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

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-10-30 at 11:24 -0700, Ihor Solodrai wrote:
> On 10/30/25 11:14 AM, Eduard Zingerman wrote:
> > 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);
> 
> I believe it's the other way around:
>      void foo_impl(struct bpf_prog_aux p__implicit);
>      __bpf_kfunc void foo(void);
> 
> foo() is callable from BPF, but foo_impl() is not.
> But we still want foo_impl() in BTF so that the verifier can find the 
> correct prototype.
> 
> Andrii, please confirm.

Oops, yes, 'foo' gets __bpf_kfunc.

> > - 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.
> 
> I think you meant "new kernel compiled with old pahole".

No, new kernel compiled with new pahole.
As it will have a different prototype for 'foo' compared to one
encoded in the old program binary 'btf'.

> 
> > 
> > Andrei and Alexei deemed this acceptable.