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>> + >> + return 0; >> +} >> +late_initcall(bpf_emit_btf_type); > > I think if we burn a dummy function on this it would be a wrong > pattern to follow. Maybe we can pick another initcall to piggyback? > This is just a nop C statement. > Typically we add BUILD_BUG_ON near the places that rely on that constraint. > There is such a function already. It's tcp_set_state() as you pointed out. > It's not using BTF of course, but I would move above BTF_TYPE_EMIT_ENUM there. > I'm not sure why you're calling it "pollute net/ipv4/tcp.c". This is the minor reason. I first coded in that place and feel awkward where we have macro referenced above and we still emit a BTF_TYPE_EMIT below although with some comments. The major reason is I think we may have some uapi type/enum's (e.g., in uapi/linux/bpf.h) which will be used in bpf program but not in kernel itself. So we cannot generate types in vmlinux btf because of this. So I used this case to find a place to generate these btf types. BPF_TCP_CLOSE is actually such an example, it happens we have a BUILD_BUG_ON in kernel to access it. Maybe I am too forward looking? > Consider that BTF_TYPE_EMIT macro is serving similar purpose > and it's scattered around net/core/filter.c