text: // 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"
  )
  
  //sysnb	raw_prctl(option int, arg2 int, arg3 int, arg4 int, arg5 int) (ret int, err Errno)
  //prctl(option _C_int, arg2 _C_long, arg3 _C_long, arg4 _C_long, arg5 _C_long) _C_int
  
  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     Pid_t
  		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 = raw_fork()
  	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 = raw_prctl(PR_SET_PDEATHSIG, int(sys.Pdeathsig), 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.
  		ppid := Getppid()
  		if ppid == 1 {
  			pid = Getpid()
  			err2 := Kill(pid, sys.Pdeathsig)
  			if err2 != nil {
  				err1 = err2.(Errno)
  				goto childerror
  			}
  		}
  	}
  
  	// Enable tracing if requested.
  	if sys.Ptrace {
  		err1 = raw_ptrace(_PTRACE_TRACEME, 0, nil, nil)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// Session ID
  	if sys.Setsid {
  		err1 = raw_setsid()
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// Set process group
  	if sys.Setpgid {
  		err1 = raw_setpgid(0, 0)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// Chroot
  	if chroot != nil {
  		err1 = raw_chroot(chroot)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// User and groups
  	if cred := sys.Credential; cred != nil {
  		ngroups := len(cred.Groups)
  		if ngroups == 0 {
  			err2 := setgroups(0, nil)
  			if err2 == nil {
  				err1 = 0
  			} else {
  				err1 = err2.(Errno)
  			}
  		} else {
  			groups := make([]Gid_t, ngroups)
  			for i, v := range cred.Groups {
  				groups[i] = Gid_t(v)
  			}
  			err2 := setgroups(ngroups, &groups[0])
  			if err2 == nil {
  				err1 = 0
  			} else {
  				err1 = err2.(Errno)
  			}
  		}
  		if err1 != 0 {
  			goto childerror
  		}
  		err2 := Setgid(int(cred.Gid))
  		if err2 != nil {
  			err1 = err2.(Errno)
  			goto childerror
  		}
  		err2 = Setuid(int(cred.Uid))
  		if err2 != nil {
  			err1 = err2.(Errno)
  			goto childerror
  		}
  	}
  
  	// Chdir
  	if dir != nil {
  		err1 = raw_chdir(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 = raw_dup2(pipe, nextfd)
  		if err1 != 0 {
  			goto childerror
  		}
  		raw_fcntl(nextfd, F_SETFD, FD_CLOEXEC)
  		pipe = nextfd
  		nextfd++
  	}
  	for i = 0; i < len(fd); i++ {
  		if fd[i] >= 0 && fd[i] < int(i) {
  			err1 = raw_dup2(fd[i], nextfd)
  			if err1 != 0 {
  				goto childerror
  			}
  			raw_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 {
  			raw_close(i)
  			continue
  		}
  		if fd[i] == int(i) {
  			// dup2(i, i) won't clear close-on-exec flag on Linux,
  			// probably not elsewhere either.
  			_, err1 = raw_fcntl(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 = raw_dup2(fd[i], 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++ {
  		raw_close(i)
  	}
  
  	// Detach fd 0 from tty
  	if sys.Noctty {
  		_, err1 = raw_ioctl(0, TIOCNOTTY, 0)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// Make fd 0 the tty
  	if sys.Setctty && sys.Ctty >= 0 {
  		_, err1 = raw_ioctl(0, TIOCSCTTY, sys.Ctty)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	// Time to exec.
  	err1 = raw_execve(argv0, &argv[0], &envv[0])
  
  childerror:
  	// send error code on pipe
  	raw_write(pipe, (*byte)(unsafe.Pointer(&err1)), int(unsafe.Sizeof(err1)))
  	for {
  		raw_exit(253)
  	}
  
  	// 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
  }
  