On Mon, Apr 4, 2022 at 1:38 AM Nikolay Borisov <nborisov@xxxxxxxx> wrote: > > Hello, > > Here are 2 patches with which I want to probe what's the sentiments towards 2 > changes: > > 1. Introduction of libbpf APIs similar to the bpf counterparts: bpf_core_field_exists, > bpf_core_type_exists and bpf_core_enum_value_exists. Of those I've implemented only > the first one and the reasoning behind this is in the patch itself. However, the > TLDR is that there can be cases where based on the kernel version we have to make a > decision in userspace what set of kprobes to use. There are currently no convenince > api's to do this so one has to essentially open code the checks that can be provided > by the aforementioned APIs. > 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 > 2. The kernel has for_each_member macro but the libbpf library doesn't provide it, > this results in having to open code members enumeration in various places such as > in find_member_by_name/find_struct_ops_kern_types/bpf_map__init_kern_struct_ops/ > parse_btf_map_def and in the newly introduced btf__field_exists. So how about > bringing the convenience macro to libbpf? see my comment, not sure it's worth it > > The reason why this series is RFC is if people agree with the proposed changed > I'd be happy to extend it to add more *exists* APIs and do the conversion to > using the for_each_member macro. > > Nikolay Borisov (2): > libbpf: Add userspace version of for_each_member macro > libbpf: Add btf__field_exists > > tools/lib/bpf/btf.c | 28 ++++++++++++++++++++++++++++ > tools/lib/bpf/btf.h | 8 ++++++++ > tools/lib/bpf/libbpf.map | 1 + > 3 files changed, 37 insertions(+) > > -- > 2.25.1 >
