Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup_elem() on an xskmap

[Martin Lau <kafai@fb.com>]

On Tue, Jun 04, 2019 at 10:25:23AM -0700, 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.
> 
> 
> > 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.
I think it would be nice and more extensible to have PTR_TO_xxx.
It could start with the existing PTR_TO_SOCKET

or starting with a new PTR_TO_XDP_SOCK from the beginning is also fine.

> 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