[lxc-devel] [PATCH] Support MS_SHARED /

[Serge Hallyn]

Quoting Michael H. Warfield (mhw at WittsEnd.com):
> Serge,
> 
> Revisiting an earlier remark...
...
> > Now I tested, and with a simple setup we can use a much simpler
> > patch which just does mount("", "/", NULL, MS_SLAVE|MS_REC, 0);
> > for the whole of chroot_into_slave() (and skips the new umount2()
> > in start.c).  The container then starts, and its mounts table
> > is clean.
> 
> Were you still looking at this?  Currently, with the MS_SHARED patch

No, I haven't been.

> work in 0.9.0, the mount table is pretty ugly and running "df" in a
> container is really ugly...
> 

...

> > Where that won't work is in a livecd or any fancy raid setup,
> > where your process's / has a parent which is MS_SHARED.
> 
> How bad is this breakage in regards to that then?

pivot_root would simply fail.

Likewise, the case where / is actually type 'rootfs', which is
not MS_USER and therefore can't be pivot_root()d from would
fail.

There is something else we could try.  Before we chroot() into
our custom MS_SLAVE /, we could fork a child.  That child sticks
around, waits for a signal saying the pivot_root+umounts are
done, then it looks through /proc/self/mounts and unmounts
anything which is not under '/root/'.

I think that might really work best.