feat: allocate real pty

This adds a new PtyHandler to handle allocating PTYs and storing them in
a platform specific field in `Pty`. This PR is backward-compatible,
it defaults to EmulatePty handler that sets the `emulatePty` field in
context and uses `PtyWriter` to preserve the current behavor.

Author: aymanbagabas

context.go

@@ -56,6 +56,10 @@ var (
 	// ContextKeyPublicKey is a context key for use with Contexts in this package.
 	// The associated value will be of type PublicKey.
 	ContextKeyPublicKey = &contextKey{"public-key"}
+
+	// ContextKeySession is a context key for use with Contexts in this package.
+	// The associated value will be of type Session.
+	ContextKeySession = &contextKey{"session"}
 )
 
 // Context is a package specific context interface. It exposes connection

go.mod

@@ -4,7 +4,9 @@ go 1.17
 
 require (
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
+	github.com/creack/pty v1.1.21
+	github.com/u-root/u-root v0.11.0
 	golang.org/x/crypto v0.17.0
 )
 
-require golang.org/x/sys v0.15.0 // indirect
+require golang.org/x/sys v0.16.0

go.sum

@@ -68,7 +68,7 @@ func HostKeyPEM(bytes []byte) Option {
 // denying PTY requests.
 func NoPty() Option {
 	return func(srv *Server) error {
-		srv.PtyCallback = func(ctx Context, pty Pty) bool {
+		srv.PtyCallback = func(Context, Pty) bool {
 			return false
 		}
 		return nil
@@ -82,3 +82,22 @@ func WrapConn(fn ConnCallback) Option {
 		return nil
 	}
 }
+
+var contextKeyEmulatePty = &contextKey{"emulate-pty"}
+
+// EmulatePty returns true if the session is set to emulate a PTY.
+func EmulatePty(ctx Context, _ Session, _ Pty) (func() error, error) {
+	ctx.SetValue(contextKeyEmulatePty, true)
+	return func() error { return nil }, nil
+}
+
+// AllocatePty returns a functional option that sets PtyCallback on the server
+// to allocate a PTY for sessions that request it.
+func AllocatePty(_ Context, s Session, pty Pty) (func() error, error) {
+	sess, ok := s.(*session)
+	if !ok {
+		return nil, nil
+	}
+
+	return sess.ptyAllocate(pty.Term, pty.Window, pty.Modes)
+}

options.go

@@ -2,9 +2,13 @@ package ssh
 
 import (
 	"bytes"
+	"errors"
 	"io"
 )
 
+// ErrUnsupported is returned when the platform does not support PTY.
+var ErrUnsupported = errors.New("pty unsupported")
+
 // NewPtyWriter creates a writer that handles when the session has a active
 // PTY, replacing the \n with \r\n.
 func NewPtyWriter(w io.Writer) io.Writer {

pty.go

@@ -0,0 +1,31 @@
+//go:build !linux && !darwin && !freebsd && !dragonfly && !netbsd && !openbsd && !solaris
+// +build !linux,!darwin,!freebsd,!dragonfly,!netbsd,!openbsd,!solaris
+
+// TODO: support Windows
+package ssh
+
+import (
+	"golang.org/x/crypto/ssh"
+)
+
+type impl struct{}
+
+func (i *impl) Read(p []byte) (n int, err error) {
+	return 0, ErrUnsupported
+}
+
+func (i *impl) Write(p []byte) (n int, err error) {
+	return 0, ErrUnsupported
+}
+
+func (i *impl) Resize(w int, h int) error {
+	return ErrUnsupported
+}
+
+func (i *impl) Close() error {
+	return nil
+}
+
+func newPty(Context, string, Window, ssh.TerminalModes) (impl, error) {
+	return impl{}, ErrUnsupported
+}

pty_other.go

@@ -0,0 +1,181 @@
+//go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package ssh
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/creack/pty"
+	"github.com/u-root/u-root/pkg/termios"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/sys/unix"
+)
+
+type impl struct {
+	// Master is the master PTY file descriptor.
+	Master *os.File
+
+	// Slave is the slave PTY file descriptor.
+	Slave *os.File
+
+	// Name is the name of the slave PTY.
+	Name string
+}
+
+// Read implements ptyInterface.
+func (i *impl) Read(p []byte) (n int, err error) {
+	return i.Master.Read(p)
+}
+
+// Write implements ptyInterface.
+func (i *impl) Write(p []byte) (n int, err error) {
+	return i.Master.Write(p)
+}
+
+func (i *impl) Close() error {
+	if err := i.Master.Close(); err != nil {
+		return err
+	}
+	return i.Slave.Close()
+}
+
+func (i *impl) Resize(w int, h int) (rErr error) {
+	conn, err := i.Master.SyscallConn()
+	if err != nil {
+		return err
+	}
+
+	return conn.Control(func(fd uintptr) {
+		rErr = termios.SetWinSize(fd, &termios.Winsize{
+			Winsize: unix.Winsize{
+				Row: uint16(h),
+				Col: uint16(w),
+			},
+		})
+	})
+}
+
+func newPty(_ Context, _ string, win Window, modes ssh.TerminalModes) (_ impl, rErr error) {
+	ptm, pts, err := pty.Open()
+	if err != nil {
+		return impl{}, err
+	}
+
+	conn, err := ptm.SyscallConn()
+	if err != nil {
+		return impl{}, err
+	}
+
+	if err := conn.Control(func(fd uintptr) {
+		rErr = applyTerminalModesToFd(fd, win.Width, win.Height, modes)
+	}); err != nil {
+		return impl{}, err
+	}
+
+	return impl{Master: ptm, Slave: pts, Name: pts.Name()}, rErr
+}
+
+func applyTerminalModesToFd(fd uintptr, width int, height int, modes ssh.TerminalModes) error {
+	// Get the current TTY configuration.
+	tios, err := termios.GTTY(int(fd))
+	if err != nil {
+		return fmt.Errorf("GTTY: %w", err)
+	}
+
+	// Apply the modes from the SSH request.
+	tios.Row = height
+	tios.Col = width
+
+	for c, v := range modes {
+		if c == ssh.TTY_OP_ISPEED {
+			tios.Ispeed = int(v)
+			continue
+		}
+		if c == ssh.TTY_OP_OSPEED {
+			tios.Ospeed = int(v)
+			continue
+		}
+		k, ok := terminalModeFlagNames[c]
+		if !ok {
+			continue
+		}
+		if _, ok := tios.CC[k]; ok {
+			tios.CC[k] = uint8(v)
+			continue
+		}
+		if _, ok := tios.Opts[k]; ok {
+			tios.Opts[k] = v > 0
+			continue
+		}
+	}
+
+	// Save the new TTY configuration.
+	if _, err := tios.STTY(int(fd)); err != nil {
+		return fmt.Errorf("STTY: %w", err)
+	}
+
+	return nil
+}
+
+// terminalModeFlagNames maps the SSH terminal mode flags to mnemonic
+// names used by the termios package.
+var terminalModeFlagNames = map[uint8]string{
+	ssh.VINTR:         "intr",
+	ssh.VQUIT:         "quit",
+	ssh.VERASE:        "erase",
+	ssh.VKILL:         "kill",
+	ssh.VEOF:          "eof",
+	ssh.VEOL:          "eol",
+	ssh.VEOL2:         "eol2",
+	ssh.VSTART:        "start",
+	ssh.VSTOP:         "stop",
+	ssh.VSUSP:         "susp",
+	ssh.VDSUSP:        "dsusp",
+	ssh.VREPRINT:      "rprnt",
+	ssh.VWERASE:       "werase",
+	ssh.VLNEXT:        "lnext",
+	ssh.VFLUSH:        "flush",
+	ssh.VSWTCH:        "swtch",
+	ssh.VSTATUS:       "status",
+	ssh.VDISCARD:      "discard",
+	ssh.IGNPAR:        "ignpar",
+	ssh.PARMRK:        "parmrk",
+	ssh.INPCK:         "inpck",
+	ssh.ISTRIP:        "istrip",
+	ssh.INLCR:         "inlcr",
+	ssh.IGNCR:         "igncr",
+	ssh.ICRNL:         "icrnl",
+	ssh.IUCLC:         "iuclc",
+	ssh.IXON:          "ixon",
+	ssh.IXANY:         "ixany",
+	ssh.IXOFF:         "ixoff",
+	ssh.IMAXBEL:       "imaxbel",
+	ssh.IUTF8:         "iutf8",
+	ssh.ISIG:          "isig",
+	ssh.ICANON:        "icanon",
+	ssh.XCASE:         "xcase",
+	ssh.ECHO:          "echo",
+	ssh.ECHOE:         "echoe",
+	ssh.ECHOK:         "echok",
+	ssh.ECHONL:        "echonl",
+	ssh.NOFLSH:        "noflsh",
+	ssh.TOSTOP:        "tostop",
+	ssh.IEXTEN:        "iexten",
+	ssh.ECHOCTL:       "echoctl",
+	ssh.ECHOKE:        "echoke",
+	ssh.PENDIN:        "pendin",
+	ssh.OPOST:         "opost",
+	ssh.OLCUC:         "olcuc",
+	ssh.ONLCR:         "onlcr",
+	ssh.OCRNL:         "ocrnl",
+	ssh.ONOCR:         "onocr",
+	ssh.ONLRET:        "onlret",
+	ssh.CS7:           "cs7",
+	ssh.CS8:           "cs8",
+	ssh.PARENB:        "parenb",
+	ssh.PARODD:        "parodd",
+	ssh.TTY_OP_ISPEED: "tty_op_ispeed",
+	ssh.TTY_OP_OSPEED: "tty_op_ospeed",
+}

pty_unix.go

@@ -43,7 +43,8 @@ type Server struct {
 	KeyboardInteractiveHandler    KeyboardInteractiveHandler    // keyboard-interactive authentication handler
 	PasswordHandler               PasswordHandler               // password authentication handler
 	PublicKeyHandler              PublicKeyHandler              // public key authentication handler
-	PtyCallback                   PtyCallback                   // callback for allowing PTY sessions, allows all if nil
+	PtyCallback                   PtyCallback                   // callback for allocating and allowing PTY sessions, ssh.EmulatePtyCallback if nil
+	PtyHandler                    PtyHandler                    // pty allocation handler, ssh.EmulatePty if nil
 	ConnCallback                  ConnCallback                  // optional callback for wrapping net.Conn before handling
 	LocalPortForwardingCallback   LocalPortForwardingCallback   // callback for allowing local port forwarding, denies all if nil
 	ReversePortForwardingCallback ReversePortForwardingCallback // callback for allowing reverse port forwarding, denies all if nil
@@ -131,6 +132,9 @@ func (srv *Server) config(ctx Context) *gossh.ServerConfig {
 	if srv.PasswordHandler == nil && srv.PublicKeyHandler == nil && srv.KeyboardInteractiveHandler == nil {
 		config.NoClientAuth = true
 	}
+	if srv.PtyHandler == nil {
+		srv.PtyHandler = EmulatePty
+	}
 	if srv.Version != "" {
 		config.ServerVersion = "SSH-2.0-" + srv.Version
 	}
@@ -304,7 +308,7 @@ func (srv *Server) HandleConn(newConn net.Conn) {
 
 	ctx.SetValue(ContextKeyConn, sshConn)
 	applyConnMetadata(ctx, sshConn)
-	//go gossh.DiscardRequests(reqs)
+	// go gossh.DiscardRequests(reqs)
 	go srv.handleRequests(ctx, reqs)
 	for ch := range chans {
 		handler := srv.ChannelHandlers[ch.ChannelType()]
@@ -381,8 +385,8 @@ func (srv *Server) SetOption(option Option) error {
 	// internal method. We can't actually lock here because if something calls
 	// (as an example) AddHostKey, it will deadlock.
 
-	//srv.mu.Lock()
-	//defer srv.mu.Unlock()
+	// srv.mu.Lock()
+	// defer srv.mu.Unlock()
 
 	return option(srv)
 }