On 6.04.22 г. 2:37 ч., Andrii Nakryiko wrote:
The problem is that what you've implemented is not a user-space equivalent of bpf_core_xxx() macros. CO-RE has extra logic around ___<flavor> suffixes, extra type checks, etc, etc. Helper you are adding does a very straightforward strings check, which isn't hard to implement and it doesn't have to be a set in stone API. So I'm a bit hesitant to add this. But I can share what I did in similar situations where I had to do some CO-RE check both on BPF side and know its result in user-space. I built a separate very simple BPF skeleton and all it did was perform various feature checks (including those that require CO-RE) and then returned the result through global variables. You can then trigger such BPF feature-checking program either through bpf_prog_test_run or through whatever other means (I actually did a simple sys_enter program in my case). See [0] for BPF program side and [1] for user-space activation/consumption of that. The benefit of this approach is that there is no way BPF and user-space sides can get "out of sync" in terms of their feature checking. With skeleton it's also extremely simple to do all this. [0]https://github.com/anakryiko/retsnoop/blob/master/src/calib_feat.bpf.c [1]https://github.com/anakryiko/retsnoop/blob/master/src/mass_attacher.c#L483-L529
That's indeed neat, however what is the minimum kernel version required to have global variables work ? AFAIU one requirement is to use a recent-enough libbpf which supports the skeleton functionality which is fine, userspace components can be updated somewhat easily than target kernels.
