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

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Date:   Sat, 7 Sep 2019 08:54:13 +0200
From:   Jiri Olsa <jolsa@...hat.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 06, 2019 at 09:09:17AM +0000, Andrii Nakryiko 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.

ok, I'll disable disable it in bpftrace code then

thanks,
jirka

> 
> > all related to deduplication code, so I believe loosing const
> > is fine there.
> > 
> > thanks,
> > jirka
> > 
> > 
> > ---
> > Jiri Olsa (7):
> >        libbpf: Use const cast for btf_int_* functions
> >        libbpf: Return const btf_array from btf_array inline function
> >        libbpf: Return const btf_enum from btf_enum inline function
> >        libbpf: Return const btf_member from btf_members inline function
> >        libbpf: Return const btf_param from btf_params inline function
> >        libbpf: Return const btf_var from btf_var inline function
> >        libbpf: Return const struct btf_var_secinfo from btf_var_secinfos inline function
> > 
> >   tools/lib/bpf/btf.c    | 21 +++++++++++----------
> >   tools/lib/bpf/btf.h    | 30 +++++++++++++++---------------
> >   tools/lib/bpf/libbpf.c |  2 +-
> >   3 files changed, 27 insertions(+), 26 deletions(-)
> > 
>