linux-kernel - Re: [PATCH v2 3/5] bpf: Save the pointer to struct perf

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Date:	Wed, 22 Jul 2015 18:08:24 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kaixu Xia <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, a.p.zijlstra@...llo.nl,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org
Cc:	wangnan0@...wei.com, linux-kernel@...r.kernel.org,
	pi3orama@....com, hekuang@...wei.com
Subject: Re: [PATCH v2 3/5] bpf: Save the pointer to struct perf_event to map

On 7/22/15 1:09 AM, Kaixu Xia wrote:
> The user space event FDs from perf_event_open() syscall are
> converted to the pointer to struct perf_event and stored in map.
...
> +static int replace_map_with_perf_event(void *value)
> +{
> +	struct perf_event *event;
> +	u32 fd;
> +
> +	fd = *(u32 *)value;
> +
> +	event = perf_event_get(fd);
> +	if (IS_ERR(event))
> +		return PTR_ERR(event);
> +
> +	/* limit the event type to PERF_TYPE_RAW
> +	 * and PERF_TYPE_HARDWARE
> +	 */
> +	if (event->attr.type != PERF_TYPE_RAW &&
> +	    event->attr.type != PERF_TYPE_HARDWARE)
> +		return -EINVAL;
> +
> +	memcpy(value, &event, sizeof(struct perf_event *));
> +
> +	return 0;
> +}
> +
> +static bool check_map_perf_event_stored(struct bpf_map *map, void *key)
> +{
> +	void *event;
> +	bool is_stored = false;
> +
> +	rcu_read_lock();
> +	event = array_map_lookup_elem(map, key);
> +	if (event && (*(unsigned long *)event))
> +		is_stored = true;
> +	rcu_read_unlock();
> +
> +	return is_stored;
> +}
> +
> +/* only called from syscall */
> +static int perf_event_array_map_update_elem(struct bpf_map *map, void *key,
> +					    void *value, u64 map_flags)
> +{
> +	/* check if the value is already stored */
> +	if (check_map_perf_event_stored(map, key))
> +		return -EINVAL;
> +
> +	if (replace_map_with_perf_event(value))
> +		return -EBADF;

the above three functions are broken due to races.
update_elem can be called by different user space processes.
Please see how prog_array solves it via xchg()

> +	if (map->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY) {
> +		if (!map->ops->map_traverse_elem)
> +			return -EPERM;
> +
> +		rcu_read_lock();
> +		if (map->ops->map_traverse_elem(bpf_map_perf_event_put, map) < 0) {
> +			rcu_read_unlock();
> +			return -EINVAL;
> +		}
> +		rcu_read_unlock();

as I mentioned as part of patch 2 the above seems unnecessary.

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d3dae34..14a9924 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8574,6 +8574,32 @@ void perf_event_delayed_put(struct task_struct *task)
>   		WARN_ON_ONCE(task->perf_event_ctxp[ctxn]);
>   }
>
> +struct perf_event *perf_event_get(unsigned int fd)
> +{
> +	struct perf_event *event;
> +	struct fd f;
> +
> +	f = fdget(fd);
> +
> +	if (!f.file)
> +		return ERR_PTR(-EBADF);
> +
> +	if (f.file->f_op != &perf_fops) {
> +		fdput(f);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	event = f.file->private_data;
> +
> +	if (!atomic_long_inc_not_zero(&event->refcount)) {

I don't understand necessity of '_not_zero' suffix. why?
How it can possibly race to zero here if we have an fd?

> +		fdput(f);
> +		return ERR_PTR(-ENOENT);
> +	}
> +
> +	fdput(f);
> +	return event;
> +}

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