gen_type_bindings.c errors on unsupported target

[jrfondren]

I'm targeting Linux 5.14 on AlmaLinux 9, and intending to use older LSM hooks, and everything basically works fine except that the generated bindings include some unsupported symbols, with errors like

gen_type_bindings.c: In function 'main':
gen_type_bindings.c:555:58: error: 'BPF_MAP_TYPE_USER_RINGBUF' undeclared (first use in this function); did you mean 'BPF_MAP_TYPE_RINGBUF'?
  555 |      enum { check_BPF_MAP_TYPE_USER_RINGBUF_const = (int)BPF_MAP_TYPE_USER_RINGBUF };
      |                                                          ^~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                          BPF_MAP_TYPE_RINGBUF
gen_type_bindings.c:555:58: note: each undeclared identifier is reported only once for each function it appears in

Quick workaround: adding print_endline "#define BPF_MAP_TYPE_USER_RINGBUF 0"; to the top of src/bindings/stubgen/gen_type_bindings.ml

In jrfondren@45bd02b I added the kernel filtering I need for this target, but a more thorough solution could associate a kernel version with everything from https://github.com/isovalent/ebpf-docs/blob/master/docs/linux/timeline/index.md

[koonwen]

The reason is because we are supporting libbpf v1.1 and above. v1.0.0 is missing the BPF_MAP_TYPE_USER_RINGBUF as you've noticed. So I'm not keen on the quick workaround because we don't expect to support libbpf 1.0.0.

However, if you'd like to work on backporting proper support for 1.0.0 I'm happy to help and review!

[jrfondren]

Oh, I was building with libbpf v1.5. Using newer version of libbpf is no problem I guess as the library tries hard to support a wide range of kernels.