Read(p[0], errbuf[:])
	Close(p[0])

	if err != nil \|\| n != 0 {
	if n != 0 {
	err = NewError(string(errbuf[:n]))
	}

	// Child failed; wait for it to exit, to make sure
	// the zombies don't accumulate.
	for wmsg.Pid != pid {
	Await(&wmsg)
	}
	return 0, err
	}

	// Read got EOF, so pipe closed on exec, so exec succeeded.
	return pid, nil
	}

	type waitErr struct {
	Waitmsg
	err error
	}

	var procs struct {
	sync.Mutex
	waits map[int]chan *waitErr
	}

	// startProcess starts a new goroutine, tied to the OS
	// thread, which runs the process and subsequently waits
	// for it to finish, communicating the process stats back
	// to any goroutines that may have been waiting on it.
	//
	// Such a dedicated goroutine is needed because on
	// Plan 9, only the parent thread can wait for a child,
	// whereas goroutines tend to jump OS threads (e.g.,
	// between starting a process and running Wait(), the
	// goroutine may have been rescheduled).
	func startProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
	type forkRet struct {
	pid int
	err error
	}

	forkc := make(chan forkRet, 1)
	go func() {
	runtime.LockOSThread()
	var ret forkRet

	ret.pid, ret.err = forkExec(argv0, argv, attr)
	// If fork fails there is nothing to wait for.
	if ret.err != nil \|\| ret.pid == 0 {
	forkc <- ret
	return
	}

	waitc := make(chan *waitErr, 1)

	// Mark that the process is running.
	procs.Lock()
	if procs.waits == nil {
	procs.waits = make(map[int]chan *waitErr)
	}
	procs.waits[ret.pid] = waitc
	procs.Unlock()

	forkc <- ret

	var w waitErr
	for w.err == nil && w.Pid != ret.pid {
	w.err = Await(&w.Waitmsg)
	}
	waitc <- &w
	close(waitc)
	}()
	ret := <-forkc
	return ret.pid, ret.err
	}

	// Combination of fork and exec, careful to be thread safe.
	func ForkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
	return startProcess(argv0, argv, attr)
	}

	// StartProcess wraps ForkExec for package os.
	func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle uintptr, err error) {
	pid, err = startProcess(argv0, argv, attr)
	return pid, 0, err
	}

	// Ordinary exec.
	func Exec(argv0 string, argv []string, envv []string) (err error) {
	if envv != nil {
	r1, _, _ := RawSyscall(SYS_RFORK, RFCENVG, 0, 0)
	if int32(r1) == -1 {
	return NewError(errstr())
	}

	for _, v := range envv {
	i := 0
	for i < len(v) && v[i] != '=' {
	i++
	}

	fd, e := Create("/env/"+v[:i], O_WRONLY, 0666)
	if e != nil {
	return e
	}

	_, e = Write(fd, []byte(v[i+1:]))
	if e != nil {
	Close(fd)
	return e
	}
	Close(fd)
	}
	}

	argv0p, err := BytePtrFromString(argv0)
	if err != nil {
	return err
	}
	argvp, err := SlicePtrFromStrings(argv)
	if err != nil {
	return err
	}
	_, _, e1 := Syscall(SYS_EXEC,
	uintptr(unsafe.Pointer(argv0p)),
	uintptr(unsafe.Pointer(&argvp[0])),
	0)

	return e1
	}

	// WaitProcess waits until the pid of a
	// running process is found in the queue of
	// wait messages. It is used in conjunction
	// with ForkExec/StartProcess to wait for a
	// running process to exit.
	func WaitProcess(pid int, w *Waitmsg) (err error) {
	procs.Lock()
	ch := procs.waits[pid]
	procs.Unlock()

	var wmsg *waitErr
	if ch != nil {
	wmsg = <-ch
	procs.Lock()
	if procs.waits[pid] == ch {
	delete(procs.waits, pid)
	}
	procs.Unlock()
	}
	if wmsg == nil {
	// ch was missing or ch is closed
	return NewError("process not found")
	}
	if wmsg.err != nil {
	return wmsg.err
	}
	if w != nil {
	*w = wmsg.Waitmsg
	}
	return nil
	}