Report correct symbolic link dentry sizes

Prior to this commit, gocryptfs's reverse mode did not report correct
directory entry sizes for symbolic links, where the dentry size needs to
be the same as the length of a string containing the target path.

This commit corrects this issue and adds a test case to verify the
correctness of the implementation.

This issue was discovered during the use of a strict file copying program
on a reverse-mounted gocryptfs file system.

internal/fusefrontend_reverse/rfs.go

@@ -221,6 +221,16 @@ func (rfs *ReverseFS) GetAttr(relPath string, context *fuse.Context) (*fuse.Attr
 	// Calculate encrypted file size
 	if a.IsRegular() {
 		a.Size = rfs.contentEnc.PlainSizeToCipherSize(a.Size)
+	} else if a.IsSymlink() {
+		var linkTarget string
+		var readlinkStatus fuse.Status
+
+		linkTarget, readlinkStatus = rfs.Readlink(relPath, context)
+		if !readlinkStatus.Ok() {
+			return nil, readlinkStatus
+		}
+
+		a.Size = uint64(len(linkTarget))
 	}
 	return a, fuse.OK
 }

tests/reverse/correctness_test.go

@@ -6,6 +6,7 @@ import (
 	"syscall"
 	"testing"
 
+	"github.com/rfjakob/gocryptfs/internal/ctlsock"
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
 )
 
@@ -51,6 +52,43 @@ func TestSymlinks(t *testing.T) {
 	}
 }
 
+// Symbolic link dentry sizes should be set to the length of the string
+// that contains the target path.
+func TestSymlinkDentrySize(t *testing.T) {
+	symlink := "a_symlink"
+
+	mnt, err := ioutil.TempDir(test_helpers.TmpDir, "reverse_mnt_")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	sock := mnt + ".sock"
+	test_helpers.MountOrFatal(t, "ctlsock_reverse_test_fs", mnt, "-reverse", "-extpass", "echo test", "-ctlsock="+sock)
+	defer test_helpers.UnmountPanic(mnt)
+
+	req := ctlsock.RequestStruct{EncryptPath: symlink}
+	symlinkResponse := test_helpers.QueryCtlSock(t, sock, req)
+	if symlinkResponse.ErrNo != 0 {
+		t.Errorf("Encrypt: %q ErrNo=%d ErrText=%s", symlink, symlinkResponse.ErrNo, symlinkResponse.ErrText)
+	}
+
+	fi, err := os.Lstat(mnt + "/" + symlinkResponse.Result)
+	if err != nil {
+		t.Errorf("Lstat: %v", err)
+	}
+
+	target, err := os.Readlink(mnt + "/" + symlinkResponse.Result)
+	if err != nil {
+		t.Errorf("Readlink: %v", err)
+	}
+
+	if fi.Size() != int64(len(target)) {
+		t.Errorf("Lstat reports that symbolic link %q's dentry size is %d, but this does not "+
+			"match the length of the string returned by readlink, which is %d.",
+			symlink, fi.Size(), len(target))
+	}
+}
+
 // .gocryptfs.reverse.conf in the plaintext dir should be visible as
 // gocryptfs.conf
 func TestConfigMapping(t *testing.T) {

tests/reverse/ctlsock_reverse_test_fs/a_symlink

@@ -0,0 +1 @@
+dir/dir/file