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, 44 insertions, 7 deletions

diff --git a/internal/fusefrontend/names.go b/internal/fusefrontend/names.go
index 3bf64d5..5ec252b 100644
--- a/internal/fusefrontend/names.go
+++ b/internal/fusefrontend/names.go
@@ -4,8 +4,11 @@ package fusefrontend
 
 import (
 	"path/filepath"
+	"strings"
+	"syscall"
 
 	"github.com/rfjakob/gocryptfs/internal/configfile"
+	"github.com/rfjakob/gocryptfs/internal/nametransform"
 	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
@@ -42,19 +45,53 @@ func (fs *FS) getBackingPath(relPath string) (string, error) {
 
 // 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.
-// openBackingDir is secure against symlink races.
+// 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) {
-	cRelPath, err := fs.encryptPath(relPath)
-	if err != nil {
-		return -1, "", err
+	// 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 parent dir
-	dirfd, err = syscallcompat.OpenDirNofollow(fs.args.Cipherdir, filepath.Dir(cRelPath))
+	// 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
 	}
-	cName = filepath.Base(cRelPath)
+	// 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 {
+			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
 }