linux-kernel - Re: [PATCH v2 03/14] notifiers: Assert that RCU is watching in notify

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Date:	Mon, 22 Jun 2015 12:48:52 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Kees Cook <keescook@...omium.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2 03/14] notifiers: Assert that RCU is watching in notify_die

On Mon, Jun 22, 2015 at 11:15 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jun 22, 2015 at 10:37:39AM -0700, Andy Lutomirski wrote:
>> But if we OOPS, we'll OOPS after the lockdep splat and the lockdep
>> splat will scroll off the screen, right?  Am I missing something here?
>
> No, you're not.
>
>> notify_die is called before the actual OOPS code is invoked in traps.c.
>
> Yes, and with this assertion, you get to potentially print two
> dump_stack()'s back-to-back instead of the one from traps.c.
>
> And if the machine is about to be wedged solid soon anyway, we want to
> dump as less (not-so-important) blurb to serial/console as possible. And
> in this case, my suspicion is not that the lockdep splat will scroll
> off the screen but that we might freeze before we even issue the whole
> thing.
>
> That's why I think we should be conservative and make the lockdep splat
> come out second, if possible.

That'll annoy people using regular consoles, though.

I think this scenario isn't that likely. If we dereference a NULL
pointer, then we really should rcu watching before we actually oops in
the page fault code.  Similarly, if we take a non-fixed-up GPF, we
should have rcu watching in the early part of do_general_protection.

I'd be all for skipping the assertion entirely if we're going to OOPS,
but we don't know whether we're actually OOPSing when notify_die is
called.  We could individually instrument everything, or we could just
drop this patch entirely, but it has helped me catch some goofs while
developing all this code.

--Andy

>
> Am I making more sense now?
>
> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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