linux-kernel - Re: [PATCH v4] sysfs: fix kobject refcount to address races with kobject removal

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Message-ID: <20210702010425.frwcbregdyuguaak@garbanzo>
Date:   Thu, 1 Jul 2021 18:04:25 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>, Tejun Heo <tj@...nel.org>
Cc:     rafael@...nel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, andriin@...com, daniel@...earbox.net,
        atenart@...nel.org, alobakin@...me, weiwan@...gle.com,
        ap420073@...il.com, jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, minchan@...nel.org,
        axboe@...nel.dk, mbenes@...e.com, jpoimboe@...hat.com,
        tglx@...utronix.de, keescook@...omium.org, jikos@...nel.org,
        rostedt@...dmis.org, peterz@...radead.org,
        linux-block@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] sysfs: fix kobject refcount to address races with
 kobject removal

On Thu, Jul 01, 2021 at 03:48:22PM -0700, Luis Chamberlain wrote:
> On Fri, Jun 25, 2021 at 02:56:03PM -0700, Luis Chamberlain wrote:
> > On Thu, Jun 24, 2021 at 01:09:03PM +0200, Greg KH wrote:
> > > thanks for making this change and sticking with it!
> > > 
> > > Oh, and with this change, does your modprobe/rmmod crazy test now work?
> > 
> > It does but I wrote a test_syfs driver and I believe I see an issue with
> > this. I'll debug a bit more and see what it was, and I'll then also use
> > the driver to demo the issue more clearly, and then verification can be
> > an easy selftest test.
> 
> OK my conclusion based on a new selftest driver I wrote is we can drop
> this patch safely. The selftest will cover this corner case well now.
> 
> In short: the kernfs active reference will ensure the store operation
> still exists. The kernfs mutex is not enough, but if the driver removes
> the operation prior to getting the active reference, the write will just
> fail. The deferencing inside of the sysfs operation is abstract to
> kernfs, and while kernfs can't do anything to prevent a driver from
> doing something stupid, it at least can ensure an open file ensure the
> op is not removed until the operation completes.

OK and now its not so clear, as it would seem the refcount can indeed
get reduced after we validated it. In any case we'll have enough tools
to reproduce any possible failure soon.

  Luis