linux-kernel - Re: [RFC] correct flags to f_mode conversion in __dentry

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Message-ID: <20080312183441.GJ27894@ZenIV.linux.org.uk>
Date:	Wed, 12 Mar 2008 18:34:41 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, alan <alan@...hat.com>,
	aviro@...hat.com, drepper@...hat.com, hch@...radead.org,
	sds@...ho.nsa.gov, jmorris@...ei.org
Subject: Re: [RFC] correct flags to f_mode conversion in __dentry_open

On Wed, Mar 12, 2008 at 02:25:27PM -0400, Eric Paris wrote:
> I recently tried to add an SELinux BUG_ON in the case where the kernel
> made a permission request for no permissions and was able to stumble
> over it with something as simple as
> 
> open("/dev/null", 3);
> 
> Notice that 3 == (O_RDWR | O_WRONLY)
> 
> First question, is 3 ever a valid flag from from userspace to sys_open?

Yes.  "Check for both read and write permissions, set neither FMODE_READ
nor FMODE_WRITE".

Don't break drivers, please - some use that for "ioctl-only" opens,
with special semantics for those.
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