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

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Date:	Tue, 6 Jan 2009 01:32:30 -0500
From:	"Mike Frysinger" <vapier.adi@...il.com>
To:	"Greg KH" <gregkh@...e.de>
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 6, 2009 at 01:19, Greg KH wrote:
> 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 :)

yeah and we have some readable but not writable addresses too :/

>> >> 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.

would it be useful for this to be a common function ?  probably not so
much considering (i'm assuming) no one else has asked so far for such
a beast ...
-mike
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