Re: [PATCH v3 1/3] bpf: Add new bpf map type to store the pointer to struct perf_event

Alexei Starovoitov
2015-07-24T02:26:07Z

On 7/23/15 7:22 PM, xiakaixu wrote:
>>> + /* check if the value is already stored */
>>> >>+ if (array->events[index])
>>> >>+ return -EINVAL;
>>> >>+
>>> >>+ /* convert the fd to the pointer to struct perf_event */
>>> >>+ event = convert_map_with_perf_event(value);
>> >
>> >imo helper name is misleading and it's too short to be separate
>> >function. Just inline it and you can reuse 'index' variable.
>> >
>>> >>+ if (!event)
>>> >>+ return -EBADF;
>>> >>+
>>> >>+ xchg(array->events + index, event);
>> >
>> >refcnt leak of old event! Please think it through.
>> >This type of bugs I shouldn't be finding.
> Maybe the commit message is not elaborate. Here I prevent
> user space from updating the existed event, so the return
> value of xchg() is NULL and no refcnt leak of old event.
> I will do the same as prog_array in next version.

I see then it's even worse.
You think that above check:
+ if (array->events[index])
+ return -EINVAL;
will protect the double insert?
It won't, since there are no locks here.
You can have two processes both seeing empty slot and
racing to do xchg.