main: add "-o" option to enable "suid" and "dev"

Device files and suid binaries are often not needed when running
gocryptfs as root. As they are potentially dangerous, let the
user enable them explicitely via the new "-o" option instead of
always enabling them when running as root.

Documentation/MANPAGE.md

@@ -85,6 +85,15 @@ continue be printed to stdout and stderr.
 :	Send USR1 to the specified process after successful mount. This is
 used internally for daemonization.
 
+**-o**
+: Pass additonal mount options to the kernel (comma-separated list).
+FUSE filesystems are mounted with "nodev,nosuid" by default. If gocryptfs
+runs as root, you can enable device files by passing the opposite mount option,
+"dev", and if you want to enable suid-binaries, pass "suid".
+"ro" (equivalent to passing the "-ro" option) and "noexec" may also be
+interesting. For a complete liste see the section
+`FILESYSTEM-INDEPENDENT MOUNT OPTIONS` in mount(8).
+
 **-openssl bool**
 :	Use OpenSSL instead of built-in Go crypto (default "auto"). Using
 built-in crypto is 4x slower unless your CPU has AES instructions and

main.go

@@ -45,7 +45,7 @@ type argContainer struct {
 	plaintextnames, quiet, nosyslog, wpanic,
 	longnames, allow_other, ro bool
 	masterkey, mountpoint, cipherdir, cpuprofile, config, extpass,
-	memprofile string
+	memprofile, o string
 	notifypid, scryptn int
 }
 
@@ -185,6 +185,7 @@ func main() {
 	flagSet.StringVar(&args.memprofile, "memprofile", "", "Write memory profile to specified file")
 	flagSet.StringVar(&args.config, "config", "", "Use specified config file instead of CIPHERDIR/gocryptfs.conf")
 	flagSet.StringVar(&args.extpass, "extpass", "", "Use external program for the password prompt")
+	flagSet.StringVar(&args.o, "o", "", "Pass additional mount options to the kernel, comma-separated list")
 	flagSet.IntVar(&args.notifypid, "notifypid", 0, "Send USR1 to the specified process after "+
 		"successful mount - used internally for daemonization")
 	flagSet.IntVar(&args.scryptn, "scryptn", configfile.ScryptDefaultLogN, "scrypt cost parameter logN. "+
@@ -402,13 +403,6 @@ func initFuseFrontend(key []byte, args argContainer, confFile *configfile.ConfFi
 		// Make the kernel check the file permissions for us
 		mOpts.Options = append(mOpts.Options, "default_permissions")
 	}
-	if os.Getuid() == 0 {
-		// FUSE filesystems are mounted with "nodev" by default. If we run as root,
-		// we can use device files by passing the opposite mount option, "dev".
-		mOpts.Options = append(mOpts.Options, "dev")
-		// Same thing for "nosuid". If we run as root, we can pass "suid".
-		mOpts.Options = append(mOpts.Options, "suid")
-	}
 	// Set values shown in "df -T" and friends
 	// First column, "Filesystem"
 	mOpts.Options = append(mOpts.Options, "fsname="+args.cipherdir)
@@ -419,7 +413,13 @@ func initFuseFrontend(key []byte, args argContainer, confFile *configfile.ConfFi
 	if args.ro {
 		mOpts.Options = append(mOpts.Options, "ro")
 	}
-
+	// Add additional mount options (if any) after the stock ones, so the user has
+	// a chance to override them.
+	if args.o != "" {
+		parts := strings.Split(args.o, ",")
+		tlog.Debug.Printf("Adding -o mount options: %v", parts)
+		mOpts.Options = append(mOpts.Options, parts...)
+	}
 	srv, err := fuse.NewServer(conn.RawFS(), args.mountpoint, &mOpts)
 	if err != nil {
 		tlog.Fatal.Printf("Mount failed: %v", err)