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

From: Martin KaFai Lau <[email-masked]>
Date: Mon, 5 Jan 2026 12:54:55 -0800

On 1/5/26 11:42 AM, Amery Hung wrote:
> On Mon, Jan 5, 2026 at 11:14 AM Alexei Starovoitov
> <[email-masked]> wrote:
>>
>> On Mon, Jan 5, 2026 at 4:15 AM Jakub Sitnicki <[email-masked]> 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.

https://lore.kernel.org/bpf/[email-masked]/