fusefrontend: make OpenDir() symlink-safe

Interestingly, little or no performance impact:

$ ./benchmark.bash
Testing gocryptfs at /tmp/benchmark.bash.39W: gocryptfs v1.6-42-g30c2349-dirty; go-fuse v20170619-66-g6df8ddc; 2018-11-04 go1.11
Downloading linux-3.0.tar.gz
/tmp/linux-3.0.tar.gz                  100%[=========================================================================>]  92.20M  2.93MB/s    in 31s
2018-11-04 21:44:44 URL:https://cdn.kernel.org/pub/linux/kernel/v3.0/linux-3.0.tar.gz [96675825/96675825] -> "/tmp/linux-3.0.tar.gz" [1]
WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 1.1808 s, 222 MB/s
READ:  262144000 bytes (262 MB, 250 MiB) copied, 0.866438 s, 303 MB/s
UNTAR: 24.745
MD5:   12.050
LS:    3.525
RM:    9.544

Note: kernel has been updated:

$ uname -a
Linux brikett 4.18.16-200.fc28.x86_64 #1 SMP Sat Oct 20 23:53:47 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

2 files changed, 19 insertions, 20 deletions

diff --git a/internal/fusefrontend/fs_dir.go b/internal/fusefrontend/fs_dir.go
index 8d27791..9486802 100644
--- a/internal/fusefrontend/fs_dir.go
+++ b/internal/fusefrontend/fs_dir.go
@@ -260,18 +260,21 @@ retry:
 	return fuse.OK
 }
 
-// OpenDir implements pathfs.FileSystem
+// OpenDir - FUSE call
+//
+// This function is symlink-safe through use of openBackingDir() and
+// ReadDirIVAt().
 func (fs *FS) OpenDir(dirName string, context *fuse.Context) ([]fuse.DirEntry, fuse.Status) {
 	tlog.Debug.Printf("OpenDir(%s)", dirName)
-	cDirName, err := fs.encryptPath(dirName)
+	parentDirFd, cDirName, err := fs.openBackingDir(dirName)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
+	defer syscall.Close(parentDirFd)
 	// Read ciphertext directory
-	cDirAbsPath := filepath.Join(fs.args.Cipherdir, cDirName)
 	var cipherEntries []fuse.DirEntry
 	var status fuse.Status
-	fd, err := syscall.Open(cDirAbsPath, syscall.O_RDONLY|syscall.O_NOFOLLOW, 0)
+	fd, err := syscallcompat.Openat(parentDirFd, cDirName, syscall.O_RDONLY|syscall.O_NOFOLLOW, 0)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
@@ -283,23 +286,16 @@ func (fs *FS) OpenDir(dirName string, context *fuse.Context) ([]fuse.DirEntry, f
 	// Get DirIV (stays nil if PlaintextNames is used)
 	var cachedIV []byte
 	if !fs.args.PlaintextNames {
-		cachedIV, _ = fs.nameTransform.DirIVCache.Lookup(dirName)
-		if cachedIV == nil {
-			// Read the DirIV from disk and store it in the cache
-			fs.dirIVLock.RLock()
-			cachedIV, err = nametransform.ReadDirIV(cDirAbsPath)
-			if err != nil {
-				fs.dirIVLock.RUnlock()
-				// The directory itself does not exist
-				if err == syscall.ENOENT {
-					return nil, fuse.ENOENT
-				}
-				// Any other problem warrants an error message
-				tlog.Warn.Printf("OpenDir %q: could not read gocryptfs.diriv: %v", cDirName, err)
-				return nil, fuse.EIO
+		// Read the DirIV from disk
+		cachedIV, err = nametransform.ReadDirIVAt(fd)
+		if err != nil {
+			// The directory itself does not exist
+			if err == syscall.ENOENT {
+				return nil, fuse.ENOENT
 			}
-			fs.nameTransform.DirIVCache.Store(dirName, cachedIV, cDirName)
-			fs.dirIVLock.RUnlock()
+			// Any other problem warrants an error message
+			tlog.Warn.Printf("OpenDir %q: could not read gocryptfs.diriv: %v", cDirName, err)
+			return nil, fuse.EIO
 		}
 	}
 	// Decrypted directory entries
diff --git a/internal/nametransform/diriv.go b/internal/nametransform/diriv.go
index c2b9bb1..b98de0c 100644
--- a/internal/nametransform/diriv.go
+++ b/internal/nametransform/diriv.go
@@ -29,6 +29,9 @@ const (
 // This function is exported because it allows for an efficient readdir implementation.
 // If the directory itself cannot be opened, a syscall error will be returned.
 // Otherwise, a fmt.Errorf() error value is returned with the details.
+//
+// TODO: this function is not symlink-safe and should be deleted once the only
+// remaining user, EncryptPathDirIV(), is gone.
 func ReadDirIV(dir string) (iv []byte, err error) {
 	fd, err := os.Open(filepath.Join(dir, DirIVFilename))
 	if err != nil {