Re: [PATCH RFC bpf-next v3 08/16] bpf/verifier: do_misc_fixups for is_bpf_timer_set_sleepable_cb_kfunc

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2024-02-27 at 17:18 +0100, Benjamin Tissoires wrote:
[...]

> Hmm, I must still be missing a piece of the puzzle:
> if I declare bpf_timer_set_sleepable_cb() to take a third "aux"
> argument, given that it is declared as kfunc, I also must declare it in
> my bpf program, or I get the following:
> 
> # libbpf: extern (func ksym) 'bpf_timer_set_sleepable_cb': func_proto [264] incompatible with vmlinux [18151]
> 
> And if I declare it, then I don't know what to pass, given that this is
> purely added by the verifier:
> 
> 43: (85) call bpf_timer_set_sleepable_cb#18152
> arg#2 pointer type STRUCT bpf_prog_aux must point to scalar, or struct with scalar

Right, something has to be done about number of arguments and we don't
have a convenient mechanism for this afaik.

The simplest way would be to have two kfuncs:
- one with 2 arguments, used form bpf program;
- another with 3 arguments, used at runtime;
- replace former by latter during rewrite.

> Maybe I should teach the verifier that this kfunc only takes 2
> arguments, and the third one is virtual, but that also means that when
> the kfunc definitions are to be included in vmlinux.h, they would also
> have this special case.

It might be a somewhat generic mechanism, e.g. btf_decl_tag("hidden")
for kfunc parameter.

imho, having two kfuncs is less hacky.

> (I just tried with a blank u64 instead of the struct bpf_prog_aux*, but
>  it crashes with KASAN complaining).

For my understanding:
- you added a 3rd param (void *) to kfunc;
- passed it as zero in BPF program;
- applied the above rewrite, so that r3 equals to prog->aux;
- and now KASAN complains, right?

Could you please provide more details on what exactly it complains about?