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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Sep 30, 2020 at 10:28:33AM +0100, Lorenz Bauer wrote:
> 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 $

That's very interesting. Thanks for the pointers.
I'm using gcc version 9.1.1 20190605 (Red Hat 9.1.1-2)
and I saw this warning while compiling selftests,
but I don't see it with above warning.c example.
clang warns correctly in both cases.

> 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.

That gcc bug has been opened since transition to ssa. That was a huge
transition for gcc. But I think the bug number is not correct. It points to a
different issue. I've checked -fdump-tree-uninit-all dump with and without
macro. They're identical. The tree-ssa-uninit pass suppose to warn, but it
doesn't. I wish I had more time to dig into it. A bit of debugging in
gcc/tree-ssa-uninit.c can probably uncover the root cause.