[PATCH 4.9 23/75] kbuild: add -fno-stack-check to kernel build options

Greg Kroah-Hartman
2018-01-01T14:32:00Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 3ce120b16cc548472f80cf8644f90eda958cf1b6 upstream.

It appears that hardened gentoo enables "-fstack-check" by default for
gcc.

That doesn't work _at_all_ for the kernel, because the kernel stack
doesn't act like a user stack at all: it's much smaller, and it doesn't
auto-expand on use. So the extra "probe one page below the stack" code
generated by -fstack-check just breaks the kernel in horrible ways,
causing infinite double faults etc.

[ I have to say, that the particular code gcc generates looks very
stupid even for user space where it works, but that's a separate
issue. ]

Reported-and-tested-by: Alexander Tsoy <alexander@tsoy.me>
Reported-and-tested-by: Toralf Förster <toralf.foerster@gmx.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
Makefile | 3 +++
1 file changed, 3 insertions(+)

--- a/Makefile
+++ b/Makefile
@@ -788,6 +788,9 @@ KBUILD_CFLAGS += $(call cc-disable-warni
# disable invalid "can't wrap" optimizations for signed / pointers
KBUILD_CFLAGS += $(call cc-option,-fno-strict-overflow)

+# Make sure -fstack-check isn't enabled (like gentoo apparently did)
+KBUILD_CFLAGS += $(call cc-option,-fno-stack-check,)
+
# conserve stack if available
KBUILD_CFLAGS += $(call cc-option,-fconserve-stack)