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Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: "J. R. Okajima" <hooanon05g@gmail.com>
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Date: Mon, 20 Feb 2017 20:07:53 -0800
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On Tue, 2017-02-21 at 11:57 +0900, J. R. Okajima wrote:
> James Bottomley:
> > I realised as I was trimming down the vestigial inode properties in 
> > the patch that actually shiftfs does use the i_ino from the 
> > underlying for userspace.  The reason why is that it comes from the 
> > getattr call in stat and that's fully what the underlying 
> > filesystem returns (including the inode number).
> 
> Let me make sure.
> - shiftfs has its own inode, but it will never be visible to 
> userspace. - the inode attr visible to users are equivalent to the 
> underlying one,   includeing dev:ino pair.
> right?

Yes, it behaves like a bind mount.

> If so, I am afraid it will make users confused. The dev:ino pair is a
> system-wide identity,

I don't believe it will, otherwise they'd have the same confusion over
a real bind mount.  The dev:inum pair identifies an inode.  An inode
may have many paths and shiftfs just adds a path.

>  but shiftfs creates the same dev:ino pair with different owner.

With a different owner view, but that's irrelevant to the underlying
inode.

>  Though I don't know whether the actual application or LSM exists or
> not who will be damaged by this situation.
> For git-status case which I wrote previously, it might not be a 
> problem as long as dev:ino is unchanged from git index.
> But such filesystem looks weird.

It behaves as much as possible like a bind mount and the user view is
standard behaviour, so it can't really be classified as "weird".  What
won't work like a classic bind mount in this scenario is NFS exporting,
but that's about the only thing.

James