Re: [PATCH bpf-next v1 0/8] bpf: magic kernel functions

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2025-10-29 at 17:44 -0700, Eduard Zingerman wrote:
> On Wed, 2025-10-29 at 12:01 -0700, Ihor Solodrai wrote:
> 
> Do we break compatibility with old pahole versions after this
> patch-set? Old paholes won't synthesize the _impl kfuncs, so:
> - binary compatibility between new-kernel/old-pahole + old-bpf
>   will be broken, as there would be no _impl kfuncs;
> - new-kernel/old-pahole + new-bpf won't work either, as kernel will
> be
>   unable to find non-_impl function names for existing kfuncs.
> 
> [...]

Point being, if we are going to break backwards compatibility the
following things need an update:
- Documentation/process/changes.rst
  minimal pahole version has to be bumped
- scripts/Makefile.btf
  All the different flags and options for different pahole
  versions can be dropped.

---

On the other hand, I'm not sure this useful but relatively obscure
feature grants such a compatibility break. Some time ago Ihor
advocated for just having two functions in the kernel, so that BTF
will be generated for both. And I think that someone suggested putting
the fake function to a discard-able section.
This way the whole thing can be done in kernel only.
E.g. it will look like so:

  __bpf_kfunc void btf_foo_impl(struct bpf_prog_aux p__implicit)
  { /* real impl here */ }

  __bpf_kfunc_proto void btf_foo(void) {}

Assuming that __bpf_kfunc_proto hides all the necessary attributes.
Not much boilerplate, and a tad easier to understand where second
prototype comes from, no need to read pahole.