linux-kernel - Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

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Message-ID: <20080424201630.GC18686@localhost.austin.ibm.com>
Date:	Thu, 24 Apr 2008 15:16:30 -0500
From:	Michael Halcrow <mhalcrow@...ibm.com>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, dave@...ux.vnet.ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 00/13] vfs: add helpers to check r/o bind mounts

On Thu, Apr 24, 2008 at 03:40:03PM -0400, Erez Zadok wrote:
> Ah, I see.  Yes: ecryptfs_init_persistent_file does have this odd
> "try to open readwrite and if that failed, try readonly" code there.
> I can't really say why it's done that way: Mike?  Maybe it was a
> workaround to not having the right permissions to open that
> persistent file?

The notion was that of "best effort," and it is sloppy.

I think the right way to do it will be to allow up to two persistent
files. If the user with read-only perms opens, then open the
persistent file RO. Then a user with write-only (but not read) perms
opens; open another persistent file WO. Allow subsequent reads or
writes by the various users to go through whichever persistent file
has the right access. Then a user with RW access opens the file; close
both the RO file and the WO file and open a new file RW. All the
while, make sure that ecryptfs_open() performs all the requisite perm
checks.

If this sounds reasonable, I will get working on a patch to do this.

Mike
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