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

[Alexei Starovoitov]

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.

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