Jonathan Lemon <jonathan.le...@gmail.com> writes: > On 4 Jun 2019, at 9:43, Jesper Dangaard Brouer wrote: > >> On Mon, 3 Jun 2019 09:38:51 -0700 >> Jonathan Lemon <jonathan.le...@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. > 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.
I think I prefer 2) over 3); since we have a verifier that can actually enforce something like read-only behaviour, actually having access to the value will probably be useful to someone. I can obviously live with 1) as well, of course (since I already did that; though I just now realise that I forgot to make the scratch space per-CPU)... :) -Toke
