fusefrontend: move openBackingDir into its own file

This function is in all fastpaths, will get a cache, and needs
its own file.
renamed:    internal/fusefrontend/names.go -> internal/fusefrontend/openbackingdir.go
renamed:    internal/fusefrontend/names_test.go -> internal/fusefrontend/openbackingdir_test.go

1 files changed, 63 insertions, 0 deletions

diff --git a/internal/fusefrontend/openbackingdir.go b/internal/fusefrontend/openbackingdir.go
new file mode 100644
index 0000000..849a486
--- /dev/null
+++ b/internal/fusefrontend/openbackingdir.go
@@ -0,0 +1,63 @@
+package fusefrontend
+
+import (
+	"path/filepath"
+	"strings"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/nametransform"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+)
+
+// openBackingDir opens the parent ciphertext directory of plaintext path
+// "relPath" and returns the dirfd and the encrypted basename.
+//
+// The caller should then use Openat(dirfd, cName, ...) and friends.
+// For convenience, if relPath is "", cName is going to be ".".
+//
+// openBackingDir is secure against symlink races by using Openat and
+// ReadDirIVAt.
+func (fs *FS) openBackingDir(relPath string) (dirfd int, cName string, err error) {
+	// With PlaintextNames, we don't need to read DirIVs. Easy.
+	if fs.args.PlaintextNames {
+		dir := nametransform.Dir(relPath)
+		dirfd, err = syscallcompat.OpenDirNofollow(fs.args.Cipherdir, dir)
+		if err != nil {
+			return -1, "", err
+		}
+		// If relPath is empty, cName is ".".
+		cName = filepath.Base(relPath)
+		return dirfd, cName, nil
+	}
+	// Open cipherdir (following symlinks)
+	dirfd, err = syscall.Open(fs.args.Cipherdir, syscall.O_RDONLY|syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+	if err != nil {
+		return -1, "", err
+	}
+	// If relPath is empty, cName is ".".
+	if relPath == "" {
+		return dirfd, ".", nil
+	}
+	// Walk the directory tree
+	parts := strings.Split(relPath, "/")
+	for i, name := range parts {
+		iv, err := nametransform.ReadDirIVAt(dirfd)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", err
+		}
+		cName = fs.nameTransform.EncryptAndHashName(name, iv)
+		// Last part? We are done.
+		if i == len(parts)-1 {
+			break
+		}
+		// Not the last part? Descend into next directory.
+		dirfd2, err := syscallcompat.Openat(dirfd, cName, syscall.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+		syscall.Close(dirfd)
+		if err != nil {
+			return -1, "", err
+		}
+		dirfd = dirfd2
+	}
+	return dirfd, cName, nil
+}