src/syscall/exec_plan9.go - go.git - 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 (
 	"internal/itoa"
 	"runtime"
 	"sync"
 	"unsafe"
 )

 // ForkLock is not used on plan9.
 var ForkLock sync.RWMutex

 // gstringb reads a non-empty string from b, prefixed with a 16-bit length in little-endian order.
 // It returns the string as a byte slice, or nil if b is too short to contain the length or
 // the full string.
 //
 //go:nosplit
 func gstringb(b []byte) []byte {
 	if len(b) < 2 {
 		return nil
 	}
 	n, b := gbit16(b)
 	if int(n) > len(b) {
 		return nil
 	}
 	return b[:n]
 }

 // Offset of the name field in a 9P directory entry - see UnmarshalDir() in dir_plan9.go
 const nameOffset = 39

 // gdirname returns the first filename from a buffer of directory entries,
 // and a slice containing the remaining directory entries.
 // If the buffer doesn't start with a valid directory entry, the returned name is nil.
 //
 //go:nosplit
 func gdirname(buf []byte) (name []byte, rest []byte) {
 	if len(buf) < 2 {
 		return
 	}
 	size, buf := gbit16(buf)
 	if size < STATFIXLEN || int(size) > len(buf) {
 		return
 	}
 	name = gstringb(buf[nameOffset:size])
 	rest = buf[size:]
 	return
 }

 // StringSlicePtr converts a slice of strings to a slice of pointers
 // to NUL-terminated byte arrays. If any string contains a NUL byte
 // this function panics instead of returning an error.
 //
 // Deprecated: Use SlicePtrFromStrings instead.
 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 arrays. 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 b := buf[:n]; len(b) > 0; {
 			var s []byte
 			s, b = gdirname(b)
 			if s == nil {
 				return nil, ErrBadStat
 			}
 			names = append(names, string(s))
 		}
 	}
 	return
 }

 // name of the directory containing names and control files for all open file descriptors
 var dupdev, _ = BytePtrFromString("#d")

 // 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.
 //
 //go:norace
 func forkAndExecInChild(argv0 *byte, argv []*byte, envv []envItem, dir *byte, attr *ProcAttr, 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
 		statbuf  [STATMAX]byte
 		dupdevfd int
 	)

 	// 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.
 	r1, _, _ = RawSyscall(SYS_OPEN, uintptr(unsafe.Pointer(dupdev)), uintptr(O_RDONLY), 0)
 	dupdevfd = int(r1)
 	if dupdevfd == -1 {
 		goto childerror
 	}
 dirloop:
 	for {
 		r1, _, _ = RawSyscall6(SYS_PREAD, uintptr(dupdevfd), uintptr(unsafe.Pointer(&statbuf[0])), uintptr(len(statbuf)), ^uintptr(0), ^uintptr(0), 0)
 		n := int(r1)
 		switch n {
 		case -1:
 			goto childerror
 		case 0:
 			break dirloop
 		}
 		for b := statbuf[:n]; len(b) > 0; {
 			var s []byte
 			s, b = gdirname(b)
 			if s == nil {
 				copy(errbuf[:], ErrBadStat.Error())
 				goto childerror1
 			}
 			if s[len(s)-1] == 'l' {
 				// control file for descriptor <N> is named <N>ctl
 				continue
 			}
 			closeFdExcept(int(atoi(s)), pipe, dupdevfd, fd)
 		}
 	}
 	RawSyscall(SYS_CLOSE, uintptr(dupdevfd), 0, 0)

 	// Write new environment variables.
 	if envv != nil {
 		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) {
 			if nextfd == pipe { // don't stomp on pipe
 				nextfd++
 			}
 			r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(nextfd), 0)
 			if int32(r1) == -1 {
 				goto childerror
 			}

 			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) {
 			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)
 childerror1:
 	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)
 	}
 }

 // close the numbered file descriptor, unless it is fd1, fd2, or a member of fds.
 //
 //go:nosplit
 func closeFdExcept(n int, fd1 int, fd2 int, fds []int) {
 	if n == fd1 || n == fd2 {
 		return
 	}
 	for _, fd := range fds {
 		if n == fd {
 			return
 		}
 	}
 	RawSyscall(SYS_CLOSE, uintptr(n), 0, 0)
 }

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

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

 	Close(p[1])
 	p[1] = 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
 	}

 	destDir := attr.Dir
 	if destDir == "" {
 		wdmu.Lock()
 		destDir = wdStr
 		wdmu.Unlock()
 	}
 	var dir *byte
 	if destDir != "" {
 		dir, err = BytePtrFromString(destDir)
 		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})
 		}
 	}

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

 	if e != nil {
 		return 0, e
 	}

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

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

 	// 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]))
 		} else if err == nil {
 			err = NewError("failed to read exec status")
 		}

 		// 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 (
	"internal/itoa"
	"runtime"
	"sync"
	"unsafe"
	)

	// ForkLock is not used on plan9.
	var ForkLock sync.RWMutex

	// gstringb reads a non-empty string from b, prefixed with a 16-bit length in little-endian order.
	// It returns the string as a byte slice, or nil if b is too short to contain the length or
	// the full string.
	//
	//go:nosplit
	func gstringb(b []byte) []byte {
	if len(b) < 2 {
	return nil
	}
	n, b := gbit16(b)
	if int(n) > len(b) {
	return nil
	}
	return b[:n]
	}

	// Offset of the name field in a 9P directory entry - see UnmarshalDir() in dir_plan9.go
	const nameOffset = 39

	// gdirname returns the first filename from a buffer of directory entries,
	// and a slice containing the remaining directory entries.
	// If the buffer doesn't start with a valid directory entry, the returned name is nil.
	//
	//go:nosplit
	func gdirname(buf []byte) (name []byte, rest []byte) {
	if len(buf) < 2 {
	return
	}
	size, buf := gbit16(buf)
	if size < STATFIXLEN \|\| int(size) > len(buf) {
	return
	}
	name = gstringb(buf[nameOffset:size])
	rest = buf[size:]
	return
	}

	// StringSlicePtr converts a slice of strings to a slice of pointers
	// to NUL-terminated byte arrays. If any string contains a NUL byte
	// this function panics instead of returning an error.
	//
	// Deprecated: Use SlicePtrFromStrings instead.
	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 arrays. 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 b := buf[:n]; len(b) > 0; {
	var s []byte
	s, b = gdirname(b)
	if s == nil {
	return nil, ErrBadStat
	}
	names = append(names, string(s))
	}
	}
	return
	}

	// name of the directory containing names and control files for all open file descriptors
	var dupdev, _ = BytePtrFromString("#d")

	// 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.
	//
	//go:norace
	func forkAndExecInChild(argv0 byte, argv []byte, envv []envItem, dir byte, attr ProcAttr, 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
	statbuf [STATMAX]byte
	dupdevfd int
	)

	// 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.
	r1, _, _ = RawSyscall(SYS_OPEN, uintptr(unsafe.Pointer(dupdev)), uintptr(O_RDONLY), 0)
	dupdevfd = int(r1)
	if dupdevfd == -1 {
	goto childerror
	}
	dirloop:
	for {
	r1, _, _ = RawSyscall6(SYS_PREAD, uintptr(dupdevfd), uintptr(unsafe.Pointer(&statbuf[0])), uintptr(len(statbuf)), ^uintptr(0), ^uintptr(0), 0)
	n := int(r1)
	switch n {
	case -1:
	goto childerror
	case 0:
	break dirloop
	}
	for b := statbuf[:n]; len(b) > 0; {
	var s []byte
	s, b = gdirname(b)
	if s == nil {
	copy(errbuf[:], ErrBadStat.Error())
	goto childerror1
	}
	if s[len(s)-1] == 'l' {
	// control file for descriptor <N> is named <N>ctl
	continue
	}
	closeFdExcept(int(atoi(s)), pipe, dupdevfd, fd)
	}
	}
	RawSyscall(SYS_CLOSE, uintptr(dupdevfd), 0, 0)

	// Write new environment variables.
	if envv != nil {
	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) {
	if nextfd == pipe { // don't stomp on pipe
	nextfd++
	}
	r1, _, _ = RawSyscall(SYS_DUP, uintptr(fd[i]), uintptr(nextfd), 0)
	if int32(r1) == -1 {
	goto childerror
	}

	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) {
	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)
	childerror1:
	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)
	}
	}

	// close the numbered file descriptor, unless it is fd1, fd2, or a member of fds.
	//
	//go:nosplit
	func closeFdExcept(n int, fd1 int, fd2 int, fds []int) {
	if n == fd1 \|\| n == fd2 {
	return
	}
	for _, fd := range fds {
	if n == fd {
	return
	}
	}
	RawSyscall(SYS_CLOSE, uintptr(n), 0, 0)
	}

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

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

	Close(p[1])
	p[1] = 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
	}

	destDir := attr.Dir
	if destDir == "" {
	wdmu.Lock()
	destDir = wdStr
	wdmu.Unlock()
	}
	var dir *byte
	if destDir != "" {
	dir, err = BytePtrFromString(destDir)
	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})
	}
	}

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

	if e != nil {
	return 0, e
	}

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

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

	// 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]))
	} else if err == nil {
	err = NewError("failed to read exec status")
	}

	// 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
	}