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

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Date:	Tue, 6 Jan 2009 16:20:54 -0500
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	"Mike Frysinger" <vapier.adi@...il.com>
Cc:	"Greg KH" <gregkh@...e.de>, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: debugfs & vfs file permission issue?

On Tue 6 Jan 2009 10:20, Mike Frysinger pondered:
> On Tue, Jan 6, 2009 at 10:12, Robin Getz wrote:
> > On Tue 6 Jan 2009 07:05, Robin Getz suggested:
> >> adding a readonly, and writeonly, and ensuring that when you call
> >> debugfs_create_*, the mode is checked, and the "correct" fops are set
> >> doesn't seem like it would be a bad idea? This would enforce the
> >> kernel programmer's view on the world, and not allow pesky root users
> >> to override things....
> >>
> >> Greg - would you take something like that?
> >
> > How about this?
> >
> > Feel free to nak it - we can do the same thing where we are calling the
> > debugfs_create_* functions - this just makes it cleaner in my opinion.
> >
> > ---
> >
> > In many SOC implementations there are hardware registers can be read only,
> > or write only. This extends the debugfs to enforce the file permissions for
> > these types of registers, by providing a set of fops which are read only
> > or write only. This assumes that the kernel developer knows more about the
> > hardware than the user (even root users) - which is normally true.
> 
> we want it for cpu registers, but i dont see any reason why this
> wouldnt also apply to external devices attached via memory interfaces
> ... fifos and such ...

Yes - Although the existing use case is for SOC on chip registers - it applies
to anything where the kernel developer really doesn't want to allow the 
user to do an access that will cause a negative side effect (crash, effect
fifos, etc).

-Robin
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