Re: [PATCH bpf-next v4 2/3] bpf: Add xdp dynptrs

Martin KaFai Lau
2022-08-26T06:51:23Z

On Thu, Aug 25, 2022 at 11:37:08PM -0700, Martin KaFai Lau wrote:
> On Thu, Aug 25, 2022 at 01:04:16AM +0200, Kumar Kartikeya Dwivedi wrote:
> > On Wed, 24 Aug 2022 at 20:11, Joanne Koong <joannelkoong@gmail.com> wrote:
> > > I'm more and more liking the idea of limiting xdp to match the
> > > constraints of skb given that both you, Kumar, and Jakub have
> > > mentioned that portability between xdp and skb would be useful for
> > > users :)
> > >
> > > What are your thoughts on this API:
> > >
> > > 1) bpf_dynptr_data()
> > >
> > > Before:
> > > for skb-type progs:
> > > - data slices in fragments is not supported
> > >
> > > for xdp-type progs:
> > > - data slices in fragments is supported as long as it is in a
> > > contiguous frag (eg not across frags)
> > >
> > > Now:
> > > for skb + xdp type progs:
> > > - data slices in fragments is not supported
> I don't think it is necessary (or help) to restrict xdp slice from getting
> a fragment. In any case, the xdp prog has to deal with the case
> that bpf_dynptr_data(xdp_dynptr, offset, len) is across two fragments.
> Although unlikely, it still need to handle it (dynptr_data returns NULL)
> properly by using bpf_dynptr_read(). The same that the skb case
> also needs to handle dynptr_data returning NULL.
>
> Something like Kumar's sample in [0] should work for both
> xdp and skb dynptr but replace the helpers with
> bpf_dynptr_{data,read,write}().
>
> [0]: https://lore.kernel.org/bpf/20220726184706.954822-1-joannelkoong@gmail.com/T/#mf082a11403bc76fa56fde4de79a25c660680285c
>
> > >
> > >
> > > 2) bpf_dynptr_write()
> > >
> > > Before:
> > > for skb-type progs:
> > > - all data slices are invalidated after a write
> > >
> > > for xdp-type progs:
> > > - nothing
> > >
> > > Now:
> > > for skb + xdp type progs:
> > > - all data slices are invalidated after a write
> I wonder if the 'Before' behavior can be kept as is.
>
> The bpf prog that runs in both xdp and bpf should be
typo: both xdp and *skb

> the one always expecting the data-slice will be invalidated and
> it has to call the bpf_dynptr_data() again after writing.
> Yes, it is unnecessary for xdp but the bpf prog needs to the
> same anyway if the verifier was the one enforcing it for
> both skb and xdp dynptr type.
>
> If the bpf prog is written for xdp alone, then it has
> no need to re-get the bpf_dynptr_data() after writing.
>
> > >
> >
> > There is also the other option: failing to write until you pull skb,
> > which looks a lot better to me if we are adding this helper anyway...