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, 152 insertions, 0 deletions

diff --git a/internal/fusefrontend/names_test.go b/internal/fusefrontend/names_test.go
new file mode 100644
index 0000000..8453e52
--- /dev/null
+++ b/internal/fusefrontend/names_test.go
@@ -0,0 +1,152 @@
+package fusefrontend
+
+import (
+	"strings"
+	"syscall"
+	"testing"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+	"github.com/rfjakob/gocryptfs/tests/test_helpers"
+)
+
+func TestOpenBackingDir(t *testing.T) {
+	cipherdir := test_helpers.InitFS(t)
+	args := Args{
+		Cipherdir: cipherdir,
+	}
+	fs := newTestFS(args)
+
+	code := fs.Mkdir("dir1", 0700, nil)
+	if !code.Ok() {
+		t.Fatal(code)
+	}
+	code = fs.Mkdir("dir1/dir2", 0700, nil)
+	if !code.Ok() {
+		t.Fatal(code)
+	}
+
+	dirfd, cName, err := fs.openBackingDir("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName != "." {
+		t.Fatal("cName should be .")
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	err = syscallcompat.Faccessat(dirfd, ".", unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+
+	dirfd, cName, err = fs.openBackingDir("dir1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName == "" {
+		t.Fatal("cName should not be empty")
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+
+	dirfd, cName, err = fs.openBackingDir("dir1/dir2")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName == "" {
+		t.Fatal("cName should not be empty")
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+
+	n255 := strings.Repeat("n", 255)
+	path := "dir1/" + n255
+	fs.Mkdir(path, 0700, nil)
+	dirfd, cName, err = fs.openBackingDir(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName == "" {
+		t.Fatal("cName should not be empty")
+	}
+	if len(cName) >= 255 {
+		t.Fatalf("cName is too long: %q", cName)
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+}
+
+func TestOpenBackingDirPlaintextNames(t *testing.T) {
+	cipherdir := test_helpers.InitFS(t, "-plaintextnames")
+	args := Args{
+		Cipherdir:      cipherdir,
+		PlaintextNames: true,
+	}
+	fs := newTestFS(args)
+
+	code := fs.Mkdir("dir1", 0700, nil)
+	if !code.Ok() {
+		t.Fatal(code)
+	}
+	code = fs.Mkdir("dir1/dir2", 0700, nil)
+	if !code.Ok() {
+		t.Fatal(code)
+	}
+
+	dirfd, cName, err := fs.openBackingDir("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName != "." {
+		t.Fatal("cName should be .")
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	err = syscallcompat.Faccessat(dirfd, ".", unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+
+	dirfd, cName, err = fs.openBackingDir("dir1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName != "dir1" {
+		t.Fatalf("wrong cName: %q", cName)
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+
+	dirfd, cName, err = fs.openBackingDir("dir1/dir2")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cName != "dir2" {
+		t.Fatalf("wrong cName: %q", cName)
+	}
+	err = syscallcompat.Faccessat(dirfd, cName, unix.R_OK)
+	if err != nil {
+		t.Error(err)
+	}
+	syscall.Close(dirfd)
+}