linux-kernel - Re: [PATCH] fbmem: copy_from/to

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Message-Id: <1233091791.6717.3.camel@brick>
Date:	Tue, 27 Jan 2009 13:29:51 -0800
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Righi <righi.andrea@...il.com>, hannes@...xchg.org,
	davej@...hat.com, rjw@...k.pl, hannes@...urebad.de,
	krzysztof.h1@...pl, stefanr@...6.in-berlin.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbmem: copy_from/to_user() with mutex held (v3)

On Tue, 2009-01-27 at 12:52 -0800, Andrew Morton wrote:
> On Tue, 20 Jan 2009 10:27:56 +0100
> Andrea Righi <righi.andrea@...il.com> wrote:
> 
> > Avoid to call copy_from/to_user() with fb_info->lock mutex held in fbmem
> > ioctl().
> > 
> > fb_mmap() is called under mm->mmap_sem (A) held, that also acquires
> > fb_info->lock (B); fb_ioctl() takes fb_info->lock (B) and does
> > copy_from/to_user() that might acquire mm->mmap_sem (A), causing a
> > deadlock.
> > 
> > NOTE: it doesn't push down the fb_info->lock in each own driver's
> > fb_ioctl(), so there're still potential deadlocks somewhere.
> > 
> 
> Looks good to me.
> 
> > ...
> >
> >  static long fb_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > -__acquires(&info->lock)
> > -__releases(&info->lock)
> 
> Should the __acquires/__releases annotation be relocated to
> do_fb_ioctl()?  I've never actually got down and understood those
> things.
> 
> 

There's not really any point as the way lock_fb_info is done, it's essentially
impossible to annotate properly.  You could look at how spin_trylock is done,
but it will require a wrapping macro.  As I suggested the first time this
was posted.

Harvey



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