Detecting the use of a mount in another namespace

Daniel P. Berrange berrange at redhat.com
Thu Jan 15 10:34:17 UTC 2015


On Thu, Jan 15, 2015 at 09:56:05AM +0100, Alexander Larsson wrote:
> This is a bit of a weird request, but I'm working on an app sandboxing
> system where each container gets /usr read-only bind mounted from a
> hardlinked tree. When i update the /usr tree I write the new tree to a
> different directory, which avoids affecting any currently running apps
> against the old one.
> 
> However, after updating I'd like to clean out the old version if it is
> not in use. I had a plan for this:
> 1) Move the old usr to a "has been deleted" location
> 2) Try to remove a file inside the user (say ".ref") which the app when
> running has bind-mounted somewhere
> 3) if the remove returned EBUSY, then the usr is in use.
> 
> However, with the recent changes to the semantics in this area this
> doesn't work. The remove always succeeds even if the file is mounted in
> some other namespace.
> 
> I realize that this is better semantics in general, but that was a quite
> useful hack. Is there any other similar way i can detect that something
> is in use in "any other namespace".

Presumably you want something more efficient than scaning /proc/$PID in
the host OS ? eg you read /proc/$PID/mounts for each process, then iterate
stating /proc/$PID/root/<mount> to lookup the st_dev+st_inode of the mount
location to see if the one you care about still exists in any process ?
Not really going to scale nicely with large numbers of $PIDs, so perhaps
you could short circuit by keeping track of your container pid leaders ?

Regards,
Daniel
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