Re: [PATCH v2 3/5] bpf: Save the pointer to struct perf_event to map

Alexei Starovoitov
2015-07-23T01:08:29Z

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;
> +}