// 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"
)
// 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(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
}
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
}
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