linux-kernel - Re: [PATCH 6/6] procfs: Kill the bkl in ioctl

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Date:	Tue, 30 Mar 2010 11:33:40 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Kacur <jkacur@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Al Viro <viro@...iv.linux.org.uk>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 6/6] procfs: Kill the bkl in ioctl

On Tuesday 30 March 2010, Frederic Weisbecker wrote:
> On Tue, Mar 30, 2010 at 09:38:11AM +0300, Alexey Dobriyan wrote:
> > On Tue, Mar 30, 2010 at 9:20 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > --- a/fs/proc/inode.c
> > > +++ b/fs/proc/inode.c
> > > @@ -231,9 +231,9 @@ static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigne
> > >                if (rv == -ENOIOCTLCMD)
> > >                        rv = -EINVAL;
> > >        } else if (ioctl) {
> > > -               lock_kernel();
> > > +               WARN_ONCE(1, "Procfs ioctl handlers must use unlocked_ioctl, "
> > > +                         "%pf will be called without the Bkl held\n", ioctl);
> > >                rv = ioctl(file->f_path.dentry->d_inode, file, cmd, arg);
> > > -               unlock_kernel();
> > 
> > Then delete the branch.
> > Or go through formal feature-removal procedure.
> 
> 
> I thought about it. I even started to write something in this
> feature-removal file but realized that I can't remove the
> .ioctl() callback from file operations. We still need to check
> the user hasn't made the mistake of implementing it.
> 
> What I can plan as a feature removal, though, is to keep the warning
> but don't actually call the ioctl.

I believe we can actually remove ioctl from file_operations. The patch I did
to convert all users to ".unlocked_ioctl = default_ioctl," should really catch
all cases, and I think we can enforce this by renaming fops->ioctl to locked_ioctl
or old_ioctl to make sure we didn't miss any, and then mandate that this one
is only used when unlocked_ioctl is set to default_ioctl.

I also remember going through procfs ioctl operations some time ago and finding
exactly three users, which I believe are the same ones that Frederic found.

	Arnd
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