Re: [PATCH v2 net-next 6/7] bpf: allow eBPF programs to use maps

From: Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>
Date: Thu, 13 Nov 2014 17:36:49 -0800

+static u64 bpf_map_lookup_elem(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+	/* verifier checked that R1 contains a valid pointer to bpf_map
+	 * and R2 points to a program stack and map->key_size bytes were
+	 * initialized
+	 */
+	struct bpf_map *map = (struct bpf_map *) (unsigned long) r1;
+	void *key = (void *) (unsigned long) r2;
+	void *value;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	value = map->ops->map_lookup_elem(map, key);
+
+	/* lookup() returns either pointer to element value or NULL
+	 * which is the meaning of PTR_TO_MAP_VALUE_OR_NULL type
+	 */
+	return (unsigned long) value;
+}

You should translate this into a true boolean '1' or '0' value so that
kernel pointers don't propagate to the user or his eBPF programs.