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SubjectRe: [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@gmail.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@toad ~/tmp $ cat warning.c
#include <stdio.h>

int main(void)
{
int duration;

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

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