netdev - Re: [PATCH bpf-next v2 2/4] selftests: bpf: Add helper to compare socket cookies

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Date:   Wed, 30 Sep 2020 10:28:33 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>, Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        kernel-team <kernel-team@...udflare.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/4] selftests: bpf: Add helper to compare
 socket cookies

On Tue, 29 Sep 2020 at 16:48, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:

...

> There was a warning. I noticed it while applying and fixed it up.
> Lorenz, please upgrade your compiler. This is not the first time such
> warning has been missed.

I tried reproducing this on latest bpf-next (b0efc216f577997) with gcc
9.3.0 by removing the initialization of duration:

make: Entering directory '/home/lorenz/dev/bpf-next/tools/testing/selftests/bpf'
  TEST-OBJ [test_progs] sockmap_basic.test.o
  TEST-HDR [test_progs] tests.h
  EXT-OBJ  [test_progs] test_progs.o
  EXT-OBJ  [test_progs] cgroup_helpers.o
  EXT-OBJ  [test_progs] trace_helpers.o
  EXT-OBJ  [test_progs] network_helpers.o
  EXT-OBJ  [test_progs] testing_helpers.o
  BINARY   test_progs
make: Leaving directory '/home/lorenz/dev/bpf-next/tools/testing/selftests/bpf'

So, gcc doesn't issue a warning. Jakub did the following little experiment:

jkbs@...d ~/tmp $ cat warning.c
#include <stdio.h>

int main(void)
{
        int duration;

        fprintf(stdout, "%d", duration);

        return 0;
}
jkbs@...d ~/tmp $ gcc -Wall -o /dev/null warning.c
warning.c: In function ‘main’:
warning.c:7:2: warning: ‘duration’ is used uninitialized in this
function [-Wuninitialized]
    7 |  fprintf(stdout, "%d", duration);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


The simple case seems to work. However, adding the macro breaks things:

jkbs@...d ~/tmp $ cat warning.c
#include <stdio.h>

#define _CHECK(duration) \
        ({                                                      \
                fprintf(stdout, "%d", duration);                \
        })
#define CHECK() _CHECK(duration)

int main(void)
{
        int duration;

        CHECK();

        return 0;
}
jkbs@...d ~/tmp $ gcc -Wall -o /dev/null warning.c
jkbs@...d ~/tmp $

Maybe this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501 ? The
problem is still there on gcc 10. Compiling test_progs with clang does
issue a warning FWIW, but it seems like other things break when doing
that.

--
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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