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

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Message-ID: <151d13d1-e894-56cc-4690-4661c8afc65e@fb.com>
Date:   Wed, 7 Aug 2019 20:01:39 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        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 8/7/19 12:59 PM, Andrii Nakryiko wrote:
> 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?

Hmm. Take a const into the helper and drop it there?
I'd like to avoid all these 'void *'.