netdev - Re: [PATCH 0/7] libbpf: Fix cast away const qualifiers in btf.h

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Date:   Fri, 6 Sep 2019 10:03:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Martin Lau <kafai@...com>
Subject: Re: [PATCH 0/7] libbpf: Fix cast away const qualifiers in btf.h

On Fri, Sep 6, 2019 at 2:09 AM Andrii Nakryiko <andriin@...com> wrote:
>
> On 9/6/19 8:31 AM, Jiri Olsa wrote:
> > hi,
> > when including btf.h in bpftrace, I'm getting -Wcast-qual warnings like:
> >
> >    bpf/btf.h: In function ‘btf_var_secinfo* btf_var_secinfos(const btf_type*)’:
> >    bpf/btf.h:302:41: warning: cast from type ‘const btf_type*’ to type
> >    ‘btf_var_secinfo*’ casts away qualifiers [-Wcast-qual]
> >      302 |  return (struct btf_var_secinfo *)(t + 1);
> >          |                                         ^
> >
> > I changed the btf.h header to comply with -Wcast-qual checks
> > and used const cast away casting in libbpf objects, where it's
>
> Hey Jiri,
>
> We made all those helper funcs return non-const structs intentionally to
> improve their usability and avoid all those casts that you added back.
>
> Also, those helpers are now part of public API, so we can't just change
> them to const, as it can break existing users easily.
>
> If there is a need to run with -Wcast-qual, we should probably disable
> those checks where appropriate in libbpf code.
>
> So this will be a NACK from me, sorry.

Same opinion. This gcc warning is bogus.