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 <[e-mail]>
Tested-by: Yafang Shao <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 					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;
 				}