[xdp-hints] Re: [PATCH bpf-next v2 2/8] bpf: XDP metadata RX kfuncs

[Stanislav Fomichev <sdf@google.com>]

 s

On Fri, Nov 25, 2022 at 9:53 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Stanislav Fomichev <sdf@google.com> writes:
>
> > There is an ndo handler per kfunc, the verifier replaces a call to the
> > generic kfunc with a call to the per-device one.
> >
> > For XDP, we define a new kfunc set (xdp_metadata_kfunc_ids) which
> > implements all possible metatada kfuncs. Not all devices have to
> > implement them. If kfunc is not supported by the target device,
> > the default implementation is called instead.
>
> BTW, this "the default implementation is called instead" bit is not
> included in this version... :)

fixup_xdp_kfunc_call should return 0 when the device doesn't have a
kfunc defined and should fallback to the default kfunc implementation,
right?
Or am I missing something?

> [...]
>
> > +#ifdef CONFIG_DEBUG_INFO_BTF
> > +BTF_SET8_START(xdp_metadata_kfunc_ids)
> > +#define XDP_METADATA_KFUNC(name, str) BTF_ID_FLAGS(func, str, 0)
> > +XDP_METADATA_KFUNC_xxx
> > +#undef XDP_METADATA_KFUNC
> > +BTF_SET8_END(xdp_metadata_kfunc_ids)
> > +
> > +static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
> > +     .owner = THIS_MODULE,
> > +     .set   = &xdp_metadata_kfunc_ids,
> > +};
> > +
> > +u32 xdp_metadata_kfunc_id(int id)
> > +{
> > +     return xdp_metadata_kfunc_ids.pairs[id].id;
> > +}
> > +EXPORT_SYMBOL_GPL(xdp_metadata_kfunc_id);
>
> So I was getting some really weird values when testing (always getting a
> timestamp value of '1'), and it turns out to be because this way of
> looking up the ID doesn't work: The set is always sorted by the BTF ID,
> not the order it was defined. Which meant that the mapping code got the
> functions mixed up, and would call a different one instead (so the
> timestamp value I was getting was really the return value of
> rx_hash_enabled()).
>
> I fixed it by building a secondary lookup table as below; feel free to
> incorporate that (or if you can come up with a better way, go ahead!).

Interesting, will take a closer look. I took this pattern from
BTF_SOCK_TYPE_xxx, which means that 'sorting by btf-id' is something
BTF_SET8_START specific...
But if it's sorted, probably easier to do a bsearch over this table
than to build another one?

> -Toke
>
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index e43f7d4ef4cf..dc0a9644dacc 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -738,6 +738,15 @@ XDP_METADATA_KFUNC_xxx
>  #undef XDP_METADATA_KFUNC
>  BTF_SET8_END(xdp_metadata_kfunc_ids)
>
> +static struct xdp_metadata_kfunc_map {
> +       const char *fname;
> +       u32 btf_id;
> +} xdp_metadata_kfunc_lookup_map[MAX_XDP_METADATA_KFUNC] = {
> +#define XDP_METADATA_KFUNC(name, str) { .fname = __stringify(str) },
> +XDP_METADATA_KFUNC_xxx
> +#undef XDP_METADATA_KFUNC
> +};
> +
>  static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
>         .owner = THIS_MODULE,
>         .set   = &xdp_metadata_kfunc_ids,
> @@ -745,13 +754,41 @@ static const struct btf_kfunc_id_set xdp_metadata_kfunc_set = {
>
>  u32 xdp_metadata_kfunc_id(int id)
>  {
> -       return xdp_metadata_kfunc_ids.pairs[id].id;
> +       return xdp_metadata_kfunc_lookup_map[id].btf_id;
>  }
>  EXPORT_SYMBOL_GPL(xdp_metadata_kfunc_id);
>
>  static int __init xdp_metadata_init(void)
>  {
> -       return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> +       const struct btf *btf;
> +       int i, j, ret;
> +
> +       ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> +       if (ret)
> +               return ret;
> +
> +       btf = bpf_get_btf_vmlinux();
> +
> +       for (i = 0; i < MAX_XDP_METADATA_KFUNC; i++) {
> +               u32 btf_id = xdp_metadata_kfunc_ids.pairs[i].id;
> +               const struct btf_type *t;
> +               const char *name;
> +
> +               t = btf_type_by_id(btf, btf_id);
> +               if (WARN_ON_ONCE(!t || !t->name_off))
> +                       continue;
> +
> +               name = btf_name_by_offset(btf, t->name_off);
> +
> +               for (j = 0; j < MAX_XDP_METADATA_KFUNC; j++) {
> +                       if (!strcmp(name, xdp_metadata_kfunc_lookup_map[j].fname)) {
> +                               xdp_metadata_kfunc_lookup_map[j].btf_id = btf_id;
> +                               break;
> +                       }
> +               }
> +       }
> +
> +       return 0;
>  }
>  late_initcall(xdp_metadata_init);
>  #else /* CONFIG_DEBUG_INFO_BTF */
>