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

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Date:	Tue, 13 Apr 2010 14:03:32 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	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 Mon, Apr 12, 2010 at 07:34:17PM +0200, Arnd Bergmann wrote:
> >  - make sure every file operation either has a ->llseek instead or
> >    calls nonseekable_open from ->open
> 
> I still think it would be better to always set llseek if we do that,
> even if nonseekable_open is already there. I can come up with scripts
> that check that case, but checking that the open function always
> calls nonseekable_open when it returns success is beyond my grep
> skills ;-)

Yes, it's not quite easily greppable.  Making no seek allowed the
implicit default will fortunately allow us to get rid of that oddness.

> >  - walk through the instances now using default_llseek and chose
> >    a better implementation for this particular instance.  Often
> >    this will be just removing the the lssek method as not allowing
> >    seeks is the right thing to do for character drivers, even if it
> >    is a behaviour change from the current version which usually
> >    is the result of sloppy coding.
> 
> This part is really hard. While in many cases, the driver maintainer
> might know what user space is potentially opening some character
> device, it's really hard to tell for outsiders whether the behaviour
> should be no_llseek (then the default) or noop_llseek to work around
> broken user space.

That's why it's last on the list.

> I think the rule set for the conversion needs to be one that can
> be done purely based on the code. How about this:
> 
> For each file operation {
> 	if (uses f_pos) {
> 		if (same module uses BKL)
> 			-> default_llseek
> 		else
> 			-> generic_file_llseek
> 	} else {
> 		if (driver maintained)
> 			-> no_llseek (with maintainer ACK)
> 		else
> 			-> noop_llseek
> 	}
> }
> 
> Once that is done, we can turn the default into nonseekable
> behavior and start removing instances of explicit no_llseek
> and nonseekable_open.

That plan sounds good to me.

> Should we also rename default_llseek to deprecated_llseek in the
> process, to go along with the approach for ioctl?

I wouldn't bother.  If you can actually work on your plan default_llseek
should be gone soon enough.

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