[PATCH] [IA64] Fix futex_atomic_cmpxchg_inatomic()

[PATCH] [IA64] Fix futex_atomic_cmpxchg_inatomic()

[Luck, Tony]

Michel Lespinasse cleaned up the futex calling conventions in
commit 37a9d912b24f96a0591773e6e6c3642991ae5a70
    futex: Sanitize cmpxchg_futex_value_locked API

But the ia64 implementation was subtly broken. Gcc does not know
that register "r8" will be updated by the fault handler if the
cmpxchg instruction takes an exception. So it feels safe in letting
the initialization of r8 slide to after the cmpxchg. Result: we
always return 0 whether the user address faulted or not.

Fix by moving the initialization of r8 into the __asm__ code so
gcc won't move it.

Reported-by: <emeric.maschino@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?idB757
Cc: stable@vger.kernel.org # v2.6.39+
Signed-off-by: Tony Luck <tony.luck@intel.com>
---
 arch/ia64/include/asm/futex.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/ia64/include/asm/futex.h b/arch/ia64/include/asm/futex.h
index 0ab82cc..841b062 100644
--- a/arch/ia64/include/asm/futex.h
+++ b/arch/ia64/include/asm/futex.h
@@ -106,10 +106,11 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 		return -EFAULT;
 
 	{
-		register unsigned long r8 __asm ("r8") = 0;
+		register unsigned long r8 __asm ("r8");
 		unsigned long prev;
 		__asm__ __volatile__(
 			"	mf;;					\n"
+			"	mov r8=r0				\n"
 			"	mov ar.ccv=%3;;				\n"
 			"[1:]	cmpxchg4.acq %0=[%1],%2,ar.ccv		\n"
 			"	.xdata4 \"__ex_table\", 1b-., 2f-.	\n"
-- 
1.7.9.rc2.1.g69204

Re: [PATCH] [IA64] Fix futex_atomic_cmpxchg_inatomic()

[Michel Lespinasse]

On Fri, Apr 13, 2012 at 8:32 PM, Luck, Tony <tony.luck@intel.com> wrote:
> Michel Lespinasse cleaned up the futex calling conventions in
> commit 37a9d912b24f96a0591773e6e6c3642991ae5a70
>    futex: Sanitize cmpxchg_futex_value_locked API
>
> But the ia64 implementation was subtly broken. Gcc does not know
> that register "r8" will be updated by the fault handler if the
> cmpxchg instruction takes an exception. So it feels safe in letting
> the initialization of r8 slide to after the cmpxchg. Result: we
> always return 0 whether the user address faulted or not.
>
> Fix by moving the initialization of r8 into the __asm__ code so
> gcc won't move it.

Looks good. Sorry for the trouble.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

Re: [PATCH] [IA64] Fix futex_atomic_cmpxchg_inatomic()

[Émeric Maschino]

Hi,

Just rebuilt kernel with the patch proposed by Tony: issue fixed :-)

Many thanks,

     Emeric


Le 14 avril 2012 00:31, Michel Lespinasse <walken@google.com> a écrit :
> On Fri, Apr 13, 2012 at 8:32 PM, Luck, Tony <tony.luck@intel.com> wrote:
>> Michel Lespinasse cleaned up the futex calling conventions in
>> commit 37a9d912b24f96a0591773e6e6c3642991ae5a70
>>    futex: Sanitize cmpxchg_futex_value_locked API
>>
>> But the ia64 implementation was subtly broken. Gcc does not know
>> that register "r8" will be updated by the fault handler if the
>> cmpxchg instruction takes an exception. So it feels safe in letting
>> the initialization of r8 slide to after the cmpxchg. Result: we
>> always return 0 whether the user address faulted or not.
>>
>> Fix by moving the initialization of r8 into the __asm__ code so
>> gcc won't move it.
>
> Looks good. Sorry for the trouble.
>
> --
> Michel "Walken" Lespinasse
> A program is never fully debugged until the last user dies.