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

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Message-ID: <CAADnVQKB5vRJM4kJC5515snR6KHweE-Ld_W1wWgPSWATgiUCwg@mail.gmail.com>
Date: Mon, 5 Jan 2026 13:47:16 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Amery Hung <ameryhung@...il.com>, 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>, 
	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 12:55 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
>
>
> On 1/5/26 11:42 AM, Amery Hung wrote:
> > 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.
>
> Similar to Amery's comment, I recalled I tried the BPF_CALL_1 in the
> qdisc but stopped at the "fn = env->ops->get_func_proto(insn->imm,
> env->prog);" in do_misc_fixups(). Potentially it could add new enum ( >
> __BPF_FUNC_MAX_ID) outside of the uapi and the user space tool should be
> able to handle unknown helper also but we went with the kfunc+filter
> approach without thinking too much about it.

hmm. BPF_EMIT_CALL() does:
#define BPF_CALL_IMM(x) ((void *)(x) - (void *)__bpf_call_base)
.imm   = BPF_CALL_IMM(FUNC)

the imm shouldn't be going through validation anymore.
none of the if (insn->imm == BPF_FUNC_...) in do_misc_fixups()
will match, so I think I see the path where get_func_proto() is
called.
But how does it work then for all cases of BPF_EMIT_CALL?
All of them happen after do_misc_fixups() ?

I guess we can mark such emitted call in insn_aux_data as finalized
and get_func_proto() isn't needed.