sftpd: add support for some SSH commands

md5sum, sha1sum are used by rclone.
cd, pwd improve the support for RemoteFiles mobile app.

These commands are all implemented inside SFTPGo so they work even
if the matching system commands are not available, for example on Windows

sftpd/ssh_cmd.go

@@ -0,0 +1,173 @@
+package sftpd
+
+import (
+	"crypto/md5"
+	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/sha512"
+	"fmt"
+	"hash"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"github.com/drakkan/sftpgo/logger"
+	"github.com/drakkan/sftpgo/utils"
+	"golang.org/x/crypto/ssh"
+)
+
+type sshCommand struct {
+	command    string
+	args       []string
+	connection Connection
+}
+
+func processSSHCommand(payload []byte, connection *Connection, channel ssh.Channel, enabledSSHCommands []string) bool {
+	var msg sshSubsystemExecMsg
+	if err := ssh.Unmarshal(payload, &msg); err == nil {
+		name, args, err := parseCommandPayload(msg.Command)
+		connection.Log(logger.LevelDebug, logSenderSSH, "new ssh command: %#v args: %v user: %v, error: %v",
+			name, args, connection.User.Username, err)
+		if err == nil && utils.IsStringInSlice(name, enabledSSHCommands) {
+			connection.command = fmt.Sprintf("%v %v", name, strings.Join(args, " "))
+			if name == "scp" && len(args) >= 2 {
+				connection.protocol = protocolSCP
+				connection.channel = channel
+				scpCommand := scpCommand{
+					sshCommand: sshCommand{
+						command:    name,
+						connection: *connection,
+						args:       args},
+				}
+				go scpCommand.handle()
+				return true
+			}
+			if name != "scp" {
+				connection.protocol = protocolSSH
+				connection.channel = channel
+				sshCommand := sshCommand{
+					command:    name,
+					connection: *connection,
+					args:       args,
+				}
+				go sshCommand.handle()
+				return true
+			}
+		} else {
+			connection.Log(logger.LevelInfo, logSenderSSH, "ssh command not enabled/supported: %#v", name)
+		}
+	}
+	return false
+}
+
+func (c *sshCommand) handle() error {
+	addConnection(c.connection)
+	defer removeConnection(c.connection)
+	updateConnectionActivity(c.connection.ID)
+	if utils.IsStringInSlice(c.command, sshHashCommands) {
+		var h hash.Hash
+		if c.command == "md5sum" {
+			h = md5.New()
+		} else if c.command == "sha1sum" {
+			h = sha1.New()
+		} else if c.command == "sha256sum" {
+			h = sha256.New()
+		} else if c.command == "sha384sum" {
+			h = sha512.New384()
+		} else {
+			h = sha512.New()
+		}
+		var response string
+		if len(c.args) == 0 {
+			// without args we need to read the string to hash from stdin
+			buf := make([]byte, 4096)
+			n, err := c.connection.channel.Read(buf)
+			if err != nil && err != io.EOF {
+				return c.sendErrorResponse(err)
+			}
+			h.Write(buf[:n])
+			response = fmt.Sprintf("%x  -\n", h.Sum(nil))
+		} else {
+			sshPath := c.getDestPath()
+			path, err := c.connection.buildPath(sshPath)
+			if err != nil {
+				return c.sendErrorResponse(err)
+			}
+			hash, err := computeHashForFile(h, path)
+			if err != nil {
+				return c.sendErrorResponse(err)
+			}
+			response = fmt.Sprintf("%v  %v\n", hash, sshPath)
+		}
+		c.connection.channel.Write([]byte(response))
+		c.sendExitStatus(nil)
+	} else if c.command == "cd" {
+		c.sendExitStatus(nil)
+	} else if c.command == "pwd" {
+		// hard coded response to "/"
+		c.connection.channel.Write([]byte("/\n"))
+		c.sendExitStatus(nil)
+	}
+	return nil
+}
+
+// for the supported command, the path, if any, is the last argument
+func (c *sshCommand) getDestPath() string {
+	if len(c.args) == 0 {
+		return ""
+	}
+	destPath := filepath.ToSlash(c.args[len(c.args)-1])
+	if !path.IsAbs(destPath) {
+		destPath = "/" + destPath
+	}
+	result := path.Clean(destPath)
+	if strings.HasSuffix(destPath, "/") && !strings.HasSuffix(result, "/") {
+		result += "/"
+	}
+	return result
+}
+
+func (c *sshCommand) sendErrorResponse(err error) error {
+	errorString := fmt.Sprintf("%v: %v %v\n", c.command, c.getDestPath(), err)
+	c.connection.channel.Write([]byte(errorString))
+	c.sendExitStatus(err)
+	return err
+}
+
+func (c *sshCommand) sendExitStatus(err error) {
+	status := uint32(0)
+	if err != nil {
+		status = uint32(1)
+	}
+	exitStatus := sshSubsystemExitStatus{
+		Status: status,
+	}
+	c.connection.Log(logger.LevelDebug, logSenderSSH, "send exit status for command %#v with args: %v user: %v err: %v",
+		c.command, c.args, c.connection.User.Username, err)
+	c.connection.channel.SendRequest("exit-status", false, ssh.Marshal(&exitStatus))
+	c.connection.channel.Close()
+}
+
+func computeHashForFile(hasher hash.Hash, path string) (string, error) {
+	hash := ""
+	f, err := os.Open(path)
+	if err != nil {
+		return hash, err
+	}
+	defer f.Close()
+	_, err = io.Copy(hasher, f)
+	if err == nil {
+		hash = fmt.Sprintf("%x", hasher.Sum(nil))
+	}
+	return hash, err
+}
+
+func parseCommandPayload(command string) (string, []string, error) {
+	parts := strings.Split(command, " ")
+	if len(parts) < 2 {
+		return parts[0], []string{}, nil
+	}
+	return parts[0], parts[1:], nil
+}