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Source file src/syscall/exec_bsd.go

Documentation: syscall

  // 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 darwin dragonfly freebsd netbsd openbsd
  
  package syscall
  
  import (
  	"runtime"
  	"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 Pgid, or, if Pgid == 0, to new pid.
  	Setctty    bool        // Set controlling terminal to fd Ctty
  	Noctty     bool        // Detach fd 0 from controlling terminal
  	Ctty       int         // Controlling TTY fd
  	Foreground bool        // Place child's process group in foreground. (Implies Setpgid. Uses Ctty as fd of controlling TTY)
  	Pgid       int         // Child's process group ID if Setpgid.
  }
  
  // Implemented in runtime package.
  func runtime_BeforeFork()
  func runtime_AfterFork()
  func runtime_AfterForkInChild()
  
  // 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.
  // For the same reason compiler does not race instrument it.
  // The calls to RawSyscall are okay because they are assembly
  // functions that do not grow the stack.
  //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, r2 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++
  
  	darwin := runtime.GOOS == "darwin"
  
  	// About to call fork.
  	// No more allocation or calls of non-assembly functions.
  	runtime_BeforeFork()
  	r1, r2, err1 = RawSyscall(SYS_FORK, 0, 0, 0)
  	if err1 != 0 {
  		runtime_AfterFork()
  		return 0, err1
  	}
  
  	// On Darwin:
  	//	r1 = child pid in both parent and child.
  	//	r2 = 0 in parent, 1 in child.
  	// Convert to normal Unix r1 = 0 in child.
  	if darwin && r2 == 1 {
  		r1 = 0
  	}
  
  	if r1 != 0 {
  		// parent; return PID
  		runtime_AfterFork()
  		return int(r1), 0
  	}
  
  	// Fork succeeded, now in child.
  
  	runtime_AfterForkInChild()
  
  	// 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 || sys.Foreground {
  		// Place child in process group.
  		_, _, err1 = RawSyscall(SYS_SETPGID, 0, uintptr(sys.Pgid), 0)
  		if err1 != 0 {
  			goto childerror
  		}
  	}
  
  	if sys.Foreground {
  		pgrp := sys.Pgid
  		if pgrp == 0 {
  			r1, _, err1 = RawSyscall(SYS_GETPID, 0, 0, 0)
  			if err1 != 0 {
  				goto childerror
  			}
  
  			pgrp = int(r1)
  		}
  
  		// Place process group in foreground.
  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
  		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]))
  		}
  		if !cred.NoSetGroups {
  			_, _, 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) {
  			if nextfd == pipe { // don't stomp on pipe
  				nextfd++
  			}
  			_, _, 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++
  		}
  	}
  
  	// 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 {
  		_, _, 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)
  	}
  }
  

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