exec_libc.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 aix solaris

// This file handles forkAndExecInChild function for OS using libc syscall like AIX or Solaris.

package syscall

import (
	"unsafe"
)

type SysProcAttr struct {
	Chroot     string      // Chroot.
	Credential *Credential // Credential.
	Setsid     bool        // Create session.
	// Setpgid sets the process group ID of the child to Pgid,
	// or, if Pgid == 0, to the new child's process ID.
	Setpgid bool
	// Setctty sets the controlling terminal of the child to
	// file descriptor Ctty. Ctty must be a descriptor number
	// in the child process: an index into ProcAttr.Files.
	// This is only meaningful if Setsid is true.
	Setctty bool
	Noctty  bool // Detach fd 0 from controlling terminal
	Ctty    int  // Controlling TTY fd
	// Foreground places the child process group in the foreground.
	// This implies Setpgid. The Ctty field must be set to
	// the descriptor of the controlling TTY.
	// Unlike Setctty, in this case Ctty must be a descriptor
	// number in the parent process.
	Foreground bool
	Pgid       int // Child's process group ID if Setpgid.
}

// Implemented in runtime package.
func runtime_BeforeFork()
func runtime_AfterFork()
func runtime_AfterForkInChild()

func chdir(path uintptr) (err Errno)
func chroot1(path uintptr) (err Errno)
func close(fd uintptr) (err Errno)
func dup2child(old uintptr, new uintptr) (val uintptr, err Errno)
func execve(path uintptr, argv uintptr, envp uintptr) (err Errno)
func exit(code uintptr)
func fcntl1(fd uintptr, cmd uintptr, arg uintptr) (val uintptr, err Errno)
func forkx(flags uintptr) (pid uintptr, err Errno)
func getpid() (pid uintptr, err Errno)
func ioctl(fd uintptr, req uintptr, arg uintptr) (err Errno)
func setgid(gid uintptr) (err Errno)
func setgroups1(ngid uintptr, gid uintptr) (err Errno)
func setsid() (pid uintptr, err Errno)
func setuid(uid uintptr) (err Errno)
func setpgid(pid uintptr, pgid uintptr) (err Errno)
func write1(fd uintptr, buf uintptr, nbyte uintptr) (n uintptr, err Errno)

// syscall defines this global on our behalf to avoid a build dependency on other platforms
func init() {
	execveLibc = execve
}

// 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.
//
// We call hand-crafted syscalls, implemented in
// ../runtime/syscall_solaris.go, rather than generated libc wrappers
// because we need to avoid lazy-loading the functions (might malloc,
// split the stack, or acquire mutexes). We can't call RawSyscall
// because it's not safe even for BSD-subsystem calls.
//go:norace
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.
	runtime_BeforeFork()
	r1, err1 = forkx(0x1) // FORK_NOSIGCHLD
	if err1 != 0 {
		runtime_AfterFork()
		return 0, err1
	}

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

	// Fork succeeded, now in child.

	runtime_AfterForkInChild()

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

	// Set process group
	if sys.Setpgid || sys.Foreground {
		// Place child in process group.
		err1 = setpgid(0, uintptr(sys.Pgid))
		if err1 != 0 {
			goto childerror
		}
	}

	if sys.Foreground {
		pgrp := _Pid_t(sys.Pgid)
		if pgrp == 0 {
			r1, err1 = getpid()
			if err1 != 0 {
				goto childerror
			}

			pgrp = _Pid_t(r1)
		}

		// Place process group in foreground.
		err1 = ioctl(uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
		if err1 != 0 {
			goto childerror
		}
	}

	// Chroot
	if chroot != nil {
		err1 = chroot1(uintptr(unsafe.Pointer(chroot)))
		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]))
		}
		if !cred.NoSetGroups {
			err1 = setgroups1(ngroups, groups)
			if err1 != 0 {
				goto childerror
			}
		}
		err1 = setgid(uintptr(cred.Gid))
		if err1 != 0 {
			goto childerror
		}
		err1 = setuid(uintptr(cred.Uid))
		if err1 != 0 {
			goto childerror
		}
	}

	// Chdir
	if dir != nil {
		err1 = chdir(uintptr(unsafe.Pointer(dir)))
		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 = dup2child(uintptr(pipe), uintptr(nextfd))
		if err1 != 0 {
			goto childerror
		}
		fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
		pipe = nextfd
		nextfd++
	}
	for i = 0; i < len(fd); i++ {
		if fd[i] >= 0 && fd[i] < int(i) {
			if nextfd == pipe { // don't stomp on pipe
				nextfd++
			}
			_, err1 = dup2child(uintptr(fd[i]), uintptr(nextfd))
			if err1 != 0 {
				goto childerror
			}
			_, err1 = fcntl1(uintptr(nextfd), F_SETFD, FD_CLOEXEC)
			if err1 != 0 {
				goto childerror
			}
			fd[i] = nextfd
			nextfd++
		}
	}

	// Pass 2: dup fd[i] down onto i.
	for i = 0; i < len(fd); i++ {
		if fd[i] == -1 {
			close(uintptr(i))
			continue
		}
		if fd[i] == int(i) {
			// dup2(i, i) won't clear close-on-exec flag on Linux,
			// probably not elsewhere either.
			_, err1 = fcntl1(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 = dup2child(uintptr(fd[i]), uintptr(i))
		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++ {
		close(uintptr(i))
	}

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

	// Set the controlling TTY to Ctty
	if sys.Setctty {
		// On AIX, TIOCSCTTY is undefined
		if TIOCSCTTY == 0 {
			err1 = ENOSYS
			goto childerror
		}
		err1 = ioctl(uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
		if err1 != 0 {
			goto childerror
		}
	}

	// Time to exec.
	err1 = execve(
		uintptr(unsafe.Pointer(argv0)),
		uintptr(unsafe.Pointer(&argv[0])),
		uintptr(unsafe.Pointer(&envv[0])))

childerror:
	// send error code on pipe
	write1(uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
	for {
		exit(253)
	}
}