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: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220902211058.60789-6-alexei.starovoitov@gmail.com

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0194a36..3dce316 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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 (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)) {