gcc/libgo/syscalls/exec.go

// exec.go -- fork/exec syscall support.

// 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 "unsafe"

func libc_fcntl(fd int, cmd int, arg int) int __asm__ ("fcntl")
func libc_fork() Pid_t __asm__ ("fork")
func libc_chdir(name *byte) int __asm__ ("chdir");
func libc_dup2(int, int) int __asm__ ("dup2")
func libc_execve(*byte, **byte, **byte) int __asm__ ("execve")
func libc_sysexit(int) __asm__ ("_exit")
func libc_wait4(Pid_t, *int, int, *Rusage) Pid_t __asm__ ("wait4")

// 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.
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.
	child := libc_fork();
	if child == -1 {
		return 0, GetErrno();
	}

	if child != 0 {
		// parent; return PID
		return int(child), 0
	}

	// Fork succeeded, now in child.

	// Enable tracing if requested.
	if traceme {
		if libc_ptrace(_PTRACE_TRACEME, 0, 0, nil) < 0 {
			goto childerror;
		}
	}

	// Chdir
	if dir != nil {
		r := libc_chdir(dir);
		if r < 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 {
		r := libc_dup2(pipe, nextfd);
		if r == -1 {
			goto childerror;
		}
		libc_fcntl(nextfd, F_SETFD, FD_CLOEXEC);
		pipe = nextfd;
		nextfd++;
	}
	for i = 0; i < len(fd); i++ {
		if fd[i] >= 0 && fd[i] < int(i) {
			r := libc_dup2(fd[i], nextfd);
			if r == -1 {
				goto childerror;
			}
			libc_fcntl(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 {
			libc_close(i);
			continue;
		}
		if fd[i] == int(i) {
			// dup2(i, i) won't clear close-on-exec flag on Linux,
			// probably not elsewhere either.
			r := libc_fcntl(fd[i], F_SETFD, 0);
			if r != 0 {
				goto childerror;
			}
			continue;
		}
		// The new fd is created NOT close-on-exec,
		// which is exactly what we want.
		r := libc_dup2(fd[i], i);
		if r == -1 {
			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++ {
		libc_close(i);
	}

	// Time to exec.
	libc_execve(argv0, &argv[0], &envv[0]);

childerror:
	// send error code on pipe
	var e uintptr = uintptr(GetErrno());
	libc_write(pipe, (*byte)(unsafe.Pointer(&e)),
		   Size_t(unsafe.Sizeof(err1)));
	for {
		libc_sysexit(253)
	}

	// 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 r1 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;
	}
	var val int;
	if val, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != 0 {
		goto error;
	}
	if val, 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 := libc_read(p[0], (*byte)(unsafe.Pointer(&err1)),
		       Size_t(unsafe.Sizeof(err1)));
	err = 0;
	if n < 0 {
		err = GetErrno();
	}
	Close(p[0]);
	if err != 0 || n != 0 {
		if int(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.
		pid1, err1 := Wait4(pid, &wstatus, 0, nil);
		for err1 == EINTR {
			pid1, 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) {
	argv_arg := StringArrayPtr(argv);
	envv_arg := StringArrayPtr(envv);
	libc_execve(StringBytePtr(argv0), &argv_arg[0], &envv_arg[0]);
	return GetErrno();
}

func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int, errno int) {
	var status int;
	r := libc_wait4(Pid_t(pid), &status, options, rusage);
	wpid = int(r);
	if r < 0 {
		errno = GetErrno();
	}
	if wstatus != nil {
		*wstatus = WaitStatus(status);
	}
	return;
}