[PATCH bpf-next 5/5] bpf: Relax the requirement to use preallocated hash maps in tracing progs.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

From: Alexei Starovoitov <ast@kernel.org>

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: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/verifier.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a20d7736a5b2..90d70304c6f1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12566,10 +12566,12 @@ 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
@@ -12579,15 +12581,26 @@ 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 (IS_ENABLED(CONFIG_PREEMPT_RT)) {
-			verbose(env, "trace type 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 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 per-cpu hash maps are unsafe."
+				" Switch to preallocated hash maps.\n");
 		}
-		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");
 	}
 
 	if (map_value_has_spin_lock(map)) {
-- 
2.30.2