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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Simon Horman , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Hao Luo , Jiri Olsa , kernel-team Subject: Re: [PATCH bpf-next v2 15/16] bpf: Realign skb metadata for TC progs using data_meta In-Reply-To: (Alexei Starovoitov's message of "Mon, 5 Jan 2026 11:14:32 -0800") References: <20260105-skb-meta-safeproof-netdevs-rx-only-v2-0-a21e679b5afa@cloudflare.com> <20260105-skb-meta-safeproof-netdevs-rx-only-v2-15-a21e679b5afa@cloudflare.com> Date: Mon, 05 Jan 2026 20:20:53 +0100 Message-ID: <87bjj7988a.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Mon, Jan 05, 2026 at 11:14 AM -08, Alexei Starovoitov wrote: > On Mon, Jan 5, 2026 at 4:15=E2=80=AFAM Jakub Sitnicki wrote: >> >> >> +__bpf_kfunc_start_defs(); >> + >> +__bpf_kfunc void bpf_skb_meta_realign(struct __sk_buff *skb_) >> +{ >> + struct sk_buff *skb =3D (typeof(skb))skb_; >> + u8 *meta_end =3D skb_metadata_end(skb); >> + u8 meta_len =3D skb_metadata_len(skb); >> + u8 *meta; >> + int gap; >> + >> + gap =3D 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 =3D meta_end - meta_len; >> + memmove(meta + gap, meta, meta_len); >> + skb_shinfo(skb)->meta_end +=3D 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_acces= s_flags, >> const struct bpf_prog *prog) >> { >> - return bpf_unclone_prologue(insn_buf, pkt_access_flags, prog, >> - TC_ACT_SHOT); >> + struct bpf_insn *insn =3D insn_buf; >> + int cnt; >> + >> + if (pkt_access_flags & PA_F_DATA_META_LOAD) { >> + /* Realign skb metadata for access through data_meta poi= nter. >> + * >> + * r6 =3D r1; // r6 will be "u64 *ctx" >> + * r0 =3D bpf_skb_meta_realign(r1); // r0 is undefined >> + * r1 =3D r6; >> + */ >> + *insn++ =3D BPF_MOV64_REG(BPF_REG_6, BPF_REG_1); >> + *insn++ =3D BPF_CALL_KFUNC(0, bpf_skb_meta_realign_ids[0= ]); >> + *insn++ =3D 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. Right. That's where I "stole" it from. > 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() ? Thanks for pointing me in the right direction. Will rework this.