Re: [PATCH] x86 page fault handler not interrupt safe

[Nigel Gamble]

On Mon, 7 May 2001, Brian Gerst wrote:
> Nigel Gamble wrote:
> > 
> > On Mon, 7 May 2001, Linus Torvalds wrote:
> > > On Mon, 7 May 2001, Brian Gerst wrote:
> > > > This patch will still cause the user process to seg fault: The error
> > > > code on the stack will not match the address in %cr2.
> > >
> > > You've convinced me. Good thinking. Let's do the irq thing.
> > 
> > I've actually seen user processes seg faulting because of this with the
> > fully preemptible kernel patch applied.  The fix we used in that patch
> > was to use an interrupt gate for the fault handler, then to simply
> > restore the interrupt state:
> 
> Keep in mind that regs->eflags could be from user space, and could have
> some undesirable flags set.  That's why I did a test/sti instead of

Good point.

> reloading eflags.  Plus my patch leaves interrupts disabled for the
> minimum time possible.

I'm not sure that it makes much difference, as interrupts are disabled
for such a short time anyway.  I'd prefer to put the test/sti in
do_page_fault(), and reduce the complexity needed in assembler routines
as much as possible, for maintainability reasons.

Nigel Gamble                                    [e-mail]
Mountain View, CA, USA.                         http://www.nrg.org/

MontaVista Software                             [e-mail]