sl wrote: > I can see a use for this in re-inserting broken mounts > higher up in the proc tree.
Yes, general "namespace repair" is one of the things I use the writable /proc/pid/ns mechanism for a lot. Especially the fact that you can apply these operations to remote systems via import of /proc can help keep a grid of machines working together smoothly if you need namespace adjustments to fix something or acquire a new resource. One application I have only worked on only in the most direct form (cpns, addns, subns) is that scripts can apply extensive namespace transformations to an arbitarary number of target processes. In theory - and note the "theory" here for this - you could make a 'fixns' cronjob that tested to make sure mounted stuff on the network was still answering, and if it wasnt, it could remove the dead files in /srv, redial, and then use namespace injection to repair things as much as possible. Even though I do some of this now by hand, I have run into one practical obstacle that limits what can be fixed in this way. It's a consequence of something which is important for making the ns rewriting work at all. Rewriting the namespace of a process does not affect the open file descriptors. Once a file descriptor is opened, changing the namespace won't break it. This is actually a very good thing for making namespace injection via /proc usable and not horribly broken as a practical matter. You can freely rewrite the namespace of running processes without corrupting the files they are currently reading and writing to, so it is very good it works this way. However, in the case of some server processes (an example is /srv/cs and the /lib/ndb/local file) the fact that the process maintains an open file descriptor to the file means that you can't use ns mod to re-connect a /srv/cs to a different /lib/ndb/local file. In practice whether the repair you need to do is on a process that has open fds that can't be fixed doesn't have a general answer in my experience at least. Some things fix up nicely but with others, since they are attached to a dead fd and not letting go, changing the ns doesn't help. I've actually thought enough about this that I've wondered what more could be done with the file descriptors and underlying chans to let you do even more to repair and modify what running processes are reading and writing from. All of that said, I can confirm that /proc namespace injection on arbitrary processes does let you fix or workaround some issues that can arise with Plan 9 networks and needing to connect new mounts or reacquire/shift attachment to older ones. Ben Kidwell "mycroftiv"