Re: [PATCH bpf-next] bpf: Allow to use kfunc XDP hints and frags together

[Larysa Zaremba]

On Thu, Sep 14, 2023 at 10:05:47AM -0700, Stanislav Fomichev wrote:
> On Thu, Sep 14, 2023 at 9:55 AM Larysa Zaremba <larysa.zaremba@xxxxxxxxx> wrote:
> >
> > On Thu, Sep 14, 2023 at 09:29:57AM -0700, Stanislav Fomichev wrote:
> > > On 09/14, Larysa Zaremba wrote:
> > > > There is no fundamental reason, why multi-buffer XDP and XDP kfunc RX hints
> > > > cannot coexist in a single program.
> > > >
> > > > Allow those features to be used together by modifying the flags conditions.
> > > >
> > > > Suggested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> > > > Link: https://lore.kernel.org/bpf/CAKH8qBuzgtJj=OKMdsxEkyML36VsAuZpcrsXcyqjdKXSJCBq=Q@xxxxxxxxxxxxxx/
> > > > Signed-off-by: Larysa Zaremba <larysa.zaremba@xxxxxxxxx>
> > > > ---
> > > >  kernel/bpf/offload.c | 6 +++++-
> > > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
> > > > index ee35f33a96d1..43aded96c79b 100644
> > > > --- a/kernel/bpf/offload.c
> > > > +++ b/kernel/bpf/offload.c
> > > > @@ -232,7 +232,11 @@ int bpf_prog_dev_bound_init(struct bpf_prog *prog, union bpf_attr *attr)
> > > >         attr->prog_type != BPF_PROG_TYPE_XDP)
> > > >             return -EINVAL;
> > > >
> > > > -   if (attr->prog_flags & ~BPF_F_XDP_DEV_BOUND_ONLY)
> > > > +   if (attr->prog_flags & ~(BPF_F_XDP_DEV_BOUND_ONLY | BPF_F_XDP_HAS_FRAGS))
> > > > +           return -EINVAL;
> > > > +
> > >
> > > [..]
> > >
> > > > +   if (attr->prog_flags & BPF_F_XDP_HAS_FRAGS &&
> > > > +       !(attr->prog_flags & BPF_F_XDP_DEV_BOUND_ONLY))
> > > >             return -EINVAL;
> > >
> > > Any reason we have 'attr->prog_flags & BPF_F_XDP_HAS_FRAGS' part here?
> > > Seems like doing '!(attr->prog_flags & BPF_F_XDP_DEV_BOUND_ONLY)' should
> > > be enough, right? We only want to bail out here when BPF_F_XDP_DEV_BOUND_ONLY
> > > is not set and we don't really care whether BPF_F_XDP_HAS_FRAGS is set
> > > or not at this point.
> >
> > If !(attr->prog_flags & BPF_F_XDP_DEV_BOUND_ONLY) at this point, program could
> > be requesting offload.
> >
> > Now I have thought about those conditions once more and they could be reduced to
> > this:
> >
> > if (attr->prog_flags & ~(BPF_F_XDP_DEV_BOUND_ONLY) &&
> >     attr->prog_flags != (BPF_F_XDP_DEV_BOUND_ONLY | BPF_F_XDP_HAS_FRAGS))
> >         return -EINVAL;
> >
> > What do you think?
> 
> Ah, so this check is here to protect against the mbuf+offloaded
> combination? (looking at that other thread with Maciej)
> Let's keep your current way with two separate checks, but let's add
> your "/* Frags are allowed only if program is dev-bound-only, but not
> if it is requesting
> bpf offload. */" as a comment to the second check?

Ok, sound good to me.