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

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Date:	Tue, 11 Sep 2012 17:44:50 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Miklos Szeredi <miklos@...redi.hu>
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

On Tue, Sep 11, 2012 at 10:56:52PM +0200, Miklos Szeredi wrote:
> "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?

Not necessarily.

(It's up to the server--and if anything servers probably want to err on
the side of returning mode bits that are an upper, not a lower, bound on
the permissions.)

> > 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.

Oh, OK, I guess I assumed you were dealing with an NFS filesystem that
had been mounted readonly on the NFS client.

If it's a read-write mount of a filesystem that's read-only on the
server side: well, there is at least an error for that case: the server
should return NFSERR_ROFS, and you should see EROFS--could you do the
copy-up only in the case you get that error?

--b.

> 
> 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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