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From: Miklos Szeredi <miklos@szeredi.hu>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Andy Whitcroft <apw@canonical.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] issues with NFS filesystems as lower layer
Date: Tue, 11 Sep 2012 22:56:52 +0200	[thread overview]
Message-ID: <87ipbkxoq3.fsf@tucsk.pomaz.szeredi.hu> (raw)
In-Reply-To: <20120907193825.GA4633@fieldses.org> (J. Bruce Fields's message of "Fri, 7 Sep 2012 15:38:26 -0400")

"J. Bruce Fields" <bfields@fieldses.org> writes:

>> > Secondly when using an NFSv3 R/O lower layer the filesystem permissions
>> > check refuses permission to write to the inode which prevents us from
>> > copying it up even though we have a writable upper layer.  (With an ext4
>> > lower layer the inode check will succeed if the inode  is writable even
>> > if the filesystem is not.)  It is not clear what the right solution is
>> > here.  One approach is to check the inode permissions only (avoiding the
>> > filesystem specific permissions op), but it is not clear we can rely on
>> > these for all underlying filesystems.  Perhaps this check should only be
>> > used for NFS.
>
> Then couldn't you for example end up circumventing ACLs on the
> underlying file to access data cached by reads from another user on the
> same system?

Ignoring ACL's should always give less access, isn't that right?

>
> Is it possible to arrange that the check for a readonly filesystem be
> done only by the vfs and not also by ->permission?

You'd need to modify NFS servers for that to work, no?  It's possible
but not practical.

Thanks,
Miklos



>
> --b.
>
>> > Perhaps it needs to be a mount option.  The second patch
>> > (for discussion) following this email implements this, using the inode
>> > permissions when the lowerlayer is read-only.  This seems to work as
>> > expected in my limited testing.
>> 
>> I fear that will create an inconsistency between the read-only and the
>> non-read-only case, even though both should behave the same.
>> 
>> I think the cleanest would be to create a mount option to always use
>> generic_permission (on both the lower and the upper fs).  That would
>> give us two, slightly different, operating modes but each would be
>> self consistent.
>> 
>> Thanks,
>> Miklos
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
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  reply	other threads:[~2012-09-11 20:55 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-06 15:56 [RFC PATCH 0/2] issues with NFS filesystems as lower layer Andy Whitcroft
2012-09-06 15:56 ` [RFC PATCH 1/2] ovl: ovl_copy_up_xattr may fail when the upper filesystem does not support the same xattrs Andy Whitcroft
2012-09-06 15:56 ` [PATCH 2/2] overlayfs: when the underlying filesystem is read-only use inode permissions Andy Whitcroft
2012-09-07  6:35 ` [RFC PATCH 0/2] issues with NFS filesystems as lower layer Miklos Szeredi
2012-09-07 19:38   ` J. Bruce Fields
2012-09-11 20:56     ` Miklos Szeredi [this message]
2012-09-11 21:44       ` J. Bruce Fields
2012-09-12 15:20         ` Miklos Szeredi
2012-09-12 16:07           ` J. Bruce Fields

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