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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

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.

> 
> - 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".

> 
> Andrei and Alexei deemed this acceptable.