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Message-ID: <20220826065059.bwoorr6mwppgop57@kafai-mbp.dhcp.thefacebook.com> Date: Thu, 25 Aug 2022 23:50:59 -0700 From: Martin KaFai Lau <kafai@...com> To: Joanne Koong <joannelkoong@...il.com> Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, Toke Høiland-Jørgensen <toke@...hat.com>, bpf@...r.kernel.org, andrii@...nel.org, daniel@...earbox.net, ast@...nel.org, kuba@...nel.org, netdev@...r.kernel.org, "brouer@...hat.com" <brouer@...hat.com>, lorenzo@...nel.org Subject: Re: [PATCH bpf-next v4 2/3] bpf: Add xdp dynptrs 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@...il.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...
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