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

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Date:	Wed, 16 Mar 2011 13:49:20 +0300
From:	Alexey Mikhailov <karma@...ois.botik.ru>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: lseek() on debugfs entries in 2.6.37

At Tue, 15 Mar 2011 15:01:41 +0100, Arnd Bergmann wrote:
Hello!

> 
> 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.

Thank you for reply. Obvious patches like this one fixed it

   static struct file_operations fops_timesync = {
      .owner = THIS_MODULE,
      .open = fop_open_timesync,
      .read = fop_read_timesync,
  +   .llseek = default_llseek,
   };

I assume there is the reason you need to specify this
explicitly as it will break much out-of-kernel code.

-- Alexey



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