netdev - Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup

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Date:   Tue, 04 Jun 2019 10:25:23 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Jesper Dangaard Brouer" <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, kernel-team@...com, bjorn.topel@...el.com,
        magnus.karlsson@...el.com, ast@...nel.org, daniel@...earbox.net
Subject: Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup_elem() on an
 xskmap

On 4 Jun 2019, at 9:43, Jesper Dangaard Brouer wrote:

> On Mon, 3 Jun 2019 09:38:51 -0700
> Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
>> Currently, the AF_XDP code uses a separate map in order to
>> determine if an xsk is bound to a queue.  Instead of doing this,
>> have bpf_map_lookup_elem() return the queue_id, as a way of
>> indicating that there is a valid entry at the map index.
>
> Just a reminder, that once we choose a return value, there the
> queue_id, then it basically becomes UAPI, and we cannot change it.

Yes - Alexei initially wanted to return the sk_cookie instead, but
that's 64 bits and opens up a whole other can of worms.

> Can we somehow use BTF to allow us to extend this later?
>
> I was also going to point out that, you cannot return a direct pointer
> to queue_id, as BPF-prog side can modify this... but Daniel already
> pointed this out.

So, I see three solutions here (for this and Toke's patchset also,
which is encountering the same problem).

1) add a scratch register (Toke's approach)
2) add a PTR_TO_<type>, which has the access checked.  This is the most
   flexible approach, but does seem a bit overkill at the moment.
3) add another helper function, say, bpf_map_elem_present() which just
   returns a boolean value indicating whether there is a valid map entry
   or not.

I was starting to do 2), but wanted to get some more feedback first.
-- 
Jonathan