linux-kernel - Re: lseek() on debugfs entries in 2.6.37

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Message-Id: <201103151501.42105.arnd@arndb.de>
Date:	Tue, 15 Mar 2011 15:01:41 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexey Mikhailov <karma@...ois.botik.ru>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: lseek() on debugfs entries in 2.6.37

On Tuesday 15 March 2011, Alexey Mikhailov wrote:
> I use simple debugfs entries for user-space <-> kernel-space
> interaction. Basically I read unsigned integers from debugfs
> files like this:
> 
>    ...
>    char buf[64];
>    lseek(timesync_fd, 0, SEEK_SET);
>    read(timesync_fd, buf, sizeof(buf));
>    ...
> 
> It works perfectly with 2.6.32 kernel. But with 2.6.37 kernel,
> lseek() fails with errno=29(Illegal seek). So second read()
> call just fails or returns garbage. Can someone please shed
> some light on it?

This is probably a result of the changes I made as part of the
BKL removal. Which file specifically are you talking about?

In older kernels, having no .llseek function meant that you
implicitly get default_llseek. New kernels now require 
that the driver explicitly chooses one llseek variant.

I did a patch to automatically convert all drivers, but
this patch may have missed some that got moved around
while I was patching them.

	Arnd
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