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Subject: Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup_elem() on an
 xskmap
To:     Jonathan Lemon <jonathan.lemon@gmail.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>
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        magnus.karlsson@intel.com, ast@kernel.org, daniel@iogearbox.net
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On 2019-06-04 19:25, Jonathan Lemon wrote:
> On 4 Jun 2019, at 9:43, Jesper Dangaard Brouer wrote:
> 
>> On Mon, 3 Jun 2019 09:38:51 -0700
>> Jonathan Lemon <jonathan.lemon@gmail.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.
>

Hmm, what other info would be useful? ifindex? Or going the the
other way, with read-only and just returning boolean?

> 
>> 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.
>

Ugh, good thing Daniel found this!


Björn

> 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.
>