Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral requests NMI-safe

On Thu, Jan 23, 2025 at 11:48:10AM -0800, Josh Poimboeuf wrote:
> On Thu, Jan 23, 2025 at 09:40:26AM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 22, 2025 at 02:49:02PM -0800, Josh Poimboeuf wrote:
> > > But also, the nmi_cookie is still needed for the case where the NMI
> > > arrives before info->cookie gets cleared by early entry-from-user.
> > 
> > So how about we clear cookie (and set nr_entries to -1) at
> 
> I think we could set nr_entries to 0 instead of -1?
> 
> > return-to-user, after we've done the work loop and have interrupts
> > disabled until we hit userspace.
> >
> > Any NMI that hits there will have to cause another entry anyway.
> 
> But there's a cookie mismatch:
> 
>     // return-to-user: IRQs disabled
>     <NMI>
> 	current->unwind_info.cookie = 0x1234
>     </NMI>
>     unwind_exit_to_user_mode()
> 	current->unwind_info.cookie = 0
>     IRET
> <IRQ>
>     task_work()
>         callback(@cookie=WRONG)

Though, assuming we're keeping the unwind_work struct, there's a simpler
alternative to nmi_cookie: store the cookie in the unwind_work.  Then
the task work can just use that instead of current->unwind_info.cookie.

-- 
Josh