syscallcompat: untangle OpenNofollow and rename to OpenDirNofollow

The function used to do two things:

1) Walk the directory tree in a manner safe from symlink attacks
2) Open the final component in the mode requested by the caller

This change drops (2), which was only used once, and lets the caller
handle it. This simplifies the function and makes it fit for reuse in
forward mode in openBackingPath(), and for using O_PATH on Linux.

internal/fusefrontend_reverse/reverse_longnames.go

@@ -63,7 +63,7 @@ func (rfs *ReverseFS) findLongnameParent(dir string, dirIV []byte, longname stri
 	if hit != "" {
 		return hit, nil
 	}
-	fd, err := syscallcompat.OpenNofollow(rfs.args.Cipherdir, dir, syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	fd, err := syscallcompat.OpenDirNofollow(rfs.args.Cipherdir, dir)
 	if err != nil {
 		tlog.Warn.Printf("findLongnameParent: opendir failed: %v\n", err)
 		return "", err

internal/fusefrontend_reverse/rfile.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io"
 	"os"
+	"path/filepath"
 	"syscall"
 
 	// In newer Go versions, this has moved to just "sync/syncmap".
@@ -46,7 +47,13 @@ func (rfs *ReverseFS) newFile(relPath string) (*reverseFile, fuse.Status) {
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
-	fd, err := syscallcompat.OpenNofollow(rfs.args.Cipherdir, pRelPath, syscall.O_RDONLY, 0)
+	dir := filepath.Dir(pRelPath)
+	dirfd, err := syscallcompat.OpenDirNofollow(rfs.args.Cipherdir, dir)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	fd, err := syscallcompat.Openat(dirfd, filepath.Base(pRelPath), syscall.O_RDONLY|syscall.O_NOFOLLOW, 0)
+	syscall.Close(dirfd)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}

internal/fusefrontend_reverse/rfs.go

@@ -305,7 +305,7 @@ func (rfs *ReverseFS) OpenDir(cipherPath string, context *fuse.Context) ([]fuse.
 		return nil, fuse.ToStatus(err)
 	}
 	// Read plaintext dir
-	fd, err := syscallcompat.OpenNofollow(rfs.args.Cipherdir, relPath, syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	fd, err := syscallcompat.OpenDirNofollow(rfs.args.Cipherdir, relPath)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}

internal/fusefrontend_reverse/rpath.go

@@ -108,7 +108,7 @@ func (rfs *ReverseFS) openBackingDir(cRelPath string) (dirfd int, pName string,
 	}
 	// Open directory, safe against symlink races
 	pDir := filepath.Dir(pRelPath)
-	dirfd, err = syscallcompat.OpenNofollow(rfs.args.Cipherdir, pDir, syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	dirfd, err = syscallcompat.OpenDirNofollow(rfs.args.Cipherdir, pDir)
 	if err != nil {
 		return -1, "", err
 	}

internal/fusefrontend_reverse/virtualfile.go

@@ -91,7 +91,7 @@ func (f *virtualFile) Read(buf []byte, off int64) (resultData fuse.ReadResult, s
 // GetAttr - FUSE call
 func (f *virtualFile) GetAttr(a *fuse.Attr) fuse.Status {
 	dir := filepath.Dir(f.parentFile)
-	dirfd, err := syscallcompat.OpenNofollow(f.cipherdir, dir, syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	dirfd, err := syscallcompat.OpenDirNofollow(f.cipherdir, dir)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}

internal/syscallcompat/open_nofollow.go

@@ -8,18 +8,18 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
-// OpenNofollow opens the file/dir at "relPath" in a way that is secure against
+// OpenDirNofollow opens the dir at "relPath" in a way that is secure against
 // symlink attacks. Symlinks that are part of "relPath" are never followed.
 // This function is implemented by walking the directory tree, starting at
 // "baseDir", using the Openat syscall with the O_NOFOLLOW flag.
 // Symlinks that are part of the "baseDir" path are followed.
-func OpenNofollow(baseDir string, relPath string, flags int, mode uint32) (fd int, err error) {
+func OpenDirNofollow(baseDir string, relPath string) (fd int, err error) {
 	if !filepath.IsAbs(baseDir) {
-		tlog.Warn.Printf("BUG: OpenNofollow called with relative baseDir=%q", baseDir)
+		tlog.Warn.Printf("BUG: OpenDirNofollow called with relative baseDir=%q", baseDir)
 		return -1, syscall.EINVAL
 	}
 	if filepath.IsAbs(relPath) {
-		tlog.Warn.Printf("BUG: OpenNofollow called with absolute relPath=%q", relPath)
+		tlog.Warn.Printf("BUG: OpenDirNofollow called with absolute relPath=%q", relPath)
 		return -1, syscall.EINVAL
 	}
 	// Open the base dir (following symlinks)
@@ -31,14 +31,11 @@ func OpenNofollow(baseDir string, relPath string, flags int, mode uint32) (fd in
 	if relPath == "" {
 		return dirfd, nil
 	}
-	// Split the path into components and separate intermediate directories
-	// and the final basename
+	// Split the path into components
 	parts := strings.Split(relPath, "/")
-	dirs := parts[:len(parts)-1]
-	final := parts[len(parts)-1]
-	// Walk intermediate directories
+	// Walk the directory tree
 	var dirfd2 int
-	for _, name := range dirs {
+	for _, name := range parts {
 		dirfd2, err = Openat(dirfd, name, syscall.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_DIRECTORY, 0)
 		syscall.Close(dirfd)
 		if err != nil {
@@ -46,8 +43,6 @@ func OpenNofollow(baseDir string, relPath string, flags int, mode uint32) (fd in
 		}
 		dirfd = dirfd2
 	}
-	defer syscall.Close(dirfd)
-	// Open the final component with the flags and permissions requested by
-	// the user plus forced NOFOLLOW.
-	return Openat(dirfd, final, flags|syscall.O_NOFOLLOW, mode)
+	// Return fd to final directory
+	return dirfd, nil
 }

internal/syscallcompat/open_nofollow_test.go

@@ -12,7 +12,11 @@ func TestOpenNofollow(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Create a file
-	fd, err := OpenNofollow(tmpDir, "d1/d2/d3/f1", syscall.O_RDWR|syscall.O_CREAT|syscall.O_EXCL, 0600)
+	dirfd, err := OpenDirNofollow(tmpDir, "d1/d2/d3")
+	if err != nil {
+		t.Fatal(err)
+	}
+	fd, err := Openat(dirfd, "f1", syscall.O_RDWR|syscall.O_CREAT|syscall.O_EXCL, 0600)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -27,7 +31,7 @@ func TestOpenNofollow(t *testing.T) {
 		t.Fatal(err)
 	}
 	os.Symlink(tmpDir+"/d1.renamed", tmpDir+"/d1")
-	fd, err = OpenNofollow(tmpDir, "d1/d2/d3/f1", syscall.O_RDWR|syscall.O_CREAT, 0600)
+	fd, err = OpenDirNofollow(tmpDir, "d1/d2/d3")
 	if err == nil {
 		t.Fatalf("should have failed")
 	}
@@ -35,7 +39,7 @@ func TestOpenNofollow(t *testing.T) {
 		t.Errorf("expected ELOOP or ENOTDIR, got %v", err)
 	}
 	// Check to see that the base dir can be opened as well
-	fd, err = OpenNofollow(tmpDir, "", syscall.O_RDONLY, 0)
+	fd, err = OpenDirNofollow(tmpDir, "")
 	if err != nil {
 		t.Errorf("cannot open base dir: %v", err)
 	} else {