Support probing for kfuncs availability

[qmonnet]

Similarly to what we do for program types, map types, helpers, and a few other components, we could look into dumping supported kfuncs with bpftool feature probe.

It is not clear yet what the probes would look like.

[danobi]

Interesting, I thought kfuncs were encoded into BTF. I see a reference to that here: https://github.com/torvalds/linux/blob/0e389834672c723435a44818ed2cabc4dad24429/kernel/bpf/btf.c#L7669

Mind double checking me on that? If kfuncs availability can be pulled from BTF feel free to give this one to me -- it's been bugging me for a while

[qmonnet]

Hey, thanks Daniel for looking into this!

As far as I'm aware, kfuncs and their prototypes are indeed available in the raw BTF, but I can't see them in the vmlinux.h generated by bpftool. In the raw BTF ouput, I don't know if there's a way to distinguish them from other non-kfuncs kernel functions.

What do you have in mind, exactly?

[danobi]

Hey, thanks Daniel for looking into this!

As far as I'm aware, kfuncs and their prototypes are indeed available in the raw BTF, but I can't see them in the vmlinux.h generated by bpftool. In the raw BTF ouput, I don't know if there's a way to distinguish them from other non-kfuncs kernel functions.

Ah makes sense. In that case, I was thinking of adding a BTF tag to all kfunc via __bpf_kfunc macro. Then bpftool could just walk all the tags and find kfunc tagged functions.

What do you have in mind, exactly?

I'm interested in two things:

1. Have bpftool support a "feature dump" of all supported kfuncs on running kernel
2. Have bpftool generate vmlinux.h with kfunc prototypes

[danobi]

Oh, BTF tags only work for clang built linux. Lemme think on it more

[qmonnet]

Would be great to have these tags, indeed. Thanks!

[danobi]

So I think this approach is probably still doable, except we cannot use BTF type tags from kernel side. Instead we'd probably want to teach pahole to parse the .BTF_ids section in vmlinux and generate type tags from that. But unfortunately not the quick win I was looking for :p w.r.t. how much time I can allocate to this.

[qmonnet]

Could be worth opening a thread on bpf@, I wouldn't be surprised if other people are interested in listing the kfuncs, too. Some may even have more suggestions, or cycles to take a look?