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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 StringArrayPtr(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
}
func CloseOnExec(fd int) { fcntl(fd, F_SETFD, FD_CLOEXEC) }
func SetNonblock(fd int, nonblocking bool) (errno int) {
flag, err := fcntl(fd, F_GETFL, 0)
if err != 0 {
return err
}
if nonblocking {
flag |= O_NONBLOCK
} else {
flag &= ^O_NONBLOCK
}
_, err = fcntl(fd, F_SETFL, flag)
return err
}
// Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
// If a dup or exec fails, write the errno int 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.
// The calls to RawSyscall are okay because they are assembly
// functions that do not grow the stack.
func forkAndExecInChild(argv0 *byte, argv []*byte, envv []*byte, traceme bool, dir *byte, fd []int, pipe int) (pid int, err int) {
// Declare all variables at top in case any
// declarations require heap allocation (e.g., err1).
var r1, r2, err1 uintptr
var nextfd int
var i int
darwin := OS == "darwin"
// About to call fork.
// No more allocation or calls of non-assembly functions.
r1, r2, err1 = RawSyscall(SYS_FORK, 0, 0, 0)
if err1 != 0 {
return 0, int(err1)
}
// On Darwin:
// r1 = child pid in both parent and child.
// r2 = 0 in parent, 1 in child.
// Convert to normal Unix r1 = 0 in child.
if darwin && r2 == 1 {
r1 = 0
}
if r1 != 0 {
// parent; return PID
return int(r1), 0
}
// Fork succeeded, now in child.
// Enable tracing if requested.
if traceme {
_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 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
}
}
// 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 {
_, _, err1 = RawSyscall(SYS_DUP2, 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) {
_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(nextfd), 0)
if err1 != 0 {
goto childerror
}
RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
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) {
// 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_DUP2, 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)
}
// 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)), uintptr(unsafe.Sizeof(err1)))
for {
RawSyscall(SYS_EXIT, 253, 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 forkExec(argv0 string, argv []string, envv []string, traceme bool, dir string, fd []int) (pid int, err int) {
var p [2]int
var n int
var err1 uintptr
var wstatus WaitStatus
p[0] = -1
p[1] = -1
// Convert args to C form.
argv0p := StringBytePtr(argv0)
argvp := StringArrayPtr(argv)
envvp := StringArrayPtr(envv)
var dirp *byte
if len(dir) > 0 {
dirp = StringBytePtr(dir)
}
// Acquire the fork lock so that no other threads
// create new fds that are not yet close-on-exec
// before we fork.
ForkLock.Lock()
// Allocate child status pipe close on exec.
if err = Pipe(p[0:]); err != 0 {
goto error
}
if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != 0 {
goto error
}
if _, err = fcntl(p[1], F_SETFD, FD_CLOEXEC); err != 0 {
goto error
}
// Kick off child.
pid, err = forkAndExecInChild(argv0p, argvp, envvp, traceme, dirp, fd, p[1])
if err != 0 {
error:
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], (*byte)(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
Close(p[0])
if err != 0 || n != 0 {
if n == unsafe.Sizeof(err1) {
err = int(err1)
}
if err == 0 {
err = EPIPE
}
// Child failed; wait for it to exit, to make sure
// the zombies don't accumulate.
_, err1 := Wait4(pid, &wstatus, 0, nil)
for err1 == EINTR {
_, err1 = Wait4(pid, &wstatus, 0, nil)
}
return 0, err
}
// Read got EOF, so pipe closed on exec, so exec succeeded.
return pid, 0
}
// Combination of fork and exec, careful to be thread safe.
func ForkExec(argv0 string, argv []string, envv []string, dir string, fd []int) (pid int, err int) {
return forkExec(argv0, argv, envv, false, dir, fd)
}
// PtraceForkExec is like ForkExec, but starts the child in a traced state.
func PtraceForkExec(argv0 string, argv []string, envv []string, dir string, fd []int) (pid int, err int) {
return forkExec(argv0, argv, envv, true, dir, fd)
}
// Ordinary exec.
func Exec(argv0 string, argv []string, envv []string) (err int) {
_, _, err1 := RawSyscall(SYS_EXECVE,
uintptr(unsafe.Pointer(StringBytePtr(argv0))),
uintptr(unsafe.Pointer(&StringArrayPtr(argv)[0])),
uintptr(unsafe.Pointer(&StringArrayPtr(envv)[0])))
return int(err1)
}