Per-instance devpts

[Alan Cox]

> > 1. /dev/ptmx would have to change to a symlink, ptmx -> pts/ptmx.
> 
> IIRC, /dev/tty also needs a similar symlink.

Making them symlinks is asking for trouble because some code does go
around using stat and the like and tools like MAKEDEV have definite ideas.

For /dev/tty the definition is precisely that it is your controlling
tty. No reference to namespace and a task whose controlling tty is in a
different namespace should still open the controlling tty not some
parallel in another universe when you open /dev/tty.

If you want to make sure the controlling tty is in the right namespace
that can be done in userspace when transferring control into a namespace.
In many cases I doubt that is even what is wanted.

> > 2. Permissions on /dev/ptmx would not be persistent, and would have to
> >    be set via devpts mount options (unless they're 0666 root.tty, which
> >    would presumably be the default.)
> > 3. The /proc/sys/kernel/pty limit would be global; a per-filesystem
> >    limit could be added on top or instead (presumably via a filesystem
> >    mount options.)
> >
> > I worry #1 would have substantial user-space impact, but I don't see a way 
> > around it, since there would be no obvious way to associate /dev/ptmx with 
> > a filesystem.

/dev/tty and /dev/ptmx already primarily operate by identifying a device
and switching the work to that. Actually putting a bit of namespace logic
in the driver code so the actual files stay as expected (device nodes
etc) seems a *lot* simpler than trying to do symlink hacks.

Alan