bpf: Transfer RCU lock state between subprog calls

Allow transferring an imbalanced RCU lock state between subprog calls
during verification. This allows patterns where a subprog call returns
with an RCU lock held, or a subprog call releases an RCU lock held by
the caller. Currently, the verifier would end up complaining if the RCU
lock is not released when processing an exit from a subprog, which is
non-ideal if its execution is supposed to be enclosed in an RCU read
section of the caller.

Instead, simply only check whether we are processing exit for frame#0
and do not complain on an active RCU lock otherwise. We only need to
update the check when processing BPF_EXIT insn, as copy_verifier_state
is already set up to do the right thing.

Suggested-by: David Vernet <void@manifault.com>
Tested-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240205055646.1112186-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7d38b2343ad40dd210f981caf72c8566cac56f61..ddaf09db117518d197ff6120d4c711f68082c1b1 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17703,8 +17703,7 @@ process_bpf_exit_full:
                                        return -EINVAL;
                                }
 
-                               if (env->cur_state->active_rcu_lock &&
-                                   !in_rbtree_lock_required_cb(env)) {
+                               if (env->cur_state->active_rcu_lock && !env->cur_state->curframe) {
                                        verbose(env, "bpf_rcu_read_unlock is missing\n");
                                        return -EINVAL;
                                }