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

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

On Thu, Oct 1, 2020 at 10:09 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Oct 1, 2020 at 12:25 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > 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.
>
> I think this might be the same problem I fixed for libbpf with [0].
> Turns out, GCC explicitly calls out (somewhere in their docs) that
> uninitialized variable warnings work only when compiled in optimized
> mode, because some internal data structures used to detect this are
> only maintained in optimized mode build.
>
> Laurenz, can you try compiling your example with -O2?

All of my experiments I did with -O2.

>   [0] https://patchwork.ozlabs.org/project/netdev/patch/20200929220604.833631-2-andriin@fb.com/
>
> >
> > > 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.