Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context

[Stanislav Fomichev <sdf@google.com>]

On Mon, Nov 7, 2022 at 2:02 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>
> On 11/3/22 8:25 PM, Stanislav Fomichev wrote:
> > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 59c9fd55699d..dba857f212d7 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -4217,9 +4217,13 @@ static inline bool skb_metadata_differs(const struct sk_buff *skb_a,
> >              true : __skb_metadata_differs(skb_a, skb_b, len_a);
> >   }
> >
> > +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len);
> > +
> >   static inline void skb_metadata_set(struct sk_buff *skb, u8 meta_len)
> >   {
> >       skb_shinfo(skb)->meta_len = meta_len;
> > +     if (meta_len)
> > +             skb_metadata_import_from_xdp(skb, meta_len);
> >   }
> >
> [ ... ]
>
> > +struct xdp_to_skb_metadata {
> > +     u32 magic; /* xdp_metadata_magic */
> > +     u64 rx_timestamp;
> > +} __randomize_layout;
> > +
> > +struct bpf_patch;
> > +
>
> [ ... ]
>
> > +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len)
> > +{
> > +     struct xdp_to_skb_metadata *meta = (void *)(skb_mac_header(skb) - len);
> > +
> > +     /* Optional SKB info, currently missing:
> > +      * - HW checksum info           (skb->ip_summed)
> > +      * - HW RX hash                 (skb_set_hash)
> > +      * - RX ring dev queue index    (skb_record_rx_queue)
> > +      */
> > +
> > +     if (len != sizeof(struct xdp_to_skb_metadata))
> > +             return;
> > +
> > +     if (meta->magic != xdp_metadata_magic)
> > +             return;
> > +
> > +     if (meta->rx_timestamp) {
> > +             *skb_hwtstamps(skb) = (struct skb_shared_hwtstamps){
> > +                     .hwtstamp = ns_to_ktime(meta->rx_timestamp),
> > +             };
> > +     }
> > +}
>
> Considering the metadata will affect the gro, should the meta be cleared after
> importing to the skb?

Yeah, good suggestion, will clear it here.

> [ ... ]
>
> > +/* Since we're not actually doing a call but instead rewriting
> > + * in place, we can only afford to use R0-R5 scratch registers.
> > + *
> > + * We reserve R1 for bpf_xdp_metadata_export_to_skb and let individual
> > + * metadata kfuncs use only R0,R4-R5.
> > + *
> > + * The above also means we _cannot_ easily call any other helper/kfunc
> > + * because there is no place for us to preserve our R1 argument;
> > + * existing R6-R9 belong to the callee.
> > + */
> > +void xdp_metadata_export_to_skb(const struct bpf_prog *prog, struct bpf_patch *patch)
> > +{
> > +     u32 func_id;
> > +
> > +     /*
> > +      * The code below generates the following:
> > +      *
> > +      * void bpf_xdp_metadata_export_to_skb(struct xdp_md *ctx)
> > +      * {
> > +      *      struct xdp_to_skb_metadata *meta;
> > +      *      int ret;
> > +      *
> > +      *      ret = bpf_xdp_adjust_meta(ctx, -sizeof(*meta));
> > +      *      if (!ret)
> > +      *              return;
> > +      *
> > +      *      meta = ctx->data_meta;
> > +      *      meta->magic = xdp_metadata_magic;
> > +      *      meta->rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
> > +      * }
> > +      *
> > +      */
> > +
> > +     bpf_patch_append(patch,
> > +             /* r2 = ((struct xdp_buff *)r1)->data_meta; */
> > +             BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> > +                         offsetof(struct xdp_buff, data_meta)),
> > +             /* r3 = ((struct xdp_buff *)r1)->data; */
> > +             BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> > +                         offsetof(struct xdp_buff, data)),
> > +             /* if (data_meta != data) return;
> > +              *
> > +              *      data_meta > data: xdp_data_meta_unsupported()
> > +              *      data_meta < data: already used, no need to touch
> > +              */
> > +             BPF_JMP_REG(BPF_JNE, BPF_REG_2, BPF_REG_3, S16_MAX),
> > +
> > +             /* r2 -= sizeof(struct xdp_to_skb_metadata); */
> > +             BPF_ALU64_IMM(BPF_SUB, BPF_REG_2,
> > +                           sizeof(struct xdp_to_skb_metadata)),
> > +             /* r3 = ((struct xdp_buff *)r1)->data_hard_start; */
> > +             BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> > +                         offsetof(struct xdp_buff, data_hard_start)),
> > +             /* r3 += sizeof(struct xdp_frame) */
> > +             BPF_ALU64_IMM(BPF_ADD, BPF_REG_3,
> > +                           sizeof(struct xdp_frame)),
> > +             /* if (data-sizeof(struct xdp_to_skb_metadata) < data_hard_start+sizeof(struct xdp_frame)) return; */
> > +             BPF_JMP_REG(BPF_JLT, BPF_REG_2, BPF_REG_3, S16_MAX),
> > +
> > +             /* ((struct xdp_buff *)r1)->data_meta = r2; */
> > +             BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2,
> > +                         offsetof(struct xdp_buff, data_meta)),
> > +
> > +             /* *((struct xdp_to_skb_metadata *)r2)->magic = xdp_metadata_magic; */
> > +             BPF_ST_MEM(BPF_W, BPF_REG_2,
> > +                        offsetof(struct xdp_to_skb_metadata, magic),
> > +                        xdp_metadata_magic),
> > +     );
> > +
> > +     /*      r0 = bpf_xdp_metadata_rx_timestamp(ctx); */
> > +     func_id = xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_TIMESTAMP);
> > +     prog->aux->xdp_kfunc_ndo->ndo_unroll_kfunc(prog, func_id, patch);
> > +
> > +     bpf_patch_append(patch,
> > +             /* r2 = ((struct xdp_buff *)r1)->data_meta; */
> > +             BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> > +                         offsetof(struct xdp_buff, data_meta)),
> > +             /* *((struct xdp_to_skb_metadata *)r2)->rx_timestamp = r0; */
> > +             BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_0,
> > +                         offsetof(struct xdp_to_skb_metadata, rx_timestamp)),
>
> Can the xdp prog still change the metadata through xdp->data_meta? tbh, I am not
> sure it is solid enough by asking the xdp prog not to use the same random number
> in its own metadata + not to change the metadata through xdp->data_meta after
> calling bpf_xdp_metadata_export_to_skb().

What do you think the usecase here might be? Or are you suggesting we
reject further access to data_meta after
bpf_xdp_metadata_export_to_skb somehow?

If we want to let the programs override some of this
bpf_xdp_metadata_export_to_skb() metadata, it feels like we can add
more kfuncs instead of exposing the layout?

bpf_xdp_metadata_export_to_skb(ctx);
bpf_xdp_metadata_export_skb_hash(ctx, 1234);
...

> Does xdp_to_skb_metadata have a use case for XDP_PASS (like patch 7) or the
> xdp_to_skb_metadata can be limited to XDP_REDIRECT only?

XDP_PASS cases where we convert xdp_buff into skb in the drivers right
now usually have C code to manually pull out the metadata (out of hw
desc) and put it into skb.

So, currently, if we're calling bpf_xdp_metadata_export_to_skb() for
XDP_PASS, we're doing a double amount of work:
skb_metadata_import_from_xdp first, then custom driver code second.

In theory, maybe we should completely skip drivers custom parsing when
there is a prog with BPF_F_XDP_HAS_METADATA?
Then both xdp->skb paths (XDP_PASS+XDP_REDIRECT) will be bpf-driven
and won't require any mental work (plus, the drivers won't have to
care either in the future).

WDYT?

> > +     );
> > +
> > +     bpf_patch_resolve_jmp(patch);
> > +}
> > +
> >   static int __init xdp_metadata_init(void)
> >   {
> > +     xdp_metadata_magic = get_random_u32() | 1;
> >       return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> >   }
> >   late_initcall(xdp_metadata_init);