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

[Marco Elver <elver@google.com>]

On Tue, 5 May 2020 at 16:23, Arnd Bergmann <arnd@arndb.de> 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]
>
> To avoid that case, add a Kconfig dependency. The dependency could
> go either way, disabling CONFIG_KCOV or CONFIG_UBSAN_BOUNDS when the
> other is set. I picked the second option here as this seems to have
> a smaller impact on the resulting kernel.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  lib/Kconfig.kcsan | 2 +-
>  lib/Kconfig.ubsan | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index ea28245c6c1d..8f856c8828d5 100644
> --- 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
> +       depends on HAVE_ARCH_KCSAN && DEBUG_KERNEL && !KASAN && !KCOV

This also disables KCOV with GCC. Why does this not work with KCSAN?

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.

>         select STACKTRACE
>         help
>           The Kernel Concurrency Sanitizer (KCSAN) is a dynamic
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index 929211039bac..f98ef029553e 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -29,6 +29,7 @@ config UBSAN_TRAP
>  config UBSAN_BOUNDS
>         bool "Perform array index bounds checking"
>         default UBSAN
> +       depends on !(CC_IS_CLANG && KCOV)

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?

Thanks,
-- Marco

>         help
>           This option enables detection of directly indexed out of bounds
>           array accesses, where the array size is known at compile time.
> --
> 2.26.0
>