configfile: add LongNameMax support

Feature flag + numeric paramater

https://github.com/rfjakob/gocryptfs/issues/499

internal/configfile/config_file.go

@@ -55,6 +55,8 @@ type ConfFile struct {
 	FeatureFlags []string
 	// FIDO2 parameters
 	FIDO2 *FIDO2Params `json:",omitempty"`
+	// LongNameMax corresponds to the -longnamemax flag
+	LongNameMax uint8 `json:",omitempty"`
 	// Filename is the name of the config file. Not exported to JSON.
 	filename string
 }
@@ -71,6 +73,7 @@ type CreateArgs struct {
 	Fido2HmacSalt      []byte
 	DeterministicNames bool
 	XChaCha20Poly1305  bool
+	LongNameMax        uint8
 }
 
 // Create - create a new config with a random key encrypted with
@@ -97,6 +100,12 @@ func Create(args *CreateArgs) error {
 		if !args.DeterministicNames {
 			cf.setFeatureFlag(FlagDirIV)
 		}
+		// 0 means to *use* the default (which means we don't have to save it), and
+		// 255 *is* the default, which means we don't have to save it either.
+		if args.LongNameMax != 0 && args.LongNameMax != 255 {
+			cf.LongNameMax = args.LongNameMax
+			cf.setFeatureFlag(FlagLongNameMax)
+		}
 		cf.setFeatureFlag(FlagEMENames)
 		cf.setFeatureFlag(FlagLongNames)
 		cf.setFeatureFlag(FlagRaw64)

internal/configfile/config_test.go

@@ -131,6 +131,30 @@ func TestCreateConfFileAESSIV(t *testing.T) {
 	}
 }
 
+func TestCreateConfLongNameMax(t *testing.T) {
+	args := &CreateArgs{
+		Filename:    "config_test/tmp.conf",
+		Password:    testPw,
+		LogN:        10,
+		Creator:     "test",
+		LongNameMax: 100,
+	}
+	err := Create(args)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, c, err := LoadAndDecrypt("config_test/tmp.conf", testPw)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !c.IsFeatureFlagSet(FlagLongNameMax) {
+		t.Error("FlagLongNameMax should be set but is not")
+	}
+	if c.LongNameMax != args.LongNameMax {
+		t.Errorf("wrong LongNameMax value: want=%d have=%d", args.LongNameMax, c.LongNameMax)
+	}
+}
+
 func TestIsFeatureFlagKnown(t *testing.T) {
 	// Test a few hardcoded values
 	testKnownFlags := []string{"DirIV", "PlaintextNames", "EMENames", "GCMIV128", "LongNames", "AESSIV"}

internal/configfile/feature_flags.go

@@ -16,6 +16,9 @@ const (
 	FlagGCMIV128
 	// FlagLongNames allows file names longer than 176 bytes.
 	FlagLongNames
+	// FlagLongNameMax sets a custom name length limit, names longer than that
+	// will be hashed.
+	FlagLongNameMax
 	// FlagAESSIV selects an AES-SIV based crypto backend.
 	FlagAESSIV
 	// FlagRaw64 enables raw (unpadded) base64 encoding for file names
@@ -40,6 +43,7 @@ var knownFlags = map[flagIota]string{
 	FlagEMENames:          "EMENames",
 	FlagGCMIV128:          "GCMIV128",
 	FlagLongNames:         "LongNames",
+	FlagLongNameMax:       "LongNameMax",
 	FlagAESSIV:            "AESSIV",
 	FlagRaw64:             "Raw64",
 	FlagHKDF:              "HKDF",