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

Eduard Zingerman
2025-10-30T18:14:25Z

On Wed, 2025-10-29 at 23:11 -0700, Eduard Zingerman wrote:
> 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.

Scheme discussed off-list for new functions with __implicit args:
- kernel source code:

__bpf_kfunc void foo(struct bpf_prog_aux p__implicit)
BTF_ID_FLAGS(foo, KF_IMPLICIT_ARGS)

- pahole:
- renames foo to foo_impl
- adds bpf-side definition for 'foo' w/o implicit args
vmlinux btf:

__bpf_kfunc void foo_impl(struct bpf_prog_aux p__implicit);
void foo(void);

- resolve_btfids puts the 'foo' (the one w/o implicit args) id to all
id lists (no changes needed for this, follows from pahole changes);
- verifier.c:add_kfunc_call()
- Sees the id of 'foo' and kfunc flags with KF_IMPLICIT_ARGS
- Replaces the id with id of 'foo_impl'.

This will break the following scenario:
- new kfunc is added with __implicit arg
- kernel is built with old pahole
- vmlinux.h is generated for such kernel
- bpf program is compiled against such vmlinux.h
- attempt to run such program on a new kernel compiled with new pahole
will fail.

Andrei and Alexei deemed this acceptable.