[PATCH 3.16 152/254] kbuild: add '-fno-stack-check' to kernel build options

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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@xxxxxxx>
Reported-and-tested-by: Toralf FÃrster <toralf.foerster@xxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

--- a/Makefile
+++ b/Makefile
@@ -736,6 +736,9 @@ 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)