x86: Disable generation of traditional x87 instructions

We recently had the case where wrongly used floating-constant 'E' caused
the generation of traditional x87 instructions in kernel code and
wreaking all kinds of havoc.

Disable the generation of those too. This will save people a lot of time
when trying to debug such issues by erroring out of the build instead of
let them manifest themselves in very spectacular and happy-crappy ways
at runtime.

We're using -mno-fp-ret-in-387 in addition to -mno-80387 (which is ==
-msoft-float) because, as the gcc manpage says:

  On machines where a function returns floating-point results in the
  80387 register stack, some floating-point opcodes may be emitted even
  if -msoft-float is used.

so we want to turn off *all* non-integer instructions involving any
architectural FPU state, unless it is absolutely necessary (and those
cases need special handling anyway).

Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Michael Matz <matz@suse.de>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1391561711-3023-1-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/Makefile

@@ -82,8 +82,8 @@ else
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64
 
-        # Don't autogenerate MMX or SSE instructions
-        KBUILD_CFLAGS += -mno-mmx -mno-sse
+        # Don't autogenerate traditional x87, MMX or SSE instructions
+        KBUILD_CFLAGS += -mno-mmx -mno-sse -mno-80387 -mno-fp-ret-in-387
 
 	# Use -mpreferred-stack-boundary=3 if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)