[PATCH] ARM: plug a race with the alignment trap handler

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Sep 14, 2010 at 11:35:27PM -0400, Nicolas Pitre wrote:
> There is however a small race window when a misaligned access in user
> space is trapped and the alignment trap disabled, but the CPU didn't
> return to user space just yet.  Any exception would be entered from kernel
> space at that point and the kernel would then execute with the alignment
> trap disabled.

This isn't good enough - you can't just disable interrupts and hope
that they'll remain that way.

Consider what happens if the threads time slice has expired, and
TIF_NEED_RESCHED is set - the result will be that we call schedule()
and possibly switch to another thread with alignment faults disabled.

I keep on toying with an idea to use prctl() for alignment faults,
and whether to revamp this code to interact with that - which means
programs can on an individual basis decide how they want alignment
faults to be dealt with.

This would mean storing a per-thread copy of the control register,
which means that the entry*.S code can deal with updating the A bit.