Re: The SMP alternatives code breaks exception fixup?

[Thomas Gleixner]

On Tue, 22 Jan 2008, Andi Kleen wrote:
> Chuck Ebbert <cebbert@xxxxxxxxxx> writes:
> > 
> > There is a fixup, so this should never happen. But the lock instruction
> > was replaced with a nop by the altinstruction code, and that makes the fixup
> > address wrong. AFAICT we don't fix up the exception table when we replace
> > a lock with a nop, which makes the fixup table point to the nop instead
> > of the cmpxchg instruction and causes us to miss the fixup.
> 
> Indeed.  Nasty issue.
> 
> A quick fix would be to add another fixup to handle both cases

Agreed.

> I checked the other LOCK_PREFIX users and they look ok.

Really ?

> Does this fix it?

No it does not. Chuck tracked it down to 

include/asm-x86/futex_32.h::futex_atomic_cmpxchg_inatomic()

And your patch is changing: __futex_atomic_op2(). That's fine, because
we need the fixup for both places.

Also your patch is against x86.git and not against mainline. Corrected
version below. x86.git needs a different fixup once this hits
mainline, as it unifies the 32/64bit versions.

That's a long standing bug in both the PI futex and the standard futex
code. Needs to go to stable as well.

Thanks,
	tglx

------------->
Subject: x86: fix missing exception entry for SMP alternatives in futex macros
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

The exception fixup for the futex macros __futex_atomic_op2 and
futex_atomic_cmpxchg_inatomic() is missing an entry when the lock
prefix is replaced by a NOP via SMP alternatives.

Chuck Ebert tracked this down from the information provided in:
https://bugzilla.redhat.com/show_bug.cgi?id=429412

The solution is to add another fixup after the LOCK_PREFIX, so both
the LOCK and NOP case have their own entry in the exception table.

The solution was pointed out by Andi Kleen.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>

---
 include/asm-x86/futex_32.h |    8 ++++----
 include/asm-x86/futex_64.h |    8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

Index: linux-2.6/include/asm-x86/futex_32.h
===================================================================
--- linux-2.6.orig/include/asm-x86/futex_32.h	2008-01-22 13:13:10.000000000 +0100
+++ linux-2.6/include/asm-x86/futex_32.h	2008-01-22 13:13:49.000000000 +0100
@@ -28,7 +28,7 @@
 "1:	movl	%2, %0\n\
 	movl	%0, %3\n"					\
 	insn "\n"						\
-"2:	" LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2:	" LOCK_PREFIX "\n 5: cmpxchgl %3, %2\n\
 	jnz	1b\n\
 3:	.section .fixup,\"ax\"\n\
 4:	mov	%5, %1\n\
@@ -36,7 +36,7 @@
 	.previous\n\
 	.section __ex_table,\"a\"\n\
 	.align	8\n\
-	.long	1b,4b,2b,4b\n\
+	.long	1b,4b,2b,4b,5b,4b\n\
 	.previous"						\
 	: "=&a" (oldval), "=&r" (ret), "+m" (*uaddr),		\
 	  "=&r" (tem)						\
@@ -111,7 +111,7 @@ futex_atomic_cmpxchg_inatomic(int __user
 		return -EFAULT;
 
 	__asm__ __volatile__(
-		"1:	" LOCK_PREFIX "cmpxchgl %3, %1		\n"
+		"1:	" LOCK_PREFIX "\n 4: cmpxchgl %3, %1	\n"
 
 		"2:	.section .fixup, \"ax\"			\n"
 		"3:	mov     %2, %0				\n"
@@ -120,7 +120,7 @@ futex_atomic_cmpxchg_inatomic(int __user
 
 		"	.section __ex_table, \"a\"		\n"
 		"	.align  8				\n"
-		"	.long   1b,3b				\n"
+		"	.long   1b,3b,4b,3b			\n"
 		"	.previous				\n"
 
 		: "=a" (oldval), "+m" (*uaddr)
Index: linux-2.6/include/asm-x86/futex_64.h
===================================================================
--- linux-2.6.orig/include/asm-x86/futex_64.h	2008-01-22 13:13:10.000000000 +0100
+++ linux-2.6/include/asm-x86/futex_64.h	2008-01-22 13:13:49.000000000 +0100
@@ -27,7 +27,7 @@
 "1:	movl	%2, %0\n\
 	movl	%0, %3\n"					\
 	insn "\n"						\
-"2:	" LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2:	" LOCK_PREFIX "\n 5: cmpxchgl %3, %2\n\
 	jnz	1b\n\
 3:	.section .fixup,\"ax\"\n\
 4:	mov	%5, %1\n\
@@ -35,7 +35,7 @@
 	.previous\n\
 	.section __ex_table,\"a\"\n\
 	.align	8\n\
-	.quad	1b,4b,2b,4b\n\
+	.quad	1b,4b,2b,4b,5b,4b\n\
 	.previous"						\
 	: "=&a" (oldval), "=&r" (ret), "=m" (*uaddr),		\
 	  "=&r" (tem)						\
@@ -101,7 +101,7 @@ futex_atomic_cmpxchg_inatomic(int __user
 		return -EFAULT;
 
 	__asm__ __volatile__(
-		"1:	" LOCK_PREFIX "cmpxchgl %3, %1		\n"
+		"1:	" LOCK_PREFIX "\n 4: cmpxchgl %3, %1	\n"
 
 		"2:	.section .fixup, \"ax\"			\n"
 		"3:	mov     %2, %0				\n"
@@ -110,7 +110,7 @@ futex_atomic_cmpxchg_inatomic(int __user
 
 		"	.section __ex_table, \"a\"		\n"
 		"	.align  8				\n"
-		"	.quad   1b,3b				\n"
+		"	.quad   1b,3b,4b,3b			\n"
 		"	.previous				\n"
 
 		: "=a" (oldval), "=m" (*uaddr)


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