Re: [PATCH RFC bpf-next v6 1/6] bpf/helpers: introduce sleepable bpf_timers

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-04-08 at 10:09 +0200, Benjamin Tissoires wrote:

[...]

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 9234174ccb21..fd05d4358b31 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1096,12 +1096,19 @@ const struct bpf_func_proto bpf_snprintf_proto = {
>   * freeing the timers when inner map is replaced or deleted by user space.
>   */
>  struct bpf_hrtimer {
> -	struct hrtimer timer;
> +	union {
> +		struct hrtimer timer;
> +		struct work_struct work;
> +	};
>  	struct bpf_map *map;
>  	struct bpf_prog *prog;
>  	void __rcu *callback_fn;
>  	void *value;
> -	struct rcu_head rcu;
> +	union {
> +		struct rcu_head rcu;
> +		struct work_struct sync_work;

Nit:
I find this name very confusing, the field is used to cancel timer
execution, is it a convention to call such things '...sync...'?

> +	};
> +	u64 flags;
>  };
>  

[...]

> +static void bpf_timer_sync_work_cb(struct work_struct *work)
> +{
> +	struct bpf_hrtimer *t = container_of(work, struct bpf_hrtimer, sync_work);
> +
> +	cancel_work_sync(&t->work);
> +
> +	kfree_rcu(t, rcu);

Sorry, I might be wrong, but this looks suspicious.
The 'rcu' field of 'bpf_hrtimer' is defined as follows:

struct bpf_hrtimer {
	...
	union {
		struct rcu_head rcu;
		struct work_struct sync_work;
	};
	...
};

And for sleepable timers the 'sync_work' field is set as follows:

BPF_CALL_3(bpf_timer_init, struct bpf_timer_kern *, timer, struct bpf_map *, map,
	   u64, flags)
{
	...
	INIT_WORK(&t->sync_work, bpf_timer_sync_work_cb);
	...
}

So, it looks like 'kfree_rcu' would be called for a non-rcu pointer.

> +}
> +