netdev - Re: [PATCH bpf-next v2 15/16] bpf: Realign skb metadata for TC progs using data

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Message-ID: <CAMB2axPivi+mZOXie=VnJM8nscqkHDjSrKT=Dhp5z_copEwxLQ@mail.gmail.com>
Date: Mon, 5 Jan 2026 11:42:48 -0800
From: Amery Hung <ameryhung@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Jakub Sitnicki <jakub@...udflare.com>, Martin KaFai Lau <martin.lau@...nel.org>, 
	bpf <bpf@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Stanislav Fomichev <sdf@...ichev.me>, 
	Simon Horman <horms@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	kernel-team <kernel-team@...udflare.com>
Subject: Re: [PATCH bpf-next v2 15/16] bpf: Realign skb metadata for TC progs
 using data_meta

On Mon, Jan 5, 2026 at 11:14 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Jan 5, 2026 at 4:15 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
> >
> >
> > +__bpf_kfunc_start_defs();
> > +
> > +__bpf_kfunc void bpf_skb_meta_realign(struct __sk_buff *skb_)
> > +{
> > +       struct sk_buff *skb = (typeof(skb))skb_;
> > +       u8 *meta_end = skb_metadata_end(skb);
> > +       u8 meta_len = skb_metadata_len(skb);
> > +       u8 *meta;
> > +       int gap;
> > +
> > +       gap = skb_mac_header(skb) - meta_end;
> > +       if (!meta_len || !gap)
> > +               return;
> > +
> > +       if (WARN_ONCE(gap < 0, "skb metadata end past mac header")) {
> > +               skb_metadata_clear(skb);
> > +               return;
> > +       }
> > +
> > +       meta = meta_end - meta_len;
> > +       memmove(meta + gap, meta, meta_len);
> > +       skb_shinfo(skb)->meta_end += gap;
> > +
> > +       bpf_compute_data_pointers(skb);
> > +}
> > +
> > +__bpf_kfunc_end_defs();
> > +
> > +BTF_KFUNCS_START(tc_cls_act_hidden_ids)
> > +BTF_ID_FLAGS(func, bpf_skb_meta_realign)
> > +BTF_KFUNCS_END(tc_cls_act_hidden_ids)
> > +
> > +BTF_ID_LIST_SINGLE(bpf_skb_meta_realign_ids, func, bpf_skb_meta_realign)
> > +
> >  static int tc_cls_act_prologue(struct bpf_insn *insn_buf, u32 pkt_access_flags,
> >                                const struct bpf_prog *prog)
> >  {
> > -       return bpf_unclone_prologue(insn_buf, pkt_access_flags, prog,
> > -                                   TC_ACT_SHOT);
> > +       struct bpf_insn *insn = insn_buf;
> > +       int cnt;
> > +
> > +       if (pkt_access_flags & PA_F_DATA_META_LOAD) {
> > +               /* Realign skb metadata for access through data_meta pointer.
> > +                *
> > +                * r6 = r1; // r6 will be "u64 *ctx"
> > +                * r0 = bpf_skb_meta_realign(r1); // r0 is undefined
> > +                * r1 = r6;
> > +                */
> > +               *insn++ = BPF_MOV64_REG(BPF_REG_6, BPF_REG_1);
> > +               *insn++ = BPF_CALL_KFUNC(0, bpf_skb_meta_realign_ids[0]);
> > +               *insn++ = BPF_MOV64_REG(BPF_REG_1, BPF_REG_6);
> > +       }
>
> I see that we already did this hack with bpf_qdisc_init_prologue()
> and bpf_qdisc_reset_destroy_epilogue().
> Not sure why we went that route back then.
>
> imo much cleaner to do BPF_EMIT_CALL() and wrap
> BPF_CALL_1(bpf_skb_meta_realign, struct sk_buff *, skb)
>
> BPF_CALL_x doesn't make it an uapi helper.
> It's still a hidden kernel function,
> while this kfunc stuff looks wrong, since kfunc isn't really hidden.
>
> I suspect progs can call this bpf_skb_meta_realign() explicitly,
> just like they can call bpf_qdisc_init_prologue() ?
>

qdisc prologue and epilogue qdisc kfuncs should be hidden from users.
The kfunc filter, bpf_qdisc_kfunc_filter(), determines what kfunc are
actually exposed.

BPF_CALL_x is simpler as there is no need for a kfunc filter to hide
it. However, IMO for qdisc they don't make too much difference since
bpf qdisc already needs the filter to limit .enqueue and .dequeue
specific kfunc.

Am I missing anything?

> cc Amery, Martin.
>