Subject: Re: [PATCH net-next] bpf: add show_fdinfo handler for maps

From: John Fastabend <[email-masked]>
Date: Mon, 23 Nov 2015 11:09:49 -0800

On 15-11-23 10:03 AM, Alexei Starovoitov wrote:
> On Mon, Nov 23, 2015 at 05:11:58PM +0100, Hannes Frederic Sowa wrote:
>>
>> Actually, that is the reason why I mentioned it, so *the admin* can see
>> something is going on. Do you want to protect ebpf from root? Skynet? ;)
> 
> correct. To me both root and non-root are users in the first place and
> they both shouldn't be allowed to misuse it.
> 
>> In my opinion the kernel never should hide any information of the admin
>> if they are accessible easily. Sampling the number of failed updates to
>> a map or printing it via procfs/ebpffs seems to be just a matter of how
>> difficult it should be done. The map has a lock, so the number is fairly
> 
> map_lookup is actually lockless. It's a critical path and should be
> as fast as possible. No extra stats just for debugging.
> 
>> accurate. Sampling and plotting size of hash maps without having kprobes
>> installed would be a nice thing, because it reduces complexity and this
>> is nice to have.
> 
> doing 'cat' from procfs is, of course, easier to use, but it's an extra
> code that permenanetly lives in memory, whereas kprobe+bpf is a run-time
> debugging.

Hopefully not jumping in off-base here (I've read most the thread), but
what I've been doing is loading programs with debug ebpf code in them
to keep a statistics map(s) and then I read that from  userspace for
stats. It works pretty well and lets me compile out the debug code when
I want and also doesn't need kprobe at all. Also I can implement
sampling so that the debug code only runs every .01% or something like
that so it can be used in "real" systems. My "real" systems are just a
couple node test setup but it seems to be ok ;)

.John

> 
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