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x86: Default to vsyscall=native for now

This UML breakage:

  linux-2.6.30.1[3800] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb9c498 ax:ffffffffff600000 si:0 di:606790
  linux-2.6.30.1[3856] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb13168 ax:ffffffffff600000 si:0 di:606790

Is caused by commit 3ae3665 ("x86-64: Rework vsyscall emulation and add
vsyscall= parameter") - the vsyscall emulation code is not fully cooked
yet as UML relies on some rather fragile SIGSEGV semantics.

Linus suggested in https://lkml.org/lkml/2011/8/9/376 to default
to vsyscall=native for now, this patch implements that.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Andrew Lutomirski <luto@mit.edu>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/20111005214047.GE14406@localhost.pp.htv.fi
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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1 parent 65112dc commit 2b666859ec323403ac9a3a441d16eab30945404b @AdrianBunk AdrianBunk committed with Ingo Molnar Oct 6, 2011
Showing with 5 additions and 4 deletions.
  1. +4 −3 Documentation/kernel-parameters.txt
  2. +1 −1 arch/x86/kernel/vsyscall_64.c
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7 Documentation/kernel-parameters.txt
@@ -2706,10 +2706,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
functions are at fixed addresses, they make nice
targets for exploits that can control RIP.
- emulate [default] Vsyscalls turn into traps and are
- emulated reasonably safely.
+ emulate Vsyscalls turn into traps and are emulated
+ reasonably safely.
- native Vsyscalls are native syscall instructions.
+ native [default] Vsyscalls are native syscall
+ instructions.
This is a little bit faster than trapping
and makes a few dynamic recompilers work
better than they would in emulation mode.
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2 arch/x86/kernel/vsyscall_64.c
@@ -56,7 +56,7 @@ DEFINE_VVAR(struct vsyscall_gtod_data, vsyscall_gtod_data) =
.lock = __SEQLOCK_UNLOCKED(__vsyscall_gtod_data.lock),
};
-static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;
+static enum { EMULATE, NATIVE, NONE } vsyscall_mode = NATIVE;
static int __init vsyscall_setup(char *str)
{

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