linux-kernel - Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

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Message-ID: <CALCETrVX289Lu6i78U0JquW2euqNCnRjrzHRc99hMAPX0FjMiQ@mail.gmail.com>
Date:	Fri, 24 Jun 2016 11:36:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Fri, Jun 24, 2016 at 10:56 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jun 24, 2016 at 10:47 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> FWIW, your patch is much more lenient than my approach.
>
> I hate big flag-days - they cause so much pain for everybody. The
> people who get it to work and can test it, can't test all the other
> cases (whether they be drivers or other architectures), so I'd much
> rather implement something that allows a gradual per-architecture
> change from having the thread_info on the stack into having the
> thread_info in the task_struct.
>
> Bit "let's just change everything at once" patches are fine (and, in
> fact, preferable) when you can test everything in one go. So something
> that can be statically verified (ie "patch makes no semantic
> difference, but changes calling convention or naming, so if it
> compiles it is fine") I much prefer just getting the pain over and
> done with rather than some lingering thing.
>
> But when it's something where "oops, I broke every other architecture,
> and I can't even test it", I'd much rather do it in a way where each
> architecture can move over to the new model one by one.

Agreed.

To clarify, though: I wasn't planning on changing all arches at once.
 I'm just saying that, for arches that switch over, they get a single
core definition of thread_info.  That way, when someone (probably
named Peter) decides down the road to move, say, thread_info::cpu into
task_struct proper to optimize cache line layout, they won't need to
do it for every architecture.

Also, I want to give people an incentive to finally move their crap
out of struct thread_info and into struct thread_struct.

--Andy