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

On 1/5/26 1:47 PM, Alexei Starovoitov wrote:

On Mon, Jan 5, 2026 at 12:55 PM Martin KaFai Lau [off-list ref] wrote:

quoted

On 1/5/26 11:42 AM, Amery Hung wrote:

quoted

On Mon, Jan 5, 2026 at 11:14 AM Alexei Starovoitov
[off-list ref] wrote:

quoted

On Mon, Jan 5, 2026 at 4:15 AM Jakub Sitnicki [off-list ref] wrote:

quoted

+__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.