syscallcompat: add OpenNofollow helper

OpenNofollow = symlink-race-safe Open

Prepares fixing https://github.com/rfjakob/gocryptfs/issues/165

internal/syscallcompat/open_nofollow.go

@@ -0,0 +1,49 @@
+package syscallcompat
+
+import (
+	"path/filepath"
+	"strings"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// OpenNofollow opens the file/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) {
+	if !filepath.IsAbs(baseDir) {
+		tlog.Warn.Printf("BUG: OpenNofollow 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)
+		return -1, syscall.EINVAL
+	}
+	// Open the base dir
+	dirfd, err := syscall.Open(baseDir, syscall.O_RDONLY, 0)
+	if err != nil {
+		return -1, err
+	}
+	// Split the path into components and separate intermediate directories
+	// and the final basename
+	parts := strings.Split(relPath, "/")
+	dirs := parts[:len(parts)-1]
+	final := parts[len(parts)-1]
+	// Walk intermediate directories
+	var dirfd2 int
+	for _, name := range dirs {
+		dirfd2, err = Openat(dirfd, name, syscall.O_RDONLY|syscall.O_NOFOLLOW, 0)
+		syscall.Close(dirfd)
+		if err != nil {
+			return -1, err
+		}
+		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)
+}

internal/syscallcompat/open_nofollow_test.go

@@ -0,0 +1,37 @@
+package syscallcompat
+
+import (
+	"os"
+	"syscall"
+	"testing"
+)
+
+func TestOpenNofollow(t *testing.T) {
+	err := os.MkdirAll(tmpDir+"/d1/d2/d3", 0700)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Create a file
+	fd, err := OpenNofollow(tmpDir, "d1/d2/d3/f1", syscall.O_RDWR|syscall.O_CREAT|syscall.O_EXCL, 0600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	syscall.Close(fd)
+	_, err = os.Stat(tmpDir + "/d1/d2/d3/f1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Replace "d1" with a symlink - open should fail with ELOOP
+	err = os.Rename(tmpDir+"/d1", tmpDir+"/d1.renamed")
+	if err != nil {
+		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)
+	if err == nil {
+		t.Fatalf("should have failed")
+	}
+	if err != syscall.ELOOP {
+		t.Errorf("expected ELOOP, got %v", err)
+	}
+}