linux-ext4 - Re: [PATCH 1/3] implement uid and gid mount options for ext2

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Message-ID: <87629130u0.fsf@xmission.com>
Date:	Thu, 02 Aug 2012 06:00:55 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Ludwig Nussel <ludwig.nussel@...e.de>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Jan Kara <jack@...e.cz> (maintainer:EXT2 FILE SYSTEM),
	Rob Landley <rob@...dley.net> (maintainer:DOCUMENTATION),
	Andrew Morton <akpm@...ux-foundation.org> (maintainer:EXT3 FILE
	SYSTEM),
	Andreas Dilger <adilger.kernel@...ger.ca> (maintainer:EXT3 FILE
	SYSTEM),
	"Theodore Ts'o" <tytso@....edu> (maintainer:EXT4 FILE SYSTEM),
	linux-ext4@...r.kernel.org (open list:EXT2 FILE SYSTEM),
	linux-doc@...r.kernel.org (open list:DOCUMENTATION)
Subject: Re: [PATCH 1/3] implement uid and gid mount options for ext2

Alan Cox <alan@...rguk.ukuu.org.uk> writes:

>> +uid=n[:m]			Make all files appear to belong to uid n.
>> +				Useful for e.g. removable media with fstab
>> +				options 'user,uid=useruid'. The optional second
>> +				uid m is actually written to the file system.
>
> So what happens if I insert a USB key containing a device node to the
> hard disk ? Why is this functionality useful in the current form ?
>
> (ie I think you need an actual rational security model first)

> Also why is this at the per fs level duplicating stuff each time rather
> than at the vfs level - this seems to be vfs level functionality.

Even more fun there is essentially a generic implementation in the user
namespaces.  What needs to be implemented to support this is support for
mounting in a non-default user namespace, and then all of the mapping
functionality is generic.  Which I presume will imply MS_NODEV.

Eric
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