fusefrontend: make openBackingDir() symlink-safe

openBackingDir() used encryptPath(), which is not symlink-safe
itself. Drop encryptPath() and implement our own directory walk.

Adds three seconds to untar and two seconds to rm:

$ ./benchmark.bash
Testing gocryptfs at /tmp/benchmark.bash.MzG: gocryptfs v1.6-36-g8fb3c2f-dirty; go-fuse v20170619-66-g6df8ddc; 2018-10-14 go1.11
WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 1.25078 s, 210 MB/s
READ:  262144000 bytes (262 MB, 250 MiB) copied, 1.0318 s, 254 MB/s
UNTAR: 20.941
MD5:   11.568
LS:    1.638
RM:    5.337

1 files changed, 6 insertions, 3 deletions

diff --git a/internal/nametransform/diriv.go b/internal/nametransform/diriv.go
index b45ae52..c2b9bb1 100644
--- a/internal/nametransform/diriv.go
+++ b/internal/nametransform/diriv.go
@@ -121,7 +121,7 @@ func WriteDirIV(dirfd int, dir string) error {
 
 // encryptAndHashName encrypts "name" and hashes it to a longname if it is
 // too long.
-func (be *NameTransform) encryptAndHashName(name string, iv []byte) string {
+func (be *NameTransform) EncryptAndHashName(name string, iv []byte) string {
 	cName := be.EncryptName(name, iv)
 	if be.longNames && len(cName) > unix.NAME_MAX {
 		return be.HashLongName(cName)
@@ -132,6 +132,9 @@ func (be *NameTransform) encryptAndHashName(name string, iv []byte) string {
 // EncryptPathDirIV - encrypt relative plaintext path "plainPath" using EME with
 // DirIV. "rootDir" is the backing storage root directory.
 // Components that are longer than 255 bytes are hashed if be.longnames == true.
+//
+// TODO: EncryptPathDirIV is NOT SAFE against symlink races. This function
+// should eventually be deleted.
 func (be *NameTransform) EncryptPathDirIV(plainPath string, rootDir string) (string, error) {
 	var err error
 	// Empty string means root directory
@@ -148,7 +151,7 @@ func (be *NameTransform) EncryptPathDirIV(plainPath string, rootDir string) (str
 	// in the tar extract benchmark.
 	parentDir := Dir(plainPath)
 	if iv, cParentDir := be.DirIVCache.Lookup(parentDir); iv != nil {
-		cBaseName := be.encryptAndHashName(baseName, iv)
+		cBaseName := be.EncryptAndHashName(baseName, iv)
 		return filepath.Join(cParentDir, cBaseName), nil
 	}
 	// We have to walk the directory tree, starting at the root directory.
@@ -166,7 +169,7 @@ func (be *NameTransform) EncryptPathDirIV(plainPath string, rootDir string) (str
 			}
 			be.DirIVCache.Store(plainWD, iv, cipherWD)
 		}
-		cipherName := be.encryptAndHashName(plainName, iv)
+		cipherName := be.EncryptAndHashName(plainName, iv)
 		cipherWD = filepath.Join(cipherWD, cipherName)
 		plainWD = filepath.Join(plainWD, plainName)
 	}