netdev - Re: [PATCH net-next] bpf: add show

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Date:	Sat, 21 Nov 2015 15:18:30 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] bpf: add show_fdinfo handler for maps

On Fri, Nov 20, 2015 at 11:30:13AM +0100, Hannes Frederic Sowa wrote:
> Hi Alexei,
> 
> > If user space can be see both 'count' and 'max_entries', it can be very
> > tempting to start assuming 'full' and 'empty' state of the map which will
> > lead to race conditions and bad design.
> > bpf programs and maps are inherently multi-thread and concurrent.
> > If userapp wants to do the counting of elements it needs to do so on its
> > own
> > and shoot itself in the foot eventually.
> > For the same reason I don't want to see BPF_MAP_GET_COUNT command.
> 
> Hmmm... I don't understand your argument. This is the same with memory
> management in general and we still report memory statistics to user
> space. I really would find it helpful to have a feeling if a map is
> nearly full or nearly empty.

memory is not the same, since it's a shared resource and knowledge
about consumption by the process gives no insight whether next malloc()
will succeed or not.

> We can also count collisions or the load in the buckets, but some
> evidence what is going on would be nice, wouldn't it?

reporting collisions may be ok, since it's probably hard to exploit
such stats, but security may become a concern in some use cases,
so may not be such a good idea at the end.

In general when user space passed kernel some numbers (like type,
element size, max_entries) it's ok to report it back via fdinfo.
Anything else I'd rather keep private.
debugging in general should be done by debugging tools.
Like I often use kprobe+bpf to debug networking+bpf :)
Unfortunately kprobe+bpf doesn't work to debug itself, but then
regular tracing and kprobe comes to rescue.

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