On 09/23/2017 12:29 AM, Arnd Bergmann wrote: > 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:2407:1: error: the frame size of 31216 bytes is larger than 2048 bytes > drivers/net/wireless/ralink/rt2x00/rt2800lib.c:5650:1: error: the frame size of 23632 bytes is larger than 2048 bytes > drivers/scsi/fnic/fnic_trace.c:451:1: error: the frame size of 5152 bytes is larger than 2048 bytes > fs/btrfs/relocation.c:1202:1: error: the frame size of 4256 bytes is larger than 2048 bytes > fs/fscache/stats.c:287:1: error: the frame size of 6552 bytes is larger than 2048 bytes > lib/atomic64_test.c:250:1: error: the frame size of 12616 bytes is larger than 2048 bytes > mm/vmscan.c:1367:1: error: the frame size of 5080 bytes is larger than 2048 bytes > net/wireless/nl80211.c:1905:1: error: the frame size of 4232 bytes is larger than 2048 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. > > A lot of warnings with KASAN_EXTRA go away if we disable KMEMCHECK, > as -fsanitize-address-use-after-scope seems to understand the builtin > memcpy, but adds checking code around an extern memcpy call. I had to work > around a circular dependency, as DEBUG_SLAB/SLUB depended on !KMEMCHECK, > while KASAN did it the other way round. Now we handle both the same way > and make KASAN and KMEMCHECK mutually exclusive. > > All patches to get the frame size below 2048 bytes with CONFIG_KASAN=y > and CONFIG_KASAN_EXTRA=n have been submitted along with this patch, 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 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715) > 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"). > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> Acked-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
