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

[Larysa Zaremba]

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?