[xdp-hints] Re: [PATCH bpf-next 06/11] xdp: Carry over xdp metadata into skb context

[Stanislav Fomichev <sdf@google.com>]

On Tue, Nov 15, 2022 at 11:04 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>
> On 11/14/22 7:02 PM, Stanislav Fomichev wrote:
> > Implement new bpf_xdp_metadata_export_to_skb kfunc which
> > prepares compatible xdp metadata for kernel consumption.
> > This kfunc should be called prior to bpf_redirect
> > or when XDP_PASS'ing the frame into the kernel (note, the drivers
> > have to be updated to enable consuming XDP_PASS'ed metadata).
> >
> > veth driver is amended to consume this metadata when converting to skb.
> >
> > Internally, XDP_FLAGS_HAS_SKB_METADATA flag is used to indicate
> > whether the frame has skb metadata. The metadata is currently
> > stored prior to xdp->data_meta. bpf_xdp_adjust_meta refuses
> > to work after a call to bpf_xdp_metadata_export_to_skb (can lift
> > this requirement later on if needed, we'd have to memmove
> > xdp_skb_metadata).
>
> It is ok to refuse bpf_xdp_adjust_meta() after bpf_xdp_metadata_export_to_skb()
> for now.  However, it will also need to refuse bpf_xdp_adjust_head().

Good catch!

> [ ... ]
>
> > +/* For the packets directed to the kernel, this kfunc exports XDP metadata
> > + * into skb context.
> > + */
> > +noinline int bpf_xdp_metadata_export_to_skb(const struct xdp_md *ctx)
> > +{
> > +     return 0;
> > +}
> > +
>
> I think it is still better to return 'struct xdp_skb_metata *' instead of
> true/false.  Like:
>
> noinline struct xdp_skb_metata *bpf_xdp_metadata_export_to_skb(const struct
> xdp_md *ctx)
> {
>         return 0;
> }
>
> The KF_RET_NULL has already been set in
> BTF_SET8_START_GLOBAL(xdp_metadata_kfunc_ids).  There is
> "xdp_btf_struct_access()" that can allow write access to 'struct xdp_skb_metata'
> What else is missing? We can try to solve it.

Ah, that KF_RET_NULL is a left-over from my previous attempt to return
pointers :-)
I can retry with returning a pointer, I don't have a preference, but I
felt like returning simple -errno might be more simple api-wise.
(with bpf_xdp_metadata_export_to_skb returning NULL vs return loggable
errno - I'd prefer the latter, but not strongly)

> Then there is no need for this double check in patch 8 selftest which is not
> easy to use:
>
> +               if (bpf_xdp_metadata_export_to_skb(ctx) < 0) {
> +                       bpf_printk("bpf_xdp_metadata_export_to_skb failed");
> +                       return XDP_DROP;
> +               }
>
> [ ... ]
>
> +               skb_metadata = ctx->skb_metadata;
> +               if (!skb_metadata) {
> +                       bpf_printk("no ctx->skb_metadata");
> +                       return XDP_DROP;
> +               }
>
> [ ... ]
>
>
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index b444b1118c4f..71e3bc7ad839 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -6116,6 +6116,12 @@ enum xdp_action {
> >       XDP_REDIRECT,
> >   };
> >
> > +/* Subset of XDP metadata exported to skb context.
> > + */
> > +struct xdp_skb_metadata {
> > +     __u64 rx_timestamp;
> > +};
> > +
> >   /* user accessible metadata for XDP packet hook
> >    * new fields must be added to the end of this structure
> >    */
> > @@ -6128,6 +6134,7 @@ struct xdp_md {
> >       __u32 rx_queue_index;  /* rxq->queue_index  */
> >
> >       __u32 egress_ifindex;  /* txq->dev->ifindex */
> > +     __bpf_md_ptr(struct xdp_skb_metadata *, skb_metadata);
>
> Once the above bpf_xdp_metadata_export_to_skb() returning a pointer works, then
> it can be another kfunc 'struct xdp_skb_metata * bpf_xdp_get_skb_metadata(const
> struct xdp_md *ctx)' to return the skb_metadata which was a similar point
> discussed in the previous RFC.

I see. I think you've mentioned it previously somewhere to have a
kfunc accessor vs uapi field and I totally forgot. Will try to address
it, ty!