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// 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 int, arg2 _C_long, arg3 _C_long, arg4 _C_long, arg5 _C_long) 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 0
        Noctty     bool        // Detach fd 0 from controlling terminal
        Pdeathsig  int         // 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.
        fd := append([]int(nil), attr.Files...)

        // 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, 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.
        nextfd = int(len(fd))
        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 {
                _, err1 = raw_ioctl(0, TIOCSCTTY, 0)
                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")
}