Re: [PATCH v2 bpf-next 03/17] libbpf: suppress compiler warning when using SEC() macro with externs

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 03/17] libbpf: suppress compiler warning when using SEC() macro with externs
Date: Wed, 21 Apr 2021 20:59:50 -0700	[thread overview]
Message-ID: <CAEf4BzbJMtqxkaJNAvv_M1OoxN6TKZgniLbJD1u5nxj1H32wKw@mail.gmail.com> (raw)
In-Reply-To: <c9367c9d-527c-ba50-bb28-59abad9f5009@fb.com>

On Wed, Apr 21, 2021 at 8:48 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> > When used on externs SEC() macro will trigger compilation warning about
> > inapplicable `__attribute__((used))`. That's expected for extern declarations,
> > so suppress it with the corresponding _Pragma.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>
> Ack with some comments below.
> Acked-by: Yonghong Song <yhs@fb.com>
>
> > ---
> >   tools/lib/bpf/bpf_helpers.h | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> > index b904128626c2..75c7581b304c 100644
> > --- a/tools/lib/bpf/bpf_helpers.h
> > +++ b/tools/lib/bpf/bpf_helpers.h
> > @@ -25,9 +25,16 @@
> >   /*
> >    * Helper macro to place programs, maps, license in
> >    * different sections in elf_bpf file. Section names
> > - * are interpreted by elf_bpf loader
> > + * are interpreted by libbpf depending on the context (BPF programs, BPF maps,
> > + * extern variables, etc).
> > + * To allow use of SEC() with externs (e.g., for extern .maps declarations),
> > + * make sure __attribute__((unused)) doesn't trigger compilation warning.
> >    */
> > -#define SEC(NAME) __attribute__((section(NAME), used))
> > +#define SEC(name) \
> > +     _Pragma("GCC diagnostic push")                                      \
> > +     _Pragma("GCC diagnostic ignored \"-Wignored-attributes\"")          \
> > +     __attribute__((section(name), used))                                \
> > +     _Pragma("GCC diagnostic pop")                                       \
>
> The 'used' attribute is mostly useful for static variable/functions
> since otherwise if not really used, the compiler could delete them
> freely. The 'used' attribute does not really have an impact on
> global variables regardless whether they are used or not in a particular
> compilation unit. We could drop 'used' here and selftests should still
> work. The only worry is that if people define something like
>     static int _version SEC("version") = 1;
>     static char _license[] SEC("license") = "GPL";
> Removing 'used' may cause failure.
>
> Since we don't want to remove 'used', then adding _Pragma to silence
> the warning is a right thing to do here.

Right, SEC() has become a universal macro used for functions,
variables, and externs. I didn't want to introduce multiple variants,
but also we can't break existing use cases. So pragma, luckily,
allowed to support all cases.

>
> >
> >   /* Avoid 'linux/stddef.h' definition of '__always_inline'. */
> >   #undef __always_inline
> >