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From: Arnd Bergmann <arndbergmann@googlemail.com>
To: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] sony_pi: Remove the BKL from sonypi_misc_open
Date: Thu, 22 Oct 2009 15:50:04 +0200
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Cc: Frederic Weisbecker <fweisbec@gmail.com>,
       Arnd Bergmann <arndbergmann@googlemail.com>,
       John Kacur <jkacur@redhat.com>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Ingo Molnar <mingo@elte.hu>
References: <alpine.LFD.2.00.0910182353480.3405@localhost.localdomain> <20091021213139.GC4880@nowhere> <20091022025514.GB17438@infradead.org>
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On Thursday 22 October 2009, Christoph Hellwig wrote:

> The right (although quite complicated) thing is to return -ESPIPE from
> vfs_llseek if no ->llseek method is present, or even better also
> disallowing pread/pwrite by default.  It'll need a quite substantial
> audit and is best done by different types of inodes - S_IFIFO is easy,
> SIFDIR at least has very few instances, S_IFREG usually wants a real
> llseek (generic_file_llseek in most cases) and directories also need
> a llseek method that takes i_mutex so it protects against namespace
> operations.

Is it safe to assume that file_operations without a read() or write()
method also don't need llseek?

There are over 200 instances of file_operations that have a no read,
write or lseek operations and we can easily detect that in vfs_llseek,
calling no_llseek by default.

Testing for S_IFREG will not work well for debugfs, which is probably
a large number of the cases that do not want an llseek method.

	Arnd <><
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