On Thu, 2024-08-15 at 14:24 -0700, Andrii Nakryiko wrote:
[...]
> > @@ -16140,6 +16140,28 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
> > }
> > }
> >
> > +/* Same as helper_nocsr_clobber_mask() but for kfuncs, see comment above */
> > +static u32 kfunc_nocsr_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
> > +{
> > + const struct btf_param *params;
> > + u32 vlen, i, mask;
> > +
> > + params = btf_params(meta->func_proto);
> > + vlen = btf_type_vlen(meta->func_proto);
> > + mask = 0;
> > + if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
> > + mask |= BIT(BPF_REG_0);
> > + for (i = 0; i < vlen; ++i)
> > + mask |= BIT(BPF_REG_1 + i);
>
> Somewhere deep in btf_dump implementation of libbpf, there is a
> special handling of `<whatever> func(void)` (no args) function as
> having vlen == 1 and type being VOID (i.e., zero). I don't know if
> that still can happen, but I believe at some point we could get this
> vlen==1 and type=VOID for no-args functions. So I wonder if we should
> handle that here as well, or is it some compiler atavism we can forget
> about?
>
I just checked BTF generated for 'int filelock_init(void)',
for gcc compiled kernel using latest pahole and func proto looks as follows:
FUNC_PROTO '(anon)' ret_type_id=12 vlen=0
So I assume this is an atavism.
[...]
