[PATCH] [v2] kasan: turn off asan-stack for clang-8 and earlier

[Arnd Bergmann]

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 issue 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.

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.

It is still possible to turn on the CONFIG_ASAN_STACK option on all versions
of clang, and it's always enabled for gcc, but when CONFIG_COMPILE_TEST is
set, the option remains invisible, so allmodconfig and randconfig builds
(which are normally done with a forced CONFIG_COMPILE_TEST) will still result
in a mostly clean build.

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>
Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Link: https://bugs.llvm.org/show_bug.cgi?id=38809
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
Changes in v2:
- allow CONFIG_KASAN_STACK to be manually enabled/disabled on all
  clang versions, just make the default version specific, and
  ensure that it's turned off for allmodconfig build testing
---
 lib/Kconfig.kasan      | 22 ++++++++++++++++++++++
 scripts/Makefile.kasan |  2 +-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 67d7d1309c52..9950b660e62d 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -103,6 +103,28 @@ config KASAN_INLINE
 
 endchoice
 
+config KASAN_STACK_ENABLE
+	bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
+	default !(CLANG_VERSION < 90000)
+	depends on KASAN
+	help
+	  The LLVM stack address sanitizer has a know problem 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.
+	  This feature is always disabled when compile-testing with clang-8
+	  or earlier to avoid cluttering the output in stack overflow
+	  warnings, but clang-8 users can still enable it for builds without
+	  CONFIG_COMPILE_TEST.  On gcc and later clang versions it is
+	  assumed to always be safe to use and enabled by default.
+
+config KASAN_STACK
+	int
+	default 1 if KASAN_STACK_ENABLE || CC_IS_GCC
+	default 0
+
 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