Re: [RFC PATCH 0/2] Add btf__field_exists

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Apr 4, 2022 at 1:38 AM Nikolay Borisov <nborisov@suse.com> 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
>