RELEASE BLOCKER: Linux doesn't follow x86/x86-64 ABI wrt direction flag

[H. Peter Anvin]

Chris Lattner wrote:
>>>> Richard Guenther wrote:
>>>>> We didn't yet run into this issue and build openSUSE with 4.3 since 
>>>>> more
>>>>> than
>>>>> three month.
>>>>
>>>> Well, how often do you take a trap inside an overlapping memmove()?
>>>
>>> How hard is it to change the kernel signal entry path from "pushf" to
>>> "pushf;cld"?  Problem solved, no?
>>
>> The problem is with old kernels, which by definition stay unfixed.
> 
> My impression was that the problem occurs in GCC compiled code in the 
> kernel itself, not in user space:

That's wrong.

The issue is that the kernel is entered (due to a trap, interrupt or 
whatever) and the state is saved.  The kernel decides to revector 
userspace to a signal handler.  The kernel modifies the userspace state 
to do so, but doesn't set DF=0.

Upon return to userspace, the modified state kicks in.  Thus the signal 
handler is entered with DF from userspace at trap time, not DF=0.

So it's an asynchronous state leak from one piece of userspace to another.

	-hpa