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// 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 (
"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
// Convert array of string to array
// of NUL-terminated byte pointer.
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
}
// gbit16 reads a 16-bit numeric value from a 9P protocol message strored in b,
// returning the value and the remaining slice of b.
func gbit16(b []byte) (uint16, []byte) {
return uint16(b[0]) | uint16(b[1])<<8, b[2:]
}
// gstring reads a string from a 9P protocol message strored in b,
// returning the value as a Go string and the remaining slice of b.
func gstring(b []byte) (string, []byte) {
n, b := gbit16(b)
return string(b[0:n]), b[n:]
}
// readdirnames returns the names of files inside the directory represented by dirfd.
func readdirnames(dirfd int) (names []string, err Error) {
result := make([]string, 0, 100)
var buf [STATMAX]byte
for {
n, e := Read(dirfd, buf[:])
if e != nil {
return []string{}, e
}
if n == 0 {
break
}
for i := 0; i < n; {
m, _ := gbit16(buf[i:])
m += 2
if m < STATFIXLEN {
return []string{}, NewError("malformed stat buffer")
}
name, _ := gstring(buf[i+41:])
result = append(result, name)
i += int(m)
}
}
return []string{}, nil
}
// 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)
fileNames, err := readdirnames(dupdevfd)
if err != nil {
return
}
fds = make([]int, 0, len(fileNames)>>1)
for _, fdstr := range fileNames {
if l := len(fdstr); l > 2 && fdstr[l-3] == 'c' && fdstr[l-2] == 't' && fdstr[l-1] == 'l' {
continue
}
fd := int(atoi([]byte(fdstr)))
if fd == 0 || fd == 1 || fd == 2 || fd == dupdevfd {
continue
}
fds = append(fds, fd)
}
return fds[0:len(fds)], nil
}
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.
fd := append([]int(nil), attr.Files...)
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 int(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 int(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 int(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 int(r1) == -1 || int(r1) != envv[i].nvalue {
goto childerror
}
r1, _, _ = RawSyscall(SYS_CLOSE, uintptr(envfd), 0, 0)
if int(r1) == -1 {
goto childerror
}
}
}
// Chdir
if dir != nil {
r1, _, _ = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
if int(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.
nextfd = int(len(fd))
if pipe < nextfd {
r1, _, _ = RawSyscall(SYS_DUP, uintptr(pipe), uintptr(nextfd), 0)
if int(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 int(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 int(r1) == -1 {
goto childerror
}
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 []int // 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 := StringBytePtr(argv0)
argvp := StringSlicePtr(argv)
var dir *byte
if attr.Dir != "" {
dir = StringBytePtr(attr.Dir)
}
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++
}
envvParsed = append(envvParsed, envItem{StringBytePtr("/env/" + v[:i]), StringBytePtr(v[i+1:]), 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))
// exclude fds opened from startup from the list of fds to be closed.
for _, fd := range openFds {
isReserved := false
for _, reservedFd := range startupFds {
if fd == reservedFd {
isReserved = true
break
}
}
if !isReserved {
fdsToClose = append(fdsToClose, fd)
}
}
// exclude fds requested by the caller from the list of fds to be closed.
for _, fd := range openFds {
isReserved := false
for _, reservedFd := range attr.Files {
if fd == reservedFd {
isReserved = true
break
}
}
if !isReserved {
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[:]))
}
// 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
}
// Combination of fork and exec, careful to be thread safe.
func ForkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err Error) {
return forkExec(argv0, argv, attr)
}
// StartProcess wraps ForkExec for package os.
func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid, handle int, err Error) {
pid, err = forkExec(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 int(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)
}
}
_, _, e := Syscall(SYS_EXEC,
uintptr(unsafe.Pointer(StringBytePtr(argv0))),
uintptr(unsafe.Pointer(&StringSlicePtr(argv)[0])),
0)
return NewError(e)
}