Subversion Repositories openrisc

// 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.

// +build darwin freebsd linux netbsd openbsd

// Fork, exec, wait, etc.

package syscall

import (
        "runtime"
        "sync"
        "unsafe"
)

//sysnb raw_fork() (pid Pid_t, err Errno)
//fork() Pid_t

//sysnb raw_setsid() (err Errno)
//setsid() Pid_t

//sysnb raw_setpgid(pid int, pgid int) (err Errno)
//setpgid(pid Pid_t, pgid Pid_t) int

//sysnb raw_chroot(path *byte) (err Errno)
//chroot(path *byte) int

//sysnb raw_chdir(path *byte) (err Errno)
//chdir(path *byte) int

//sysnb raw_fcntl(fd int, cmd int, arg int) (val int, err Errno)
//fcntl(fd int, cmd int, arg int) int

//sysnb raw_close(fd int) (err Errno)
//close(fd int) int

//sysnb raw_ioctl(fd int, cmd int, val int) (rval int, err Errno)
//ioctl(fd int, cmd int, val int) int

//sysnb raw_execve(argv0 *byte, argv **byte, envv **byte) (err Errno)
//execve(argv0 *byte, argv **byte, envv **byte) int

//sysnb raw_write(fd int, buf *byte, count int) (err Errno)
//write(fd int, buf *byte, count Size_t) Ssize_t

//sysnb raw_exit(status int)
//_exit(status int)

//sysnb raw_dup2(oldfd int, newfd int) (err Errno)
//dup2(oldfd int, newfd int) int

// Note: not raw, returns error rather than Errno.
//sys   read(fd int, p *byte, np int) (n int, err error)
//read(fd int, buf *byte, count Size_t) Ssize_t

// 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 (GNU/Linux).
//             Otherwise, live with the race.
// 5) Dup.     Does not block.  Use the ForkLock.
//             On GNU/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
}

func CloseOnExec(fd int) { fcntl(fd, F_SETFD, FD_CLOEXEC) }

func SetNonblock(fd int, nonblocking bool) (err error) {
        flag, err := fcntl(fd, F_GETFL, 0)
        if err != nil {
                return err
        }
        if nonblocking {
                flag |= O_NONBLOCK
        } else {
                flag &= ^O_NONBLOCK
        }
        _, err = fcntl(fd, F_SETFL, flag)
        return err
}

// Credential holds user and group identities to be assumed
// by a child process started by StartProcess.
type Credential struct {
        Uid    uint32   // User ID.
        Gid    uint32   // Group ID.
        Groups []uint32 // Supplementary group IDs.
}

// ProcAttr holds attributes that will be applied to a new process started
// by StartProcess.
type ProcAttr struct {
        Dir   string   // Current working directory.
        Env   []string // Environment.
        Files []int    // File descriptors.
        Sys   *SysProcAttr
}

var zeroProcAttr ProcAttr
var zeroSysProcAttr SysProcAttr

func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
        var p [2]int
        var n int
        var err1 Errno
        var wstatus WaitStatus

        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)
        envvp := StringSlicePtr(attr.Env)

        if runtime.GOOS == "freebsd" && len(argv[0]) > len(argv0) {
                argvp[0] = argv0p
        }

        var chroot *byte
        if sys.Chroot != "" {
                chroot = StringBytePtr(sys.Chroot)
        }
        var dir *byte
        if attr.Dir != "" {
                dir = StringBytePtr(attr.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 != nil {
                goto error
        }
        if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
                goto error
        }
        if _, err = fcntl(p[1], F_SETFD, FD_CLOEXEC); err != nil {
                goto error
        }

        // Kick off child.
        pid, err1 = forkAndExecInChild(argv0p, argvp, envvp, chroot, dir, attr, sys, p[1])
        if err1 != 0 {
                goto error
        }
        ForkLock.Unlock()

        // Read child error status from pipe.
        Close(p[1])
        n, err = read(p[0], (*byte)(unsafe.Pointer(&err1)), int(unsafe.Sizeof(err1)))
        Close(p[0])
        if err != nil || n != 0 {
                if n == int(unsafe.Sizeof(err1)) {
                        err = Errno(err1)
                }
                if err == nil {
                        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, nil

error:
        if p[0] >= 0 {
                Close(p[0])
                Close(p[1])
        }
        ForkLock.Unlock()
        return 0, err
}

// 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 int, handle uintptr, err error) {
        pid, err = forkExec(argv0, argv, attr)
        return pid, 0, err
}

// Ordinary exec.
func Exec(argv0 string, argv []string, envv []string) (err error) {
        err1 := raw_execve(StringBytePtr(argv0),
                &StringSlicePtr(argv)[0],
                &StringSlicePtr(envv)[0])
        return Errno(err1)
}