bpf: Allow usage of BPF ringbuffer in sleepable programs

The BPF ringbuffer map is pre-allocated and the implementation logic
does not rely on disabling preemption or per-cpu data structures. Using
the BPF ringbuffer sleepable LSM and tracing programs does not trigger
any warnings with DEBUG_ATOMIC_SLEEP, DEBUG_PREEMPT,
PROVE_RCU and PROVE_LOCKING and LOCKDEP enabled.

This allows helpers like bpf_copy_from_user and bpf_ima_inode_hash to
write to the BPF ring buffer from sleepable BPF programs.
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210204193622.3367275-2-kpsingh@kernel.org

kernel/bpf/verifier.c

@@ -10024,9 +10024,11 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
 				return -EINVAL;
 			}
 			break;
+		case BPF_MAP_TYPE_RINGBUF:
+			break;
 		default:
 			verbose(env,
-				"Sleepable programs can only use array and hash maps\n");
+				"Sleepable programs can only use array, hash, and ringbuf maps\n");
 			return -EINVAL;
 		}