bpf: introduce the bpf_get_local_storage() helper function

The bpf_get_local_storage() helper function is used
to get a pointer to the bpf local storage from a bpf program.

It takes a pointer to a storage map and flags as arguments.
Right now it accepts only cgroup storage maps, and flags
argument has to be 0. Further it can be extended to support
other types of local storage: e.g. thread local storage etc.

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/helpers.c

@@ -193,4 +193,24 @@ const struct bpf_func_proto bpf_get_current_cgroup_id_proto = {
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
 };
+
+DECLARE_PER_CPU(void*, bpf_cgroup_storage);
+
+BPF_CALL_2(bpf_get_local_storage, struct bpf_map *, map, u64, flags)
+{
+	/* map and flags arguments are not used now,
+	 * but provide an ability to extend the API
+	 * for other types of local storages.
+	 * verifier checks that their values are correct.
+	 */
+	return (unsigned long) this_cpu_read(bpf_cgroup_storage);
+}
+
+const struct bpf_func_proto bpf_get_local_storage_proto = {
+	.func		= bpf_get_local_storage,
+	.gpl_only	= false,
+	.ret_type	= RET_PTR_TO_MAP_VALUE,
+	.arg1_type	= ARG_CONST_MAP_PTR,
+	.arg2_type	= ARG_ANYTHING,
+};
 #endif