linux-kernel - Re: [PATCH 0/4] treewide: fix interrupted release

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Message-ID: <20191014084847.GD11828@phenom.ffwll.local>
Date:   Mon, 14 Oct 2019 10:48:47 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Johan Hovold <johan@...nel.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Rob Clark <robdclark@...il.com>,
        Sean Paul <sean@...rly.run>,
        Fabien Dessenne <fabien.dessenne@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        David Airlie <airlied@...ux.ie>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-s390@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/4] treewide: fix interrupted release

On Fri, Oct 11, 2019 at 11:36:33AM +0200, Johan Hovold wrote:
> On Thu, Oct 10, 2019 at 03:50:43PM +0200, Daniel Vetter wrote:
> > On Thu, Oct 10, 2019 at 03:13:29PM +0200, Johan Hovold wrote:
> > > Two old USB drivers had a bug in them which could lead to memory leaks
> > > if an interrupted process raced with a disconnect event.
> > > 
> > > Turns out we had a few more driver in other subsystems with the same
> > > kind of bug in them.
> 
> > Random funny idea: Could we do some debug annotations (akin to
> > might_sleep) that splats when you might_sleep_interruptible somewhere
> > where interruptible sleeps are generally a bad idea? Like in
> > fops->release?
> 
> There's nothing wrong with interruptible sleep in fops->release per se,
> it's just that drivers cannot return -ERESTARTSYS and friends and expect
> to be called again later.

Do you have a legit usecase for interruptible sleeps in fops->release?

I'm not even sure killable is legit in there, since it's an fd, not a
process context ...

> The return value from release() is ignored by vfs, and adding a splat in
> __fput() to catch these buggy drivers might be overkill.

Ime once you have a handful of instances of a broken pattern, creating a
check for it (under a debug option only ofc) is very much justified.
Otherwise they just come back to life like the undead, all the time. And
there's a _lot_ of fops->release callbacks in the kernel.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch