Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name

Linus Torvalds
2017-08-24T19:22:23Z

On Thu, Aug 24, 2017 at 12:01 PM, Christian Brauner
<christian.brauner@canonical.com> wrote:
>
> I've touched on this in my original message, I wonder whether we currently
> support mounting devpts at a different a location and expect an open on a
> newly created slave to work.

Yes. That is very much intended to work.

> Say I mount devpts at /mnt and to open("/mnt/ptmx", O_RDWR | O_NOCTTY) and get a new slave pty at /mnt/1 do we
> expect open("/mnt/1, O_RDWR | O_NOCTTY) to work?

Yes.

Except you actually don't want to use "/mnt/ptmx". That ptmx node
inside the pts filesystem is garbage that we never actually used,
because the permissions aren't sane. It should probably be removed,
but because somebody *might* have used it, we have left it alone.

So what you're actually *supposed* to do is

- create a ptmx node and a pts directory in /mnt

- mount devpts on /mnt/pts

- use /mnt/ptmx to create new pty's, which should just look up that
pts mount directly.

And yes, the pathname should then be /mnt/pts/X for the slave side,
and /mnt/ptmx for the master.

In fact, I just tested that TIOCGPTPEER, including using your original
test-program (this is me as root in my home directory):

[root@i7 torvalds]# mkdir dummy
[root@i7 torvalds]# cd dummy/
[root@i7 dummy]# mknod ptmx c 5 2
[root@i7 dummy]# mkdir pts
[root@i7 dummy]# mount -t devpts devpts pts
[root@i7 dummy]# ../a.out
I point to "/home/torvalds/dummy/pts/0"
[root@i7 dummy]# umount pts/
[root@i7 dummy]# cd ..
[root@i7 torvalds]# rm -rf dummy

There's two things to note there:

- look at that "I point to" - it's not hardcoded to /dev/pts/X

- look at the pts number: each pts filesystem has its own private
numbers, so despite the fact that in another window I *also* have that
ptx/0:

[torvalds@i7 linux]$ tty
/dev/pts/0

that new devpts instance has its *own* pts/0, which is a
completely different pty.

this is one of those big cleanups we did with the pts filesystem some time ago.

Linus