[RFC 0/1] shiftfs: uid/gid shifting filesystem

[James Bottomley <James.Bottomley@HansenPartnership.com>]

This is currently an RFC because the patch applies to Linus head, but
needs altering for the vfs tree, so I'll respin and resend after the
merge window closes.

My use case for this is that I run a lot of unprivileged architectural
emulation containers on my system using user namespaces.  Details here:

http://blog.hansenpartnership.com/unprivileged-build-containers/

They're mostly for building non-x86 stuff (like aarch64 and arm secure
boot and mips images).  For builds, I have all the environments in my
home directory with downshifted uids; however, sometimes I need to use
them to administer real images that run on systems, meaning the uids
are the usual privileged ones not the downshifted ones.  The only
current choice I have is to start the emulation as root so the uid/gids
match.  The reason for this filesystem is to use my standard
unprivileged containers to maintain these images.  The way I do this is
crack the image with a loop and then shift the uids before bringing up
the container.  I usually loop mount into /var/tmp/images/, so it's
owned by real root there:

jarvis:~ # ls -l /var/tmp/images/mips|head -4
total 0
drwxr-xr-x 1 root root 8192 May 12 08:33 bin
drwxr-xr-x 1 root root    6 May 12 08:33 boot
drwxr-xr-x 1 root root  167 May 12 08:33 dev

And I usually run my build containers with a uid_map of 

         0     100000       1000
      1000       1000          1
     65534     101000          1

(maps 0-999 shifted, then shifts nobody to 1000 and keeps my uid [1000]
fixed so I can mount my home directory into the namespace) and
something similar with gid_map. So I shift mount the mips image with

mount -t shiftfs -o uidmap=0:100000:1000,uidmap=65534:101000:1,gidmap=0:100000:100,gidmap=101:100101:899,gidmap=65533:101000:2 /var/tmp/images/mips /home/jejb/containers/mips

and I now see it as

jejb@jarvis:~> ls -l containers/mips|head -4
total 0
drwxr-xr-x 1 100000 100000 8192 May 12 08:33 bin/
drwxr-xr-x 1 100000 100000    6 May 12 08:33 boot/
drwxr-xr-x 1 100000 100000  167 May 12 08:33 dev/

Like my usual unprivileged build roots and I can now use an
unprivileged container to enter and administer the image.

It seems like a lot of container systems need to do something similar
when they try and provide unprivileged access to standard images. 
 Right at the moment, the security mechanism only allows root in the
host to use this, but it's not impossible to come up with a scheme for
marking trees that can safely be shift mounted by unprivileged user
namespaces.

James

---

 fs/Kconfig                 |   8 +
 fs/Makefile                |   1 +
 fs/shiftfs.c               | 833 +++++++++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/magic.h |   2 +
 4 files changed, 844 insertions(+)