Re: [PATCH bpf-next v1 6/8] selftests/bpf: test autocreate behavior for struct_ops maps

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-02-28 at 16:02 -0800, Andrii Nakryiko wrote:
> On Wed, Feb 28, 2024 at 3:55 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
[...]
> > On Wed, 2024-02-28 at 15:43 -0800, Andrii Nakryiko wrote:
> > > > +SEC(".struct_ops.link")
> > > 
> > > can you please also have a test where we use SEC("?.struct_ops.link")
> > > which set autoload to false by default?
> > 
> > As far as I understand, that would be a new behavior, currently '?'
> > works only for programs. I'll add a separate patch to support this.
> > 
> 
> Yep, thanks!

So, I have a draft for v2 with support for this feature in [0].
But there is a gotcha:

    libbpf: BTF loading error: -22
    libbpf: -- BEGIN BTF LOAD LOG ---
    ...
    [23] DATASEC ?.struct_ops size=8 vlen=1 Invalid name
    
    -- END BTF LOAD LOG --
    libbpf: Error loading .BTF into kernel: -22. BTF is mandatory, can't proceed.

Kernel rejects DATASEC name with '?'.
The options are:
1. Tweak kernel to allow '?' as a first char in DATASEC names;
2. Use some different name, e.g. ".struct_ops.opt";
3. Do some kind of BTF rewrite in libbpf to merge
   "?.struct_ops" and ".struct_ops" DATASECs as a single DATASEC.
  
(1) is simple, but downside is requirement for kernel upgrade;
(2) is simple, but goes against current convention for program section names;
(3) idk, will check if that is feasible tomorrow.

[0] https://github.com/eddyz87/bpf/tree/structops-multi-version