[v3,11/11] kasan: rework Kconfig settings

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/acpi/nfit/core.c:2686:1: warning: the frame size of 4080 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/gpu/drm/amd/amdgpu/si.c:1756:1: warning: the frame size of 7304 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/gpu/drm/i915/gvt/handlers.c:2200:1: warning: the frame size of 43752 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/gpu/drm/vmwgfx/vmwgfx_drv.c:952:1: warning: the frame size of 6032 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/isdn/hardware/avm/b1.c:637:1: warning: the frame size of 13200 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/media/dvb-frontends/stv090x.c:3089:1: warning: the frame size of 5880 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/media/i2c/cx25840/cx25840-core.c:4964:1: warning: the frame size of 93992 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/net/wireless/ralink/rt2x00/rt2800lib.c:4994:1: warning: the frame size of 23928 bytes is larger than 2048 bytes [-Wframe-larger-than=]
drivers/staging/dgnc/dgnc_tty.c:2788:1: warning: the frame size of 7072 bytes is larger than 2048 bytes [-Wframe-larger-than=]
fs/ntfs/mft.c:2762:1: warning: the frame size of 7432 bytes is larger than 2048 bytes [-Wframe-larger-than=]
lib/atomic64_test.c:242:1: warning: the frame size of 12648 bytes is larger than 2048 bytes [-Wframe-larger-than=]

To reduce this risk, -fsanitize-address-use-after-scope is now split out
into a separate Kconfig option, vhich cannot be selected at the same
time as KMEMCHECK, 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.

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.

This reverts parts of commit commit 3f181b4 ("lib/Kconfig.debug:
disable -Wframe-larger-than warnings with KASAN=y").

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 and 1536 when CONFIG_KASAN (but not KASAN_EXTRA) is
enabled, this requires another ~25 patches to address the additional
warnings. I also have patches for all KASAN_EXTRA warnings, but we
should look at those separately and then decide whether to remove
it completely, leaving out -fsanitize-address-use-after-scope.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 lib/Kconfig.debug      |  4 ++--
 lib/Kconfig.kasan      | 11 ++++++++++-
 lib/Kconfig.kmemcheck  |  1 +
 scripts/Makefile.kasan |  3 +++
 4 files changed, 16 insertions(+), 3 deletions(-)