[uml-user] mapping root uid for hostfs

[D. J. A. <de...@ra...>]

Inspired by the news that UML is now self-hosting, among other
things, I thought I'd try to run UML on its own nice big partition of
a new disk, via hostfs, before I clutter the disk up with other
stuff.

I mount a downloaded root_fs on loopback, and used rsync to copy all
of it over to the intended hostfs space, /mnt/uml.

I change the ownership of the files similarly to what is suggested by
this:

  http://user-mode-linux.sourceforge.net/UserModeLinux-HOWTO-9.html#ss9.1

  find /mnt/uml -uid 0 | xargs chown deejoe

I started UML as user 'deejoe' with

   linux eth0=tuntap,,,192.168.0.97 ubd0=/mnt/uml

So far, so good.

The host system is Mandrake, but the original root_fs I'm working
from is root_fs_debian2.2_small.  I'd like to start filling out the
debian system with additional packages.

With a loopback root filesystem, I've been able to do 'apt-get
update'/'apt-get install packagename' or 'apt-get upgrade' just fine
and I hoped to be able to do this through hostfs.[1]  

But, the Debian scripts want to make certain newly-installed files
owned by root, apparently, and this kind of error occurs:

tar: ./postinst: Cannot change ownership to uid 0, gid 0: Operation
not permitted

If I go back and reverse the ownership change above:

    find /mnt/uml/ -user deejoe | xargs chown root

and then invoke user-mode-linux at the command line as 'root' on the
host system, apt-get works just fine, which makes sense, since
everything is uid 0 at that point:  the user in uml, the host-context
user running uml, and the ownership of the files.

What would be really nice, though, would be for the uid=0 from the
uml context to be mapped to the unprivileged uid in the
host context which executed the instance of uml ('deejoe' in this
case).

If I'm missing something about how to make this work the way I
expect it to (ie, as a loopback filesystem would) or why it would be
a Bad Thing for it to work the way I want, I'd appreciate some
pointers.

Thanks,

--Joe

[1] I'd rather use hostfs than a loopback filesystem file here, because
with loopback I lose the flexibility of having a) enough space to
install all the files I want but b) without having to commit up front
to a given size for the loopback filesystem.  A loopback filesystem
pressed into use for non-uml storage is suboptimal, whereas the same
disk space used via hostfs in uml is just another filesystem when not
used by uml.

[2] I'm running the penultimate binary uml rpm, user_mode_linux-2.4.18.2um-0