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

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Date:   Fri, 19 Jul 2019 15:34:17 -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 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 :-)

- 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
 EXTRA_WARNINGS += -fno-strict-aliasing
+EXTRA_WARNINGS += -Wno-shadow
+else
+EXTRA_WARNINGS += -Wshadow
 endif
 
 ifneq ($(findstring $(MAKEFLAGS), w),w)