Re: [PATCH] private mounts

[Jamie Lokier]

Pavel Machek wrote:
> > > Actually, after you add right mount xyzzy /foo lines into .profile,
> > > you can just . ~/.profile ;-).
> > 
> > Is there a mount command that can do that?  We're talking about
> > private mounts - invisible to other namespaces, which includes the
> > other shells.
> > 
> > If there was a /proc/NNN/namespace, that would do the trick :)
> 
> Sounds like the solution, then. I do not think Al Viro is going to
> kill you for /proc/NNN/namespace...

Looking closer, I think we already have it.

It's called /proc/NNN/root.

Does chroot into /proc/NNN/root cause the chroot'ing process to adopt
the namespace of NNN?  Looking at the code, I think it does.

Furthermore, I think a daemon can acquire file descriptors for
multiple namespaces already, by open("/") and passing descriptors
between processes.  And the chroot can be done using /proc/self/fd/N
after receiving a descriptor.

This is because file descriptors, and current->fs->pwd and
current->fs->root, record the vfsmnt as well as the dentry that they
opened.

So no new system calls are needed.  A daemon to hand out per-user
namespaces (or any other policy) can be written using existing
kernels, and those namespaces can be joined using chroot.

That's the theory anyway.  It's always possible I misread the code (as
I don't use namespaces and don't have tools handy to try them).

-- Jamie
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