1 files changed, 60 insertions, 0 deletions
diff --git a/internal/fusefrontend/root_node.go b/internal/fusefrontend/root_node.go
index 2689e05..cf23169 100644
--- a/internal/fusefrontend/root_node.go
+++ b/internal/fusefrontend/root_node.go
@@ -2,6 +2,8 @@ package fusefrontend
 
 import (
 	"os"
+	"path/filepath"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -185,3 +187,61 @@ func (rn *RootNode) openWriteOnlyFile(dirfd int, cName string, newFlags int) (rw
 	}()
 	return syscallcompat.Openat(dirfd, cName, newFlags, 0)
 }
+
+// openBackingDir opens the parent ciphertext directory of plaintext path
+// "relPath". It returns the dirfd (opened with O_PATH) 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 (rn *RootNode) openBackingDir(relPath string) (dirfd int, cName string, err error) {
+	dirRelPath := nametransform.Dir(relPath)
+	// With PlaintextNames, we don't need to read DirIVs. Easy.
+	if rn.args.PlaintextNames {
+		dirfd, err = syscallcompat.OpenDirNofollow(rn.args.Cipherdir, dirRelPath)
+		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(rn.args.Cipherdir, 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, err = rn.nameTransform.EncryptAndHashName(name, iv)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", err
+		}
+		// 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_NOFOLLOW|syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+		syscall.Close(dirfd)
+		if err != nil {
+			return -1, "", err
+		}
+		dirfd = dirfd2
+	}
+	return dirfd, cName, nil
+}