fusefrontend: use OpenDirNofollow in openBackingDir

Rename openBackingPath to openBackingDir and use OpenDirNofollow
to be safe against symlink races. Note that openBackingDir is
not used in several important code paths like Create().

But it is used in Unlink, and the performance impact in the RM benchmark
to be acceptable:

Before

	$ ./benchmark.bash
	Testing gocryptfs at /tmp/benchmark.bash.bYO: gocryptfs v1.6-12-g930c37e-dirty; go-fuse v20170619-49-gb11e293; 2018-09-08 go1.10.3
	WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 1.07979 s, 243 MB/s
	READ:  262144000 bytes (262 MB, 250 MiB) copied, 0.882413 s, 297 MB/s
	UNTAR: 16.703
	MD5:   7.606
	LS:    1.349
	RM:    3.237

After

	$ ./benchmark.bash
	Testing gocryptfs at /tmp/benchmark.bash.jK3: gocryptfs v1.6-13-g84d6faf-dirty; go-fuse v20170619-49-gb11e293; 2018-09-08 go1.10.3
	WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 1.06261 s, 247 MB/s
	READ:  262144000 bytes (262 MB, 250 MiB) copied, 0.947228 s, 277 MB/s
	UNTAR: 17.197
	MD5:   7.540
	LS:    1.364
	RM:    3.410

3 files changed, 18 insertions, 14 deletions

diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 45b5b40..d68939b 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -250,7 +250,7 @@ func (fs *FS) Chmod(path string, mode uint32, context *fuse.Context) (code fuse.
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(path)
+	dirfd, cName, err := fs.openBackingDir(path)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
@@ -266,7 +266,7 @@ func (fs *FS) Chown(path string, uid uint32, gid uint32, context *fuse.Context)
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(path)
+	dirfd, cName, err := fs.openBackingDir(path)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
@@ -291,7 +291,7 @@ func (fs *FS) Mknod(path string, mode uint32, dev uint32, context *fuse.Context)
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(path)
+	dirfd, cName, err := fs.openBackingDir(path)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
@@ -409,7 +409,7 @@ func (fs *FS) Unlink(path string, context *fuse.Context) (code fuse.Status) {
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(path)
+	dirfd, cName, err := fs.openBackingDir(path)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
@@ -447,7 +447,7 @@ func (fs *FS) Symlink(target string, linkName string, context *fuse.Context) (co
 	if fs.isFiltered(linkName) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(linkName)
+	dirfd, cName, err := fs.openBackingDir(linkName)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
@@ -579,12 +579,12 @@ func (fs *FS) Link(oldPath string, newPath string, context *fuse.Context) (code
 	if fs.isFiltered(newPath) {
 		return fuse.EPERM
 	}
-	oldDirFd, cOldName, err := fs.openBackingPath(oldPath)
+	oldDirFd, cOldName, err := fs.openBackingDir(oldPath)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
 	defer oldDirFd.Close()
-	newDirFd, cNewName, err := fs.openBackingPath(newPath)
+	newDirFd, cNewName, err := fs.openBackingDir(newPath)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
diff --git a/internal/fusefrontend/fs_dir.go b/internal/fusefrontend/fs_dir.go
index b7bc790..b9266dd 100644
--- a/internal/fusefrontend/fs_dir.go
+++ b/internal/fusefrontend/fs_dir.go
@@ -54,7 +54,7 @@ func (fs *FS) Mkdir(newPath string, mode uint32, context *fuse.Context) (code fu
 	if fs.isFiltered(newPath) {
 		return fuse.EPERM
 	}
-	dirfd, cName, err := fs.openBackingPath(newPath)
+	dirfd, cName, err := fs.openBackingDir(newPath)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
diff --git a/internal/fusefrontend/names.go b/internal/fusefrontend/names.go
index 9be2623..d7fbdce 100644
--- a/internal/fusefrontend/names.go
+++ b/internal/fusefrontend/names.go
@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 
 	"github.com/rfjakob/gocryptfs/internal/configfile"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -40,18 +41,21 @@ func (fs *FS) getBackingPath(relPath string) (string, error) {
 	return cAbsPath, nil
 }
 
-// openBackingPath - get the absolute encrypted path of the backing file
-// and open the corresponding directory
-func (fs *FS) openBackingPath(relPath string) (*os.File, string, error) {
-	cPath, err := fs.getBackingPath(relPath)
+// 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.
+func (fs *FS) openBackingDir(relPath string) (*os.File, string, error) {
+	cRelPath, err := fs.encryptPath(relPath)
 	if err != nil {
 		return nil, "", err
 	}
-	dirfd, err := os.Open(filepath.Dir(cPath))
+	// Open parent dir
+	dirfd, err := syscallcompat.OpenDirNofollow(fs.args.Cipherdir, filepath.Dir(cRelPath))
 	if err != nil {
 		return nil, "", err
 	}
-	return dirfd, filepath.Base(cPath), nil
+	return os.NewFile(uintptr(dirfd), cRelPath), filepath.Base(cRelPath), nil
 }
 
 // encryptPath - encrypt relative plaintext path