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Message-ID: <CAMB2axM+Z9npytoRDb-D1xVQSSx__nW0GOPMOP_uMNU-ZE=AZA@mail.gmail.com>
Date: Mon, 5 Jan 2026 15:19:44 -0800
From: Amery Hung <ameryhung@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@...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 2:26 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
>
>
> On 1/5/26 1:47 PM, Alexei Starovoitov wrote:
> > 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() ?
>
> 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.
>
Hmm, insn_aux_data has to be marked in gen_{pro,epi}logue since this
is the only place we know whether the call needs fixup or not. However
insn_aux_data is not available yet in gen_{pro,epi}logue because we
haven't resized insn_aux_data.
Can we do some hack based on the fact that calls emitted by
BPF_EMIT_CALL() are finalized while calls emitted by BPF_RAW_INSN()
most likely are not?
Let BPF_EMIT_CALL() mark the call insn as finalized temporarily (e.g.,
.off = 1). Then, when do_misc_fixups() encounters it just reset off to
0 and don't call get_func_proto().
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