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

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Message-ID: <CAADnVQJbGosoXOCdyi=NZar966FVibKYobBgQ9BiyEH3=-HOsw@mail.gmail.com>
Date: Mon, 5 Jan 2026 11:14:32 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>, Amery Hung <ameryhung@...il.com>, 
	Martin KaFai Lau <martin.lau@...nel.org>
Cc: 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 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() ?

cc Amery, Martin.