exec_linux.go

// Copyright 2011 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 linux

package syscall

import (
	"unsafe"
)

type SysProcAttr struct {
	Chroot     string      // Chroot.
	Credential *Credential // Credential.
	Ptrace     bool        // Enable tracing.
	Setsid     bool        // Create session.
	Setpgid    bool        // Set process group ID to new pid (SYSV setpgrp)
	Setctty    bool        // Set controlling terminal to fd Ctty (only meaningful if Setsid is set)
	Noctty     bool        // Detach fd 0 from controlling terminal
	Ctty       int         // Controlling TTY fd (Linux only)
	Pdeathsig  Signal      // Signal that the process will get when its parent dies (Linux only)
}

// Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
// If a dup or exec fails, write the errno error 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, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
	// Declare all variables at top in case any
	// declarations require heap allocation (e.g., err1).
	var (
		r1     uintptr
		err1   Errno
		nextfd int
		i      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++

	// About to call fork.
	// No more allocation or calls of non-assembly functions.
	r1, _, err1 = RawSyscall(SYS_FORK, 0, 0, 0)
	if err1 != 0 {
		return 0, err1
	}

	if r1 != 0 {
		// parent; return PID
		return int(r1), 0
	}

	// Fork succeeded, now in child.

	// Parent death signal
	if sys.Pdeathsig != 0 {
		_, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_PDEATHSIG, uintptr(sys.Pdeathsig), 0, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}

		// Signal self if parent is already dead. This might cause a
		// duplicate signal in rare cases, but it won't matter when
		// using SIGKILL.
		r1, _, _ = RawSyscall(SYS_GETPPID, 0, 0, 0)
		if r1 == 1 {
			pid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
			_, _, err1 := RawSyscall(SYS_KILL, pid, uintptr(sys.Pdeathsig), 0)
			if err1 != 0 {
				goto childerror
			}
		}
	}

	// Enable tracing if requested.
	if sys.Ptrace {
		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Session ID
	if sys.Setsid {
		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Set process group
	if sys.Setpgid {
		_, _, err1 = RawSyscall(SYS_SETPGID, 0, 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Chroot
	if chroot != nil {
		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// User and groups
	if cred := sys.Credential; cred != nil {
		ngroups := uintptr(len(cred.Groups))
		groups := uintptr(0)
		if ngroups > 0 {
			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
		}
		_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, groups, 0)
		if err1 != 0 {
			goto childerror
		}
		_, _, err1 = RawSyscall(SYS_SETGID, uintptr(cred.Gid), 0, 0)
		if err1 != 0 {
			goto childerror
		}
		_, _, err1 = RawSyscall(SYS_SETUID, uintptr(cred.Uid), 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.
	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)
	}

	// Detach fd 0 from tty
	if sys.Noctty {
		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCNOTTY), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Set the controlling TTY to Ctty
	if sys.Setctty && sys.Ctty >= 0 {
		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// 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)), 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")
}

// Try to open a pipe with O_CLOEXEC set on both file descriptors.
func forkExecPipe(p []int) (err error) {
	err = Pipe2(p, O_CLOEXEC)
	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
	// might not be implemented.
	if err == ENOSYS {
		if err = Pipe(p); err != nil {
			return
		}
		if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
			return
		}
		_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
	}
	return
}