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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: Andy 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2015-07-10 08:34:23 -07:00 committed by Ingo Molnar
parent 3490565b63
commit 5aef51c340
1 changed files with 28 additions and 7 deletions
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35
arch/x86/Kconfig
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@ -996,15 +996,36 @@ config X86_THERMAL_VECTOR
def_bool y def_bool y
depends on X86_MCE_INTEL depends on X86_MCE_INTEL
config VM86 config X86_LEGACY_VM86
bool "Enable VM86 support" if EXPERT bool "Legacy VM86 support (obsolete)"
default y default n
depends on X86_32 depends on X86_32
---help--- ---help---
This option is required by programs like DOSEMU to run This option allows user programs to put the CPU into V8086
16-bit real mode legacy code on x86 processors. It also may mode, which is an 80286-era approximation of 16-bit real mode.
be needed by software like XFree86 to initialize some video
cards via BIOS. Disabling this option saves about 6K. 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 config X86_16BIT
bool "Enable support for 16-bit segments" if EXPERT bool "Enable support for 16-bit segments" if EXPERT