Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

[Andrey Konovalov]

On Tue, Feb 19, 2019 at 10:49 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> Building an arm64 allmodconfig kernel with clang results in over 140 warnings
> about overly large stack frames, the worst ones being:
>
> drivers/gpu/drm/panel/panel-sitronix-st7789v.c:196:12: error: stack frame size of 20224 bytes in function 'st7789v_prepare'
> drivers/video/fbdev/omap2/omapfb/displays/panel-tpo-td028ttec1.c:196:12: error: stack frame size of 13120 bytes in function 'td028ttec1_panel_enable'
> drivers/usb/host/max3421-hcd.c:1395:1: error: stack frame size of 10048 bytes in function 'max3421_spi_thread'
> drivers/net/wan/slic_ds26522.c:209:12: error: stack frame size of 9664 bytes in function 'slic_ds26522_probe'
> drivers/crypto/ccp/ccp-ops.c:2434:5: error: stack frame size of 8832 bytes in function 'ccp_run_cmd'
> drivers/media/dvb-frontends/stv0367.c:1005:12: error: stack frame size of 7840 bytes in function 'stv0367ter_algo'
>
> None of these happen with gcc today, and almost all of these are the result
> of a single known bug in llvm.  Hopefully it will eventually get fixed with the
> clang-9 release.
>
> In the meantime, the best idea I have is to turn off asan-stack for clang-8
> and earlier, so we can produce a kernel that is safe to run.

Hi Arnd,

I don't think it's good to disable KASAN stack instrumentation for the
whole kernel by default with clang. It makes more sense to disable
stack instrumentation only for these few drivers.

Thanks!


>
> I have posted three patches that address the frame overflow warnings that are
> not addressed by turning off asan-stack, so in combination with this change,
> we get much closer to a clean allmodconfig build, which in turn is necessary
> to do meaningful build regression testing.
>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: Qian Cai <cai@xxxxxx>
> Link: https://bugs.llvm.org/show_bug.cgi?id=38809
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  lib/Kconfig.kasan      | 13 +++++++++++++
>  scripts/Makefile.kasan |  2 +-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index 67d7d1309c52..219cddc913ac 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -103,6 +103,19 @@ config KASAN_INLINE
>
>  endchoice
>
> +config KASAN_STACK
> +       int
> +       default 0 if CC_IS_CLANG && (CLANG_VERSION < 90000)
> +       default 1
> +       help
> +         The LLVM stack address sanitizer has a know bug that
> +         causes excessive stack usage in a lot of functions, see
> +         https://bugs.llvm.org/show_bug.cgi?id=38809
> +         Disabling asan-stack makes it safe to run kernels build
> +         with clang-8 with KASAN enabled, though it loses some of
> +         the functionality.  We assume that clang-9 will have a fix,
> +         so the feature can be used.
> +
>  config KASAN_S390_4_LEVEL_PAGING
>         bool "KASan: use 4-level paging"
>         depends on KASAN && S390
> diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
> index f1fb8e502657..6410bd22fe38 100644
> --- a/scripts/Makefile.kasan
> +++ b/scripts/Makefile.kasan
> @@ -26,7 +26,7 @@ else
>         CFLAGS_KASAN := $(CFLAGS_KASAN_SHADOW) \
>          $(call cc-param,asan-globals=1) \
>          $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
> -        $(call cc-param,asan-stack=1) \
> +        $(call cc-param,asan-stack=$(CONFIG_KASAN_STACK)) \
>          $(call cc-param,asan-instrument-allocas=1)
>  endif
>
> --
> 2.20.0
>