On Wed, Jan 20, 2021 at 7:27 AM Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: > > >> Would it make sense with some kind of BPF-specific "supported > >> features" mechanism? Something else with a bigger scope (whole > >> kernel)? > > > > Heh, in my opinion, yeah. Seems like we'll finally get it for XDP, but > > for BPF in general the approach has always been probing AFAICT. > > > > For the particular case of arguments to helpers, I suppose the verifier > > could technically validate value ranges for flags arguments, say. That > > would be nice as an early reject anyway, but I'm not sure if it is > > possible to add after-the-fact without breaking existing programs > > because the verifier can't prove the argument is within the valid range. > > And of course it doesn't help you with compatibility with > > already-released kernels. > > > > Hmm, think I have a way forward. I'll use BPF_PROG_TEST_RUN. > > If the load fail for the new helper, fallback to bpf_redirect_map(). Use > BPF_PROG_TEST_RUN to make sure that "action via flags" passes. +1 to Toke's point. No version checks please. One way to detect is to try prog_load. Search for FEAT_* in libbpf. Another approach is to scan vmlinux BTF for necessary helpers. Currently libbpf is relying on the former. I think going forward would be good to detect features via BTF. It's going to be much faster and won't create noise for audit that could be looking at prog_load calls.
