Re: [RFC PATCH v9 02/13] x86: always set IF before oopsing from page fault

[Andy Lutomirski <luto@kernel.org>]

On Wed, Apr 3, 2019 at 6:42 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> On Wed, Apr 03, 2019 at 05:12:56PM -0700, Andy Lutomirski wrote:
> > On Wed, Apr 3, 2019 at 10:36 AM Khalid Aziz <khalid.aziz@oracle.com> wrote:
> > >
> > > From: Tycho Andersen <tycho@tycho.ws>
> > >
> > > Oopsing might kill the task, via rewind_stack_do_exit() at the bottom, and
> > > that might sleep:
> > >
> >
> >
> > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > > index 9d5c75f02295..7891add0913f 100644
> > > --- a/arch/x86/mm/fault.c
> > > +++ b/arch/x86/mm/fault.c
> > > @@ -858,6 +858,12 @@ no_context(struct pt_regs *regs, unsigned long error_code,
> > >         /* Executive summary in case the body of the oops scrolled away */
> > >         printk(KERN_DEFAULT "CR2: %016lx\n", address);
> > >
> > > +       /*
> > > +        * We're about to oops, which might kill the task. Make sure we're
> > > +        * allowed to sleep.
> > > +        */
> > > +       flags |= X86_EFLAGS_IF;
> > > +
> > >         oops_end(flags, regs, sig);
> > >  }
> > >
> >
> >
> > NAK.  If there's a bug in rewind_stack_do_exit(), please fix it in
> > rewind_stack_do_exit().
>
> [I trimmed the CC list since google rejected it with E2BIG :)]
>
> I guess the problem is really that do_exit() (or really
> exit_signals()) might sleep. Maybe we should put an irq_enable() at
> the beginning of do_exit() instead and fix this problem for all
> arches?
>

Hmm.  do_exit() isn't really meant to be "try your best to leave the
system somewhat usable without returning" -- it's a function that,
other than in OOPSes, is called from a well-defined state.  So I think
rewind_stack_do_exit() is probably a better spot.  But we need to
rewind the stack and *then* turn on IRQs, since we otherwise risk
exploding quite badly.