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x86: Disable generation of traditional x87 instructions

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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>
This commit is contained in:
Borislav Petkov 2014-02-05 01:55:11 +01:00 committed by H. Peter Anvin
parent 38dbfb59d1
commit b399fe355b
1 changed files with 2 additions and 2 deletions
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arch/x86/Makefile
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@ -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)