bpf: Allow sleepable programs to use LPM trie maps directly

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.

VPVlad Poenaru committed 5 months ago