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

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Date:   Wed, 5 Jun 2019 10:45:56 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Martin Lau <kafai@...com>
Cc:     Jonathan Lemon <jonathan.lemon@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH v4 bpf-next 1/2] bpf: Allow bpf_map_lookup_elem() on an xskmap

On Tue, 4 Jun 2019 at 20:13, Martin Lau <kafai@...com> wrote:
>
> 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@...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.
> 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.
>

Doesn't the PTR_TO_SOCKET path involve taking a ref and mandating
sk_release() from the fast path? :-(


Björn


> > 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