linux-kernel - Re: debugfs & vfs file permission issue?

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Date:	Mon, 5 Jan 2009 22:19:46 -0800
From:	Greg KH <gregkh@...e.de>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Robin Getz <rgetz@...ckfin.uclinux.org>, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: debugfs & vfs file permission issue?

On Tue, Jan 06, 2009 at 12:54:58AM -0500, Mike Frysinger wrote:
> On Tue, Jan 6, 2009 at 00:48, Greg KH wrote:
> > On Mon, Jan 05, 2009 at 09:57:07PM -0500, Robin Getz wrote:
> >> On 2.6.28-rc2, If I create a debugfs file with
> >>
> >>     debugfs_create_x16("SPORT1_TX", 0200 , parent, 0xFFC00910);
> >
> > Um, are you are passing in a pointer to a known memory location
> > properly?  Why would it be ok for the kernel to directly read that
> > location?
> 
> it's a nommu system and the 0xffc00000+ addresses are always available

Writable but not readable?  Isn't hardware fun :)

> >> Although the file shows up as write only (no read):
> >>
> >> root:/> ls -l /sys/kernel/debug/blackfin/SPORT/SPORT1_TX
> >> --w-------    1 root     root            0 Jan  1
> >> 2007 /sys/kernel/debug/blackfin/SPORT/SPORT1_TX
> >>
> >> root:/> cat /sys/kernel/debug/blackfin/SPORT/SPORT1_TX
> >>
> >> Still works - and causes the read to occur, which crashes :(
> >
> > You're root, you can do anything :)
> 
> any thoughts on how to declare debugfs files that are read or write
> only ?  we'll have to add new helper functions or have it be a
> parameter or declare our own debugfs file ?

Just use debugfs_create_file() and use your own read/write functions to
prevent a read or write from happening no matter what.  No new debugfs
infrastructure should be needed.

good luck,

greg k-h
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