[patch 2.6.17-rc3] i386: fix broken FP exception handling

[patch 2.6.17-rc3] i386: fix broken FP exception handling

[Chuck Ebbert]

The FXSAVE information leak patch introduced a bug in FP exception
handling: it clears FP exceptions only when there are already
none outstanding.  Mikael Pettersson reported that causes problems
with the Erlang runtime and has tested this fix.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Acked-by: Mikael Pettersson <mikpe@it.uu.se>

---

The same bug is in 2.6.16.9+ and this patch applies there as well.


--- 2.6.17-rc3-d4.orig/include/asm-i386/i387.h
+++ 2.6.17-rc3-d4/include/asm-i386/i387.h
@@ -58,13 +58,13 @@ static inline void __save_init_fpu( stru
 	alternative_input(
 		"fnsave %[fx] ;fwait;" GENERIC_NOP8 GENERIC_NOP4,
 		"fxsave %[fx]\n"
-		"bt $7,%[fsw] ; jc 1f ; fnclex\n1:",
+		"bt $7,%[fsw] ; jnc 1f ; fnclex\n1:",
 		X86_FEATURE_FXSR,
 		[fx] "m" (tsk->thread.i387.fxsave),
 		[fsw] "m" (tsk->thread.i387.fxsave.swd) : "memory");
 	/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
 	   is pending.  Clear the x87 state here by setting it to fixed
-   	   values. __per_cpu_offset[0] is a random variable that should be in L1 */
+   	   values. safe_address is a random variable that should be in L1 */
 	alternative_input(
 		GENERIC_NOP8 GENERIC_NOP2,
 		"emms\n\t"	  	/* clear stack tags */
-- 
Chuck
"Penguins don't come from next door, they come from the Antarctic!"

Re: [patch 2.6.17-rc3] i386: fix broken FP exception handling

[Andi Kleen]

On Saturday 29 April 2006 20:07, Chuck Ebbert wrote:
> The FXSAVE information leak patch introduced a bug in FP exception
> handling: it clears FP exceptions only when there are already
> none outstanding.  Mikael Pettersson reported that causes problems
> with the Erlang runtime and has tested this fix.

Patch looks correct, thanks. All other cases (x86-64, 2.4) should
be already ok.

-Andi

Re: [stable] [patch 2.6.17-rc3] i386: fix broken FP exception handling

[Greg KH]

On Sat, Apr 29, 2006 at 02:07:49PM -0400, Chuck Ebbert wrote:
> The FXSAVE information leak patch introduced a bug in FP exception
> handling: it clears FP exceptions only when there are already
> none outstanding.  Mikael Pettersson reported that causes problems
> with the Erlang runtime and has tested this fix.
> 
> Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
> Acked-by: Mikael Pettersson <mikpe@it.uu.se>
> 
> ---
> 
> The same bug is in 2.6.16.9+ and this patch applies there as well.

Thanks, this got included in 2.6.16.12

greg k-h

Re: [patch 2.6.17-rc3] i386: fix broken FP exception handling

[Mikael Pettersson]

On Sat, 29 Apr 2006 14:07:49 -0400, Chuck Ebbert wrote:
>The FXSAVE information leak patch introduced a bug in FP exception
>handling: it clears FP exceptions only when there are already
>none outstanding.  Mikael Pettersson reported that causes problems
>with the Erlang runtime and has tested this fix.
>
>Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
>Acked-by: Mikael Pettersson <mikpe@it.uu.se>

To clarify: the problem the bug caused was not that it broke
Erlang, but that running Erlang (which enables and uses FP
exceptions) caused the kernel to oops badly and hang.

/Mikael