Re: [PATCH] lib: Kconfig: disable dynamic sanitizers for test builds

[Marco Elver]

On Wed, 18 Oct 2023 at 17:32, 'Hamza Mahfooz' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
> kasan, kcsan and kmsan all have the tendency to blow up the stack
> and there isn't a lot of value in having them enabled for test builds,
> since they are intended to be useful for runtime debugging. So, disable
> them for test builds.
>
> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@xxxxxxx>
> ---
>  lib/Kconfig.kasan | 1 +
>  lib/Kconfig.kcsan | 1 +
>  lib/Kconfig.kmsan | 1 +
>  3 files changed, 3 insertions(+)

Do you have links to discussions that motivate this change? This has
been discussed in the past. One recommendation is to adjust the
build/test scripts to exclude some combination of configs if they are
causing issues. Or we increase CONFIG_FRAME_WARN if one of them is
enabled (KMSAN sets it to 0, 32-bit KASAN increases it a bit).

That being said, we're aware of KASAN having had more issues and there
are some suboptions that have been disabled because of that (like
KASAN_STACK). I'm not sure if Clang's KASAN instrumentation has had
some recent improvements (we did investigate it, but I can't recall
what the outcome was [1]) - maybe try a more recent compiler? However,
KCSAN and KMSAN shouldn't have any issues (if KMSAN is enabled,
FRAME_WARN is 0). And having build tests with them enabled isn't
useless at all: we're making sure that these tools (even though only
for debugging), still work. We _want_ them to work during random build
testing!

Please share the concrete problem you're having, because this change
will make things worse for everyone in the long run.

[1] https://github.com/llvm/llvm-project/issues/38157

> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index fdca89c05745..fbd85c4872c0 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -38,6 +38,7 @@ menuconfig KASAN
>                     CC_HAS_WORKING_NOSANITIZE_ADDRESS) || \
>                    HAVE_ARCH_KASAN_HW_TAGS
>         depends on (SLUB && SYSFS && !SLUB_TINY) || (SLAB && !DEBUG_SLAB)
> +       depends on !COMPILE_TEST
>         select STACKDEPOT_ALWAYS_INIT
>         help
>           Enables KASAN (Kernel Address Sanitizer) - a dynamic memory safety

This also disables KASAN_HW_TAGS, which is actually enabled in
production kernels and does not use any compiler instrumentation.

> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index 609ddfc73de5..7bcefdbfb46f 100644
> --- a/lib/Kconfig.kcsan
> +++ b/lib/Kconfig.kcsan
> @@ -14,6 +14,7 @@ menuconfig KCSAN
>         bool "KCSAN: dynamic data race detector"
>         depends on HAVE_ARCH_KCSAN && HAVE_KCSAN_COMPILER
>         depends on DEBUG_KERNEL && !KASAN
> +       depends on !COMPILE_TEST
>         select CONSTRUCTORS
>         select STACKTRACE
>         help
> diff --git a/lib/Kconfig.kmsan b/lib/Kconfig.kmsan
> index ef2c8f256c57..eb05c885d3fd 100644
> --- a/lib/Kconfig.kmsan
> +++ b/lib/Kconfig.kmsan
> @@ -13,6 +13,7 @@ config KMSAN
>         depends on HAVE_ARCH_KMSAN && HAVE_KMSAN_COMPILER
>         depends on SLUB && DEBUG_KERNEL && !KASAN && !KCSAN
>         depends on !PREEMPT_RT
> +       depends on !COMPILE_TEST

KMSAN already selects FRAME_WARN of 0 and should not cause you any
issues during build testing.

Nack.