netdev - Re: [PATCH bpf] libbpf: fix missing _

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Date:   Fri, 19 Jul 2019 17:27:03 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>, Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...il.com>
Subject: Re: [PATCH bpf] libbpf: fix missing __WORDSIZE definition

Em Fri, Jul 19, 2019 at 01:04:32PM -0700, Andrii Nakryiko escreveu:
> On Fri, Jul 19, 2019 at 11:34 AM Arnaldo Carvalho de Melo
> <arnaldo.melo@...il.com> wrote:
> >
> > Em Fri, Jul 19, 2019 at 11:26:50AM -0700, Andrii Nakryiko escreveu:
> > > On Fri, Jul 19, 2019 at 11:14 AM Arnaldo Carvalho de Melo <arnaldo.melo@...il.com> wrote:
> > > > Em Fri, Jul 19, 2019 at 10:54:44AM -0700, Andrii Nakryiko escreveu:
> > > > > Ok, did some more googling. This warning (turned error in your setup)
> > > > > is emitted when -Wshadow option is enabled for GCC/clang. It appears
> > > > > to be disabled by default, so it must be enabled somewhere for perf
> > > > > build or something.
> >
> > > > Right, I came to the exact same conclusion, doing tests here:
> >
> > > > [perfbuilder@...8896a648d tmp]$ gcc -Wshadow shadow_global_decl.c   -o shadow_global_decl
> > > > shadow_global_decl.c: In function 'main':
> > > > shadow_global_decl.c:9: warning: declaration of 'link' shadows a global declaration
> > > > shadow_global_decl.c:4: warning: shadowed declaration is here
> > > > [perfbuilder@...8896a648d tmp]$ gcc --version |& head -1
> > > > gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23)
> > > > [perfbuilder@...8896a648d tmp]$ gcc shadow_global_decl.c   -o shadow_global_decl
> > > > [perfbuilder@...8896a648d tmp]$
> >
> > > > So I'm going to remove this warning from the places where it causes
> > > > problems.
> >
> > > > > Would it be possible to disable it at least for libbpf when building
> > > > > from perf either everywhere or for those systems where you see this
> > > > > warning? I don't think this warning is useful, to be honest, just
> > > > > random name conflict between any local and global variables will cause
> > > > > this.
> >
> > > > Yeah, I might end up having this applied.
> >
> > > Thanks!
> >
> > So, I'm ending up with the patch below, there is some value after all in
> > Wshadow, that is, from gcc 4.8 onwards :-)
> 
> I agree with the intent, but see below.
> 
> >
> > - Arnaldo
> >
> > diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> > index 495066bafbe3..ded7a950dc40 100644
> > --- a/tools/scripts/Makefile.include
> > +++ b/tools/scripts/Makefile.include
> > @@ -32,7 +32,6 @@ EXTRA_WARNINGS += -Wno-system-headers
> >  EXTRA_WARNINGS += -Wold-style-definition
> >  EXTRA_WARNINGS += -Wpacked
> >  EXTRA_WARNINGS += -Wredundant-decls
> > -EXTRA_WARNINGS += -Wshadow
> >  EXTRA_WARNINGS += -Wstrict-prototypes
> >  EXTRA_WARNINGS += -Wswitch-default
> >  EXTRA_WARNINGS += -Wswitch-enum
> > @@ -69,8 +68,16 @@ endif
> >  # will do for now and keep the above -Wstrict-aliasing=3 in place
> >  # in newer systems.
> >  # Needed for the __raw_cmpxchg in tools/arch/x86/include/asm/cmpxchg.h
> > +#
> > +# See https://lkml.org/lkml/2006/11/28/253 and https://gcc.gnu.org/gcc-4.8/changes.html,
> > +# that takes into account Linus's comments (search for Wshadow) for the reasoning about
> > +# -Wshadow not being interesting before gcc 4.8.
> > +
> >  ifneq ($(filter 3.%,$(MAKE_VERSION)),)  # make-3
> 
> This is checking make version, not GCC version. So code comment and
> configurations are not in sync?

Ah, I should have added a few lines back:

# Hack to avoid type-punned warnings on old systems such as RHEL5:
# We should be changing CFLAGS and checking gcc version, but this
# will do for now and keep the above -Wstrict-aliasing=3 in place
# in newer systems.
# Needed for the __raw_cmpxchg in tools/arch/x86/include/asm/cmpxchg.h
#
# See https://lkml.org/lkml/2006/11/28/253 and https://gcc.gnu.org/gcc-4.8/changes.html,
# that takes into account Linus's comments (search for Wshadow) for the reasoning about
# -Wshadow not being interesting before gcc 4.8.


In time I'll try and get it to use the gcc version to be strict.

- Arnaldo