[PATCH bpf v2 2/2] bpf, lpm_trie: Allow sleepable programs to use LPM trie maps directly

[Vlad Poenaru]

The previous change relaxed the rcu_dereference annotations in
lpm_trie.c so the trie walks no longer trip lockdep when reached from a
sleepable BPF program holding only rcu_read_lock_trace().  By itself
that only helps tries reached as the inner map of a map-of-maps, or
from the classic-RCU syscall path: a sleepable program that references
an LPM trie directly is still rejected at load time by
check_map_prog_compatibility(), whose sleepable whitelist omits
BPF_MAP_TYPE_LPM_TRIE:

  Sleepable programs can only use array, hash, ringbuf and local storage maps

LPM trie nodes are allocated from a bpf_mem_alloc (trie->ma) and freed
with bpf_mem_cache_free_rcu(), which chains a regular RCU grace period
into a Tasks Trace grace period before the node -- and the value
embedded in it that trie_lookup_elem() returns to the program -- is
released.  That is the same reclaim discipline BPF_MAP_TYPE_HASH relies
on for sleepable access, so a value handed to a sleepable reader cannot
be freed while the program is still running under rcu_read_lock_trace().
The writer paths take trie->lock across the walk and never relied on the
RCU read-side lock to keep nodes alive.

Add BPF_MAP_TYPE_LPM_TRIE to the sleepable map whitelist so these
programs can use LPM tries directly.

Signed-off-by: Vlad Poenaru <vlad.wing@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7fb88e1cd7c4..71c1e59e4df4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -18122,6 +18122,7 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
 		case BPF_MAP_TYPE_PERCPU_HASH:
 		case BPF_MAP_TYPE_PERCPU_ARRAY:
 		case BPF_MAP_TYPE_LRU_PERCPU_HASH:
+		case BPF_MAP_TYPE_LPM_TRIE:
 		case BPF_MAP_TYPE_ARRAY_OF_MAPS:
 		case BPF_MAP_TYPE_HASH_OF_MAPS:
 		case BPF_MAP_TYPE_RINGBUF:
-- 
2.53.0-Meta