netdev - Re: [PATCH v4 bpf-next 02/14] libbpf: convert libbpf code to use new btf helpers

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Date:   Wed, 7 Aug 2019 12:59:53 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 02/14] libbpf: convert libbpf code to use new
 btf helpers

On Wed, Aug 7, 2019 at 12:30 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Aug 06, 2019 at 10:37:54PM -0700, Andrii Nakryiko wrote:
> > Simplify code by relying on newly added BTF helper functions.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> ..
> >
> > -     for (i = 0, vsi = (struct btf_var_secinfo *)(t + 1);
> > -          i < vars; i++, vsi++) {
> > +     for (i = 0, vsi = (void *)btf_var_secinfos(t); i < vars; i++, vsi++) {
>
> > +                     struct btf_member *m = (void *)btf_members(t);
> ...
> >               case BTF_KIND_ENUM: {
> > -                     struct btf_enum *m = (struct btf_enum *)(t + 1);
> > -                     __u16 vlen = BTF_INFO_VLEN(t->info);
> > +                     struct btf_enum *m = (void *)btf_enum(t);
> > +                     __u16 vlen = btf_vlen(t);
> ...
> >               case BTF_KIND_FUNC_PROTO: {
> > -                     struct btf_param *m = (struct btf_param *)(t + 1);
> > -                     __u16 vlen = BTF_INFO_VLEN(t->info);
> > +                     struct btf_param *m = (void *)btf_params(t);
> > +                     __u16 vlen = btf_vlen(t);
>
> So all of these 'void *' type hacks are only to drop const-ness ?

Yes.

> May be the helpers shouldn't be taking const then?
>

Probably not, because then we'll have much wider-spread problem of
casting const pointers into non-const when passing btf_type into
helpers.
I think const as a default is the right choice, because normally BTF
is immutable and btf__type_by_id is returning const pointer, etc.
That's typical and expected use-case. btf_dedup and BTF sanitization +
datasec size setting pieces are an exception that have to modify BTF
types in place before passing it to user.

So realistically I think we can just leave it as (void *), or I can do
explicit non-const type casts, or we can just not use helpers for
mutable cases. Do you have a preference?