Commit 34dd3bad authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Daniel Borkmann
Browse files

bpf: Relax the requirement to use preallocated hash maps in tracing progs. 



Since bpf hash map was converted to use bpf_mem_alloc it is safe to use
from tracing programs and in RT kernels.
But per-cpu hash map is still using dynamic allocation for per-cpu map
values, hence keep the warning for this map type.
In the future alloc_percpu_gfp can be front-end-ed with bpf_mem_cache
and this restriction will be completely lifted.
perf_event (NMI) bpf programs have to use preallocated hash maps,
because free_htab_elem() is using call_rcu which might crash if re-entered.

Sleepable bpf programs have to use preallocated hash maps, because
life time of the map elements is not protected by rcu_read_lock/unlock.
This restriction can be lifted in the future as well.

Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220902211058.60789-6-alexei.starovoitov@gmail.com
parent 89dc8d0c

kernel/bpf/verifier.c

+22 −9
Original line number Diff line number Diff line
@@ -12629,10 +12629,12 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env, 
	 * For programs attached to PERF events this is mandatory as the

	 * perf NMI can hit any arbitrary code sequence.

	 *

	 * All other trace types using preallocated hash maps are unsafe as

	 * well because tracepoint or kprobes can be inside locked regions

	 * of the memory allocator or at a place where a recursion into the

	 * memory allocator would see inconsistent state.

	 * All other trace types using non-preallocated per-cpu hash maps are

	 * unsafe as well because tracepoint or kprobes can be inside locked

	 * regions of the per-cpu memory allocator or at a place where a

	 * recursion into the per-cpu memory allocator would see inconsistent

	 * state. Non per-cpu hash maps are using bpf_mem_alloc-tor which is

	 * safe to use from kprobe/fentry and in RT.

	 *

	 * On RT enabled kernels run-time allocation of all trace type

	 * programs is strictly prohibited due to lock type constraints. On
@@ -12642,15 +12644,26 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env, 
	 */

	if (is_tracing_prog_type(prog_type) && !is_preallocated_map(map)) {

		if (prog_type == BPF_PROG_TYPE_PERF_EVENT) {

			/* perf_event bpf progs have to use preallocated hash maps

			 * because non-prealloc is still relying on call_rcu to free

			 * elements.

			 */

			verbose(env, "perf_event programs can only use preallocated hash map\n");

			return -EINVAL;

		}

		if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||

		    (map->inner_map_meta &&

		     map->inner_map_meta->map_type == BPF_MAP_TYPE_PERCPU_HASH)) {

			if (IS_ENABLED(CONFIG_PREEMPT_RT)) {

			verbose(env, "trace type programs can only use preallocated hash map\n");

				verbose(env,

					"trace type programs can only use preallocated per-cpu hash map\n");

				return -EINVAL;

			}

			WARN_ONCE(1, "trace type BPF program uses run-time allocation\n");

		verbose(env, "trace type programs with run-time allocated hash maps are unsafe. Switch to preallocated hash maps.\n");

			verbose(env,

				"trace type programs with run-time allocated per-cpu hash maps are unsafe."

				" Switch to preallocated hash maps.\n");

		}

	}



	if (map_value_has_spin_lock(map)) {