console.go

//go:build !windows

/*
   Copyright The containerd Authors.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package runc

import (
	"fmt"
	"net"
	"os"
	"path/filepath"

	"github.com/containerd/console"
	"golang.org/x/sys/unix"
)

// NewConsoleSocket creates a new unix socket at the provided path to accept a
// pty master created by runc for use by the container
func NewConsoleSocket(path string) (*Socket, error) {
	abs, err := filepath.Abs(path)
	if err != nil {
		return nil, err
	}
	addr, err := net.ResolveUnixAddr("unix", abs)
	if err != nil {
		return nil, err
	}
	l, err := net.ListenUnix("unix", addr)
	if err != nil {
		return nil, err
	}
	return &Socket{
		l: l,
	}, nil
}

// NewTempConsoleSocket returns a temp console socket for use with a container
// On Close(), the socket is deleted
func NewTempConsoleSocket() (*Socket, error) {
	runtimeDir := os.Getenv("XDG_RUNTIME_DIR")
	dir, err := os.MkdirTemp(runtimeDir, "pty")
	if err != nil {
		return nil, err
	}
	abs, err := filepath.Abs(filepath.Join(dir, "pty.sock"))
	if err != nil {
		return nil, err
	}
	addr, err := net.ResolveUnixAddr("unix", abs)
	if err != nil {
		return nil, err
	}
	l, err := net.ListenUnix("unix", addr)
	if err != nil {
		return nil, err
	}
	if runtimeDir != "" {
		if err := os.Chmod(abs, 0o755|os.ModeSticky); err != nil {
			return nil, err
		}
	}
	return &Socket{
		l:     l,
		rmdir: true,
	}, nil
}

// Socket is a unix socket that accepts the pty master created by runc
type Socket struct {
	rmdir bool
	l     *net.UnixListener
}

// Path returns the path to the unix socket on disk
func (c *Socket) Path() string {
	return c.l.Addr().String()
}

// recvFd waits for a file descriptor to be sent over the given AF_UNIX
// socket. The file name of the remote file descriptor will be recreated
// locally (it is sent as non-auxiliary data in the same payload).
func recvFd(socket *net.UnixConn) (*os.File, error) {
	const MaxNameLen = 4096
	oobSpace := unix.CmsgSpace(4)

	name := make([]byte, MaxNameLen)
	oob := make([]byte, oobSpace)

	n, oobn, _, _, err := socket.ReadMsgUnix(name, oob)
	if err != nil {
		return nil, err
	}

	if n >= MaxNameLen || oobn != oobSpace {
		return nil, fmt.Errorf("recvfd: incorrect number of bytes read (n=%d oobn=%d)", n, oobn)
	}

	// Truncate.
	name = name[:n]
	oob = oob[:oobn]

	scms, err := unix.ParseSocketControlMessage(oob)
	if err != nil {
		return nil, err
	}
	if len(scms) != 1 {
		return nil, fmt.Errorf("recvfd: number of SCMs is not 1: %d", len(scms))
	}
	scm := scms[0]

	fds, err := unix.ParseUnixRights(&scm)
	if err != nil {
		return nil, err
	}
	if len(fds) != 1 {
		return nil, fmt.Errorf("recvfd: number of fds is not 1: %d", len(fds))
	}
	fd := uintptr(fds[0])

	return os.NewFile(fd, string(name)), nil
}

// ReceiveMaster blocks until the socket receives the pty master
func (c *Socket) ReceiveMaster() (console.Console, error) {
	conn, err := c.l.Accept()
	if err != nil {
		return nil, err
	}
	defer conn.Close()
	uc, ok := conn.(*net.UnixConn)
	if !ok {
		return nil, fmt.Errorf("received connection which was not a unix socket")
	}
	f, err := recvFd(uc)
	if err != nil {
		return nil, err
	}
	return console.ConsoleFromFile(f)
}

// Close closes the unix socket
func (c *Socket) Close() error {
	err := c.l.Close()
	if c.rmdir {
		if rerr := os.RemoveAll(filepath.Dir(c.Path())); err == nil {
			err = rerr
		}
	}
	return err
}