linux-kernel - Re: [RFC PATCH 0/2] issues with NFS filesystems as lower layer

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Date:	Tue, 11 Sep 2012 22:56:52 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Andy Whitcroft <apw@...onical.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] issues with NFS filesystems as lower layer

"J. Bruce Fields" <bfields@...ldses.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
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