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 14:25:40 -0800

On 1/5/26 1:47 PM, Alexei Starovoitov wrote:
> On Mon, Jan 5, 2026 at 12:55 PM Martin KaFai Lau <[email-masked]> wrote:
>>
>>
>>
>> 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.
> 
> 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() ?

yeah, I think most (all?) of them (e.g. map_gen_lookup) happens in 
do_misc_fixups which then does "goto next_insn;" to skip the 
get_func_proto().

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

It is a good idea.