linux-kernel - Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YXlh5K7FyeBXNafM@bombadil.infradead.org>
Date:   Wed, 27 Oct 2021 07:27:48 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Ming Lei <ming.lei@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Petr Mladek <pmladek@...e.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, tj@...nel.org,
        gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        minchan@...nel.org, jeyu@...nel.org, shuah@...nel.org,
        bvanassche@....org, dan.j.williams@...el.com, joe@...ches.com,
        tglx@...utronix.de, keescook@...omium.org, rostedt@...dmis.org,
        linux-spdx@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

On Wed, Oct 27, 2021 at 01:57:40PM +0200, Miroslav Benes wrote:
> On Tue, 26 Oct 2021, Luis Chamberlain wrote:
> 
> > On Tue, Oct 26, 2021 at 11:37:30PM +0800, Ming Lei wrote:
> > > OK, then Luis shouldn't consider livepatching as one such issue to solve
> > > with one generic solution.
> > 
> > It's not what I was told when the deadlock was found with zram, so I was
> > informed quite the contrary.
> 
> From my perspective, it is quite easy to get it wrong due to either a lack 
> of generic support, or missing rules/documentation.

Indeed. I agree some level of guidence is needed, even if subtle, rather
than tribal knowledge. I'll start off with the test_sysfs demo'ing what
not to do and documenting this there. I don't think it makes sense to
formalize yet documentation for "though shalt not do this" generically
until a full depth search is done with Coccinelle.

> So if this thread 
> leads to "do not share locks between a module removal and a sysfs 
> operation" strict rule, it would be at least something.

I think that's where we are at. I'll wait to complete my coccinelle
deadlock hunt patch to complete the full search, and that could be
useful to *warn* aboute new use cases, so to prevent this deadlock
in the future. Until then I agree that the complexity introduced is
not worth it given the evidence of users, but the full evidence of
actual users still remains to be determined. A perfect job left to
advances with Coccinelle.

> In the same 
> manner as Luis proposed to document try_module_get() expectations.

Right and so sysfs ops using try_module_get() *still* remains safe,
and so will keep that patch in my next iteration because there *are*
*many* uses cases for that.

  Luis