In a spice grinder or with a mortar and pestle, grind together the cumin, red-pepper flakes, and salt. Add the ginger and garlic and grind again to form a paste. Rub the paste all over the pork and inside any cavities, and place the pork, fat side up, in a small roasting pan. Cover the pan loosely with foil and let it rest in the refrigerator for at least 2 and up to 8 hours. Remove the pork from the refrigerator 1 hour before cooking.
Preheat the oven to 250°. Roast the pork, uncovered, in the middle of the oven for 3½ hours, or until an instant-read thermometer registers 145°. Transfer the roast to a cutting board and let it stand for 15 minutes before slicing or shredding. Serve immediately.
Makes 8 to 10 servings