Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

[Vivek Goyal]

On Thu, Feb 16, 2017 at 07:51:58AM -0800, James Bottomley wrote:

[..]
> > Two levels of checks will simplify this a bit. Top level inode will 
> > belong to the user namespace of caller and checks should pass. And 
> > mounter's creds will have ownership over the real inode so no 
> > additional namespace shifting required there.
> 
> That's the problem: for a marked mount, they don't.

In this new model it does not fit directly. 

I was playing with a slightly different approach and modified patches so
that real root still does the mounting and takes an mount option which
specifies which user namespace we want to shift into. Thanks to Eric for
the idea.

mount -t shiftfs -o userns_fd=<fd> source shifted-fs

In this case real-root is mounter and notion of using mounter's creds on
real-inode works.

This requires a user namespace to be created before shiftfs can be mounted
and then container admin should be able to bind mount shifted-fs.

In this model, intervention of real-root is still required to setup
container and shiftfs. I guess that might not satisfy your needs where
unprivileged user should be able to launch container and be able to
make use of shiftfs, IIUC.

Vivek