linux-kernel - Re: [PATCH] macintosh: Explicitly set llseek to no

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Date:	Mon, 16 Nov 2009 11:54:27 +0100
From:	Christoph Hellwig <hch@....de>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Arnd Bergmann <arndbergmann@...glemail.com>,
	Jonathan Corbet <corbet@....net>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	John Kacur <jkacur@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] macintosh: Explicitly set llseek to no_llseek in ans-lcd

On Mon, Nov 02, 2009 at 04:51:52PM +0100, Arnd Bergmann wrote:
> I looked at what places in the code manipulate file->f_pos directly
> and found that almost all the uses in driver code are broken because
> they don't take any locks. Most of them are in driver specific
> lseek operations. Others are in read/write methods and are even
> more broken because the change gets immediately overwritten by
> vfs_read/vfs_write when the driver method returns.
> 
> IMHO, we should complete the pushdown into all ioctl methods
> that John and Thomas have started working on, fixing the lseek
> methods in those files we touch.
> 
> When that is done, all interaction between default_llseek and
> driver locking has to be with explicit calls to lock_kernel
> in those drivers, so we can reasonably well script the search
> for drivers needing the BKL in llseek: everyhing that
>  a) defines file_operations without an llseek function,
>  b) touches f_pos somewhere, and
>  c) calls lock_kernel() somewhere
> That should only be a small number and when they are fixed,
> we can remove default_llseek and instead call generic_file_llseek
> for any file operation without a separate llseek method.

As mentioned before making generic_file_llseek the new default is
probably a bad idea.  The majority of our file_operations instances
don't actually support seeking, so no_llseek should become the new
default if you spend some effort on converting things.  Anything that
wants to allow seeking will have to set a llseek method.  This also
mirrors what we do for other file operations.  None of the major ones
has a non-trivial default, it's either silently succeeding for a
selected few like open or release or returning an error for operatings
that actually do something like read and write.

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