netdev - Re: [PATCH v1 bpf-next] selftests/bpf : Clean up feature/ when make clean

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Date:   Mon, 10 Jun 2019 19:02:56 +0000
From:   Hechao Li <hechaol@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        "Kernel Team" <Kernel-team@...com>
Subject: Re: [PATCH v1 bpf-next] selftests/bpf : Clean up feature/ when make
 clean

Andrii Nakryiko <andrii.nakryiko@...il.com> wrote on Mon [2019-Jun-10 11:05:28 -0700]:
> On Mon, Jun 10, 2019 at 9:57 AM Hechao Li <hechaol@...com> wrote:
> >
> > I got an error when compiling selftests/bpf:
> >
> > libbpf.c:411:10: error: implicit declaration of function 'reallocarray';
> > did you mean 'realloc'? [-Werror=implicit-function-declaration]
> >   progs = reallocarray(progs, nr_progs + 1, sizeof(progs[0]));
> >
> > It was caused by feature-reallocarray=1 in FEATURE-DUMP.libbpf and it
> > was fixed by manually removing feature/ folder. This diff adds feature/
> > to EXTRA_CLEAN to avoid this problem.
> >
> > Signed-off-by: Hechao Li <hechaol@...com>
> > ---
> 
> There is no need to include v1 into patch prefix for a first version
> of a patch. Only v2 and further versions are added.
> 
> 
> >  tools/testing/selftests/bpf/Makefile | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> > index 2b426ae1cdc9..44fb61f4d502 100644
> > --- a/tools/testing/selftests/bpf/Makefile
> > +++ b/tools/testing/selftests/bpf/Makefile
> > @@ -279,4 +279,5 @@ $(OUTPUT)/verifier/tests.h: $(VERIFIER_TESTS_DIR) $(VERIFIER_TEST_FILES)
> >                  ) > $(VERIFIER_TESTS_H))
> >
> >  EXTRA_CLEAN := $(TEST_CUSTOM_PROGS) $(ALU32_BUILD_DIR) \
> > -       $(VERIFIER_TESTS_H) $(PROG_TESTS_H) $(MAP_TESTS_H)
> > +       $(VERIFIER_TESTS_H) $(PROG_TESTS_H) $(MAP_TESTS_H) \
> > +       feature
> 
> It doesn't seem any of linux's Makefile do that. From brief reading of
> build/Makefile.feature, it seems like it is supposed to handle
> transparently the case where environment changes and thus a set of
> supported features changes. I also verified that FEATURE-DUMP.libbpf
> is re-generated every single time I run make in
> tools/testing/selftests/bpf, even if nothing changed at all. So I
> don't think this patch is necessary.
> 
> I'm not sure what was the cause of your original problem, though.
> 
> > --
> > 2.17.1
> >

# Background:

My default GCC version is 4.8.5, which caused the following error when I
run make under selftests/bpf:
libbpf.c:39:10: fatal error: libelf.h: No such file or directory

To fix it, I have to run:

make CC=<Path to GCC 7.x>

The I got reallocarray not found error. By deleting feature/ folder
under selftests/bpf, it was fixed.

# Root Cause:

Now I found the root cause. When I run "make", which uses GCC 4.8.5, it
generates feature/test-reallocarray.d which indicates reallocarray is
enabled. However, when I switched to GCC 7.0, this file was not
re-generated and thus even FEATURE-DUMP.libbpf was re-generated, 
feature-reallocarray is still 1 in it.

This can be reproduced by the following steps:
$ cd tools/testing/selftests/bpf
$ make clean && make CC=<Path to GCC 4.8.5>
(Fail due to libelf.h not found)
$ make clean && make CC=<Path to GCC 7.x>
(Should succeed but actually fail with reallocarray not defined)

If adding feature to EXTRA_CLEAN is not the way to go, do you have any
suggestion to fix such problem? I spent some time debugging this and I
hope to fix it so that other people with similar situation won't have to
waste time on this issue.

Thanks,
Hechao