Re: [PATCH bpf-next 0/2] bpf: add csum/ip_summed fields to __sk_buff

[Menglong Dong]

On Wed, Jan 3, 2024 at 11:55 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
> On 1/2/24 6:54 PM, Menglong Dong wrote:
> > On Wed, Jan 3, 2024 at 8:52 AM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
> >> On 1/2/24 10:11 AM, Stanislav Fomichev wrote:
> >>> On 12/29, Menglong Dong wrote:
> >>>> For now, we have to call some helpers when we need to update the csum,
> >>>> such as bpf_l4_csum_replace, bpf_l3_csum_replace, etc. These helpers are
> >>>> not inlined, which causes poor performance.
> >>>>
> >>>> In fact, we can define our own csum update functions in BPF program
> >>>> instead of bpf_l3_csum_replace, which is totally inlined and efficient.
> >>>> However, we can't do this for bpf_l4_csum_replace for now, as we can't
> >>>> update skb->csum, which can cause skb->csum invalid in the rx path with
> >>>> CHECKSUM_COMPLETE mode.
> >>>>
> >>>> What's more, we can't use the direct data access and have to use
> >>>> skb_store_bytes() with the BPF_F_RECOMPUTE_CSUM flag in some case, such
> >>>> as modifing the vni in the vxlan header and the underlay udp header has
> >>>> no checksum.
> >> There is bpf_csum_update(), does it work?
> >> A helper call should be acceptable comparing with the csum calculation itself.
> > Yeah, this helper works in this case! Now we miss the last
> > piece for the tx path: ip_summed. We need to know if it is
> > CHECKSUM_PARTIAL to decide if we should update the
> > csum in the packet. In the tx path, the csum in the L4 is the
> > pseudo header only if skb->ip_summed is CHECKSUM_PARTIAL.
> >
> > Maybe we can introduce a lightweight kfunc to get its
> > value? Such as bpf_skb_csum_mode(). As we need only call
> > it once, there shouldn't be overhead on it.
>
> You don't need kfunc, you can do checking like
>    struct sk_buff *kskb = bpf_cast_to_kern_ctx(skb);
>    if (kskb->ip_summed == CHECKSUM_PARTIAL) ...
>    ...
>

Great, this is exactly what I need! Thanks~