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

From: Alexei Starovoitov <ast@plumgrid.com>
To: xiakaixu <xiakaixu@huawei.com>
Cc: davem@davemloft.net, acme@kernel.org, mingo@redhat.com,
	a.p.zijlstra@chello.nl, masami.hiramatsu.pt@hitachi.com,
	jolsa@kernel.org, wangnan0@huawei.com,
	linux-kernel@vger.kernel.org, pi3orama@163.com,
	hekuang@huawei.com
Subject: Re: [PATCH v3 1/3] bpf: Add new bpf map type to store the pointer to struct perf_event
Date: Thu, 23 Jul 2015 19:26:02 -0700	[thread overview]
Message-ID: <55B1A23A.5060303@plumgrid.com> (raw)
In-Reply-To: <55B1A159.2090106@huawei.com>

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.