Re: [PATCH RFC bpf-next v2 02/10] bpf/helpers: introduce sleepable timers

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Feb 16, 2024 at 10:50:10AM +0100, Benjamin Tissoires wrote:
>  static bool is_rbtree_lock_required_kfunc(u32 btf_id)
>  {
>  	return is_bpf_rbtree_api_kfunc(btf_id);
> @@ -12140,6 +12143,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  		}
>  	}
>  
> +	if (is_bpf_timer_set_sleepable_cb_kfunc(meta.func_id)) {
> +		err = push_callback_call(env, insn, insn_idx, meta.subprogno,
> +					 set_timer_callback_state);
> +		if (err) {
> +			verbose(env, "kfunc %s#%d failed callback verification\n",
> +				func_name, meta.func_id);
> +			return err;
> +		}
> +	}

All makes sense so far.
Please squash all the fix and repost.
It's hard to do a proper review in this shape of the patch.
As far as rcu_read_lock/unlock that is done in callback...
it feels buggy and unnecessary.
bpf prog and timer won't disappear while work is queued.
array and hash map will call bpf_obj_free_timer() before going away.

And things like:
+       rcu_read_lock();
+       callback_fn = rcu_dereference(t->sleepable_cb_fn);
+       rcu_read_unlock();
+       if (!callback_fn)
+               return;

is 99% broken. if (!callback_fn) line is UAF.