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

[Marco Elver]

On Tue, 5 May 2020 at 17:29, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Tue, May 5, 2020 at 5:20 PM 'Marco Elver' via Clang Built Linux
> <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> > > --- 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)
> >
> > I wonder if we can just add this:  depends on !(KCOV &&
> > !$(cc-option,-Werror -fsanitize=thread -fsanitize-coverage=trace-pc))
> >
> > Similarly for UBSAN.
> >
> > That way, once Clang supports this combination, we don't need another
> > patch to fix it.
>
> Good idea. It probably get a little more complicated because kcov uses
> different flags depending on other options:
>
> kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC)     += -fsanitize-coverage=trace-pc
> kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)    += -fsanitize-coverage=trace-cmp
> kcov-flags-$(CONFIG_GCC_PLUGIN_SANCOV)          +=
> -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
>
> Do you have any preference on whether we should make KCSAN or KCOV
> conditional in this case? It may be easier to move the compiletime check
> into CONFIG_KCOV_ENABLE_COMPARISONS and
> CONFIG_CC_HAS_SANCOV_TRACE_PC.

Whichever is easier. I think if we have a config that tries to set
both, but then one gets silently disabled, it likely already breaks
the usecase. It'd be nice if there was a way to warn about only one
being selected so that a developer can then go back and choose the one
they're most interested in (or change compiler).

Thanks,
-- Marco