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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 v3 16/17] bpf: Realign skb metadata for TC progs using data_meta In-Reply-To: (Alexei Starovoitov's message of "Wed, 7 Jan 2026 14:01:12 -0800") References: <20260107-skb-meta-safeproof-netdevs-rx-only-v3-0-0d461c5e4764@cloudflare.com> <20260107-skb-meta-safeproof-netdevs-rx-only-v3-16-0d461c5e4764@cloudflare.com> Date: Thu, 08 Jan 2026 20:54:54 +0100 Message-ID: <87a4ynj2wh.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: bpf@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 Wed, Jan 07, 2026 at 02:01 PM -08, Alexei Starovoitov wrote: > On Wed, Jan 7, 2026 at 6:28=E2=80=AFAM Jakub Sitnicki wrote: >> >> >> +static void bpf_skb_meta_realign(struct sk_buff *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); >> +} >> + >> 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; >> + */ >> + BUILD_BUG_ON(!__same_type(&bpf_skb_meta_realign, >> + (void (*)(struct sk_buff *))NU= LL)); >> + *insn++ =3D BPF_MOV64_REG(BPF_REG_6, BPF_REG_1); >> + *insn++ =3D BPF_EMIT_CALL(bpf_skb_meta_realign); > > Not quite. drop this BUILD_BUG_ON(), since it's pointless and misleading. Will do. > bpf_skb_meta_realign() has to be the one done with BPF_CALL_1(...). > Otherwise above will work only on x86. > In this case on arm64 too, but it's by accident. > BPF_CALL* has to do ABI conversion from BPF to native. > > For kfuncs that's what JITs do via btf_func_model machinery. Oh, 32-bit ARM has different register mapping. TIL. Thanks.