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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"
    )
    
    
    // SysProcIDMap holds Container ID to Host ID mappings used for User Namespaces in Linux.
    // See user_namespaces(7).
    type SysProcIDMap struct {
    	ContainerID int // Container ID.
    	HostID      int // Host ID.
    	Size        int // Size.
    }
    
    
    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)
    	Cloneflags  uintptr        // Flags for clone calls (Linux only)
    	UidMappings []SysProcIDMap // User ID mappings for user namespaces.
    	GidMappings []SysProcIDMap // Group ID mappings for user namespaces.
    
    // Implemented in runtime package.
    func runtime_BeforeFork()
    func runtime_AfterFork()
    
    
    // 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.
    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
    
    	// Record parent PID so child can test if it has died.
    	ppid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
    
    
    	// 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))
    
    	for i, ufd := range attr.Files {
    
    		fd[i] = int(ufd)
    	}
    
    	// Allocate another pipe for parent to child communication for
    	// synchronizing writing of User ID/Group ID mappings.
    	if sys.UidMappings != nil || sys.GidMappings != nil {
    		if err := forkExecPipe(p[:]); err != nil {
    			return 0, err.(Errno)
    		}
    	}
    
    
    	// About to call fork.
    	// No more allocation or calls of non-assembly functions.
    
    	runtime_BeforeFork()
    
    	r1, _, err1 = RawSyscall6(SYS_CLONE, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0, 0)
    
    		runtime_AfterFork()
    
    		return 0, err1
    	}
    
    	if r1 != 0 {
    		// parent; return PID
    
    		runtime_AfterFork()
    
    		if sys.UidMappings != nil || sys.GidMappings != nil {
    			Close(p[0])
    			err := writeUidGidMappings(pid, sys)
    			if err != nil {
    				err2 = err.(Errno)
    			}
    			RawSyscall(SYS_WRITE, uintptr(p[1]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
    			Close(p[1])
    		}
    
    
    	// Wait for User ID/Group ID mappings to be written.
    	if sys.UidMappings != nil || sys.GidMappings != nil {
    		if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(p[1]), 0, 0); err1 != 0 {
    			goto childerror
    		}
    		r1, _, err1 = RawSyscall(SYS_READ, uintptr(p[0]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
    		if err1 != 0 {
    			goto childerror
    		}
    		if r1 != unsafe.Sizeof(err2) {
    			err1 = EINVAL
    			goto childerror
    		}
    		if err2 != 0 {
    			err1 = err2
    			goto childerror
    		}
    	}
    
    
    	// 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)
    
    			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))
    
    		var groups unsafe.Pointer
    
    			groups = unsafe.Pointer(&cred.Groups[0])
    
    		_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, uintptr(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_dup, 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_dup, 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_dup, 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)
    	}
    }
    
    
    // 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
    }
    
    
    // writeIDMappings writes the user namespace User ID or Group ID mappings to the specified path.
    func writeIDMappings(path string, idMap []SysProcIDMap) error {
    	fd, err := Open(path, O_RDWR, 0)
    	if err != nil {
    		return err
    	}
    
    	data := ""
    	for _, im := range idMap {
    		data = data + itoa(im.ContainerID) + " " + itoa(im.HostID) + " " + itoa(im.Size) + "\n"
    	}
    
    	bytes, err := ByteSliceFromString(data)
    	if err != nil {
    		Close(fd)
    		return err
    	}
    
    	if _, err := Write(fd, bytes); err != nil {
    		Close(fd)
    		return err
    	}
    
    	if err := Close(fd); err != nil {
    		return err
    	}
    
    	return nil
    }
    
    // writeUidGidMappings writes User ID and Group ID mappings for user namespaces
    // for a process and it is called from the parent process.
    func writeUidGidMappings(pid int, sys *SysProcAttr) error {
    	if sys.UidMappings != nil {
    		uidf := "/proc/" + itoa(pid) + "/uid_map"
    		if err := writeIDMappings(uidf, sys.UidMappings); err != nil {
    			return err
    		}
    	}
    
    	if sys.GidMappings != nil {
    		gidf := "/proc/" + itoa(pid) + "/gid_map"
    		if err := writeIDMappings(gidf, sys.GidMappings); err != nil {
    			return err
    		}
    	}
    
    	return nil
    }