Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.

From: Linus Torvalds
Date: Wed Apr 20 2016 - 00:49:57 EST

On Tue, Apr 19, 2016 at 9:36 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Wed, Apr 20, 2016 at 6:04 AM, Eric W. Biederman
>> The kernel.pty.reserve sysctl is neutered with no way currently
>> implemented to be able to use the reserved ptys.
> I think we could convert this into reserve for init user namespace,
> ssh in host will work even if containers eaten all ptys.

Yes. That's basically how it effectively worked before (ie everything
but the initial non-newinstance devpts mount would be limited to the
non-reserved numbers).

We required the non-init namespaces to do a newinstance mount, so the
whole test for "newinstance" was effectively the same thing as just
checking for the init namespace from a security standpoint.

And in fact, rewriting it in that form (ie checking for init_ns) would
just make it much more obvious what the intent it.