Re: [PATCH] ubsan, kcsan: don't combine sanitizer with kcov

[Arnd Bergmann <arnd@arndb.de>]

On Tue, May 5, 2020 at 4:50 PM 'Dmitry Vyukov' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
> On Tue, May 5, 2020 at 4:36 PM Marco Elver <elver@google.com> wrote:
> > > Clang does not allow -fsanitize-coverage=trace-{pc,cmp} together
> > > with -fsanitize=bounds or with ubsan:
> > >
> > > clang: error: argument unused during compilation: '-fsanitize-coverage=trace-pc' [-Werror,-Wunused-command-line-argument]
> > > clang: error: argument unused during compilation: '-fsanitize-coverage=trace-cmp' [-Werror,-Wunused-command-line-argument]
> > >
> > >  menuconfig KCSAN
> > >         bool "KCSAN: dynamic data race detector"
> > > -       depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN
> > > +       depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !KCOV
> >
> > This also disables KCOV with GCC. Why does this not work with KCSAN?

My mistake, this should be kept enabled for gcc. If we can get the combination
to work in clang, that's something that should also get enabled.

> > This is a huge problem for us, since syzbot requires KCOV. In fact
> > I've always been building KCSAN kernels with CONFIG_KCOV=y (with GCC
> > or Clang) and cannot reproduce the problem.

I have some local patches that change the way we pick the warning options
for each compiler, and enable more of the warnings that are normally disabled.

Maybe -Wunused-command-line-argument is disabled by default?
I only started seeing this problem recently. It's also possible that there
are some other options that interact with it so only Kcov+FOO leads to
KCSAN being ignored.

> > Ditto, we really need KCOV for all sanitizers. I also just tried to
> > reproduce the problem but can't.
> >
> > Which version of clang is causing this? I'm currently using Clang 9.
> > My guess is that we should not fix this by disallowing KCOV, but
> > rather make Clang work with these configs.
> >
> > Dmitry, can you comment?
>
> FWIW I can reproduce both with clang:
>
> $ clang /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=bounds
> clang-11: warning: argument unused during compilation:
> '-fsanitize-coverage=trace-pc' [-Wunused-command-line-argument]
>
> $ clang /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=thread
> clang-11: warning: argument unused during compilation:
> '-fsanitize-coverage=trace-pc' [-Wunused-command-line-argument]
>
> with both my disto's 9.0.1 and fresher 11.0.0
> (7b80cb7cf45faf462d6193cc41c2cb7ad556600d.
>
> But both work with gcc
>
> $ gcc /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=thread
> $ gcc /tmp/test.c -c -fsanitize-coverage=trace-pc -fsanitize=bounds
>
> Is it a known issue in clang?
>
> Can we somehow disable it only for clang and not gcc?
>
> This will immediately break KCSAN on syzbot as it enables KCSAN and KCOV:
> https://syzkaller.appspot.com/upstream?manager=ci2-upstream-kcsan-gce

I can respin the patch with this fixup if you like:

--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -5,7 +5,7 @@ config HAVE_ARCH_KCSAN

 menuconfig KCSAN
        bool "KCSAN: dynamic data race detector"
-       depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !KCOV
+       depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !(KCOV
&& CC_IS_CLANG)
        select STACKTRACE
        help
          The Kernel Concurrency Sanitizer (KCSAN) is a dynamic

As you both say, the combination seems to be quite important, so maybe there
is something else that can be to also enable it with clang.

      Arnd