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Commit 5aef51c3 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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x86/kconfig/32: Rename CONFIG_VM86 and default it to 'n' 



VM86 is entirely broken if ptrace, syscall auditing, or
NOHZ_FULL is in use.  The code is a big undocumented mess, it's
a real PITA to test, and it looks like a big chunk of vm86_32.c
is dead code.  It also plays awful games with the entry asm.

No one should be using it anyway. Use DOSBOX or KVM instead.

Let's accelerate its slow death.  Remove it from EXPERT and
default it to n.  Distros should not enable it.  In the unlikely
event that some user needs it, they can easily re-enable it.

While we're at it, rename it to CONFIG_X86_LEGACY_VM86 so that 'make
oldconfig' users will be prompted again.  I left CONFIG_VM86 as
an alias to avoid a treewide replacement of the names.  We can
clean that up once the current asm and vm86 code churn settles
down.

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/d29c6cc442d32d4df58849d2f8c89fb39ff88d61.1436542295.git.luto@kernel.org


[ Refined it some more. ]
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3490565b

arch/x86/Kconfig

+28 −7
Original line number Diff line number Diff line
@@ -996,15 +996,36 @@ config X86_THERMAL_VECTOR 
	def_bool y

	depends on X86_MCE_INTEL



config VM86

	bool "Enable VM86 support" if EXPERT

	default y

config X86_LEGACY_VM86

	bool "Legacy VM86 support (obsolete)"

	default n

	depends on X86_32

	---help---

	  This option is required by programs like DOSEMU to run

	  16-bit real mode legacy code on x86 processors. It also may

	  be needed by software like XFree86 to initialize some video

	  cards via BIOS. Disabling this option saves about 6K.

	  This option allows user programs to put the CPU into V8086

	  mode, which is an 80286-era approximation of 16-bit real mode.



	  Some very old versions of X and/or vbetool require this option

	  for user mode setting.  Similarly, DOSEMU will use it if

	  available to accelerate real mode DOS programs.  However, any

	  recent version of DOSEMU, X, or vbetool should be fully

	  functional even without kernel VM86 support, as they will all

	  fall back to (pretty well performing) software emulation.



	  Anything that works on a 64-bit kernel is unlikely to need

	  this option, as 64-bit kernels don't, and can't, support V8086

	  mode.  This option is also unrelated to 16-bit protected mode

	  and is not needed to run most 16-bit programs under Wine.



	  Enabling this option adds considerable attack surface to the

	  kernel and slows down system calls and exception handling.



	  Unless you use very old userspace or need the last drop of

	  performance in your real mode DOS games and can't use KVM,

	  say N here.



config VM86

       bool

       default X86_LEGACY_VM86



config X86_16BIT

	bool "Enable support for 16-bit segments" if EXPERT