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From: Arnd Bergmann <>
To: Andrew Morton <>
Cc: Arnd Bergmann <>,,
	Mauro Carvalho Chehab <>,
	Andrey Ryabinin <>,
	Alexander Potapenko <>,
	Dmitry Vyukov <>,,
	Masahiro Yamada <>,
	Michal Marek <>,
	Ingo Molnar <>,
	Byungchul Park <>,
	"Paul E. McKenney" <>,
	Peter Zijlstra <>,
	Geert Uytterhoeven <>,
	Josh Poimboeuf <>,
	Thomas Gleixner <>,
	Greg Kroah-Hartman <>,,
Subject: [PATCH] [v4] kasan: rework Kconfig settings
Date: Thu, 21 Dec 2017 14:46:39 +0100
Message-ID: <> (raw)

We get a lot of very large stack frames using gcc-7.0.1 with the default
-fsanitize-address-use-after-scope --param asan-stack=1 options, which
can easily cause an overflow of the kernel stack, e.g.

drivers/gpu/drm/i915/gvt/handlers.c:2434:1: warning: the frame size of 46176 bytes is larger than 3072 bytes
drivers/net/wireless/ralink/rt2x00/rt2800lib.c:5650:1: warning: the frame size of 23632 bytes is larger than 3072 bytes
lib/atomic64_test.c:250:1: warning: the frame size of 11200 bytes is larger than 3072 bytes
drivers/gpu/drm/i915/gvt/handlers.c:2621:1: warning: the frame size of 9208 bytes is larger than 3072 bytes
drivers/media/dvb-frontends/stv090x.c:3431:1: warning: the frame size of 6816 bytes is larger than 3072 bytes
fs/fscache/stats.c:287:1: warning: the frame size of 6536 bytes is larger than 3072 bytes

To reduce this risk, -fsanitize-address-use-after-scope is now split
out into a separate CONFIG_KASAN_EXTRA Kconfig option, leading to stack
frames that are smaller than 2 kilobytes most of the time on x86_64. An
earlier version of this patch also prevented combining KASAN_EXTRA with
KASAN_INLINE, but that is no longer necessary with gcc-7.0.1.

All patches to get the frame size below 2048 bytes with CONFIG_KASAN=y and
CONFIG_KASAN_EXTRA=n have been merged by maintainers now, so we can bring
back that default now. KASAN_EXTRA=y still causes lots of warnings but now
defaults to !COMPILE_TEST to disable it in allmodconfig, and it remains
disabled in all other defconfigs since it is a new option.  I arbitrarily
raise the warning limit for KASAN_EXTRA to 3072 to reduce the noise,
but an allmodconfig kernel still has around 50 warnings on gcc-7.

I experimented a bit more with smaller stack frames and have another
follow-up series that reduces the warning limit for 64-bit architectures
to 1280 bytes (without CONFIG_KASAN).

With earlier versions of this patch series, I also had patches to
address the warnings we get with KASAN and/or KASAN_EXTRA, using a
"noinline_if_stackbloat" annotation. That annotation now got replaced with
a gcc-8 bugfix (see
and a workaround for older compilers, which means that KASAN_EXTRA is
now just as bad as before and will lead to an instant stack overflow in
a few extreme cases.

This reverts parts of commit commit 3f181b4 ("lib/Kconfig.debug: disable
-Wframe-larger-than warnings with KASAN=y").
Two patches in linux-next should be merged first to avoid introducing
warnings in an allmodconfig build:
3cd890dbe2a4 ("media: dvb-frontends: fix i2c access helpers for KASAN")
16c3ada89cff ("media: r820t: fix r820t_write_reg for KASAN")

Cc: Mauro Carvalho Chehab <>
Cc: Andrey Ryabinin <>
Cc: Alexander Potapenko <>
Cc: Dmitry Vyukov <>
Signed-off-by: Arnd Bergmann <>
 lib/Kconfig.debug      |  2 +-
 lib/Kconfig.kasan      | 11 +++++++++++
 scripts/Makefile.kasan |  3 +++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 811e5019b36a..84b4ffa67596 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -217,7 +217,7 @@ config ENABLE_MUST_CHECK
 config FRAME_WARN
 	int "Warn for stack frames larger than (needs gcc 4.4)"
 	range 0 8192
-	default 0 if KASAN
+	default 3072 if KASAN_EXTRA
 	default 1280 if (!64BIT && PARISC)
 	default 1024 if (!64BIT && !PARISC)
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index e39de4191480..8613a6fc4bda 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -21,6 +21,17 @@ config KASAN
 	  Currently CONFIG_KASAN doesn't work with CONFIG_DEBUG_SLAB
 	  (the resulting kernel does not boot).
+	bool "KAsan: extra checks"
+	help
+	  This enables further checks in the kernel address sanitizer, for now
+	  it only includes the address-use-after-scope check that can lead
+	  to excessive kernel stack usage, frame size warnings and longer
+	  compile time.
+ has more
 	prompt "Instrumentation type"
 	depends on KASAN
diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
index 1ce7115aa499..02f4d2974073 100644
--- a/scripts/Makefile.kasan
+++ b/scripts/Makefile.kasan
@@ -30,5 +30,8 @@ else
 CFLAGS_KASAN += $(call cc-option, -fsanitize-address-use-after-scope)

             reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-12-21 13:46 Arnd Bergmann [this message]
2017-12-22 22:41 ` Andrew Morton
2017-12-23 10:14   ` Arnd Bergmann
2017-12-23 12:42 ` Andrey Ryabinin

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