Groups > linux.kernel > #1719634

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

From	ebiederm@xmission.com (Eric W. Biederman)
Newsgroups	linux.kernel
Subject	Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name
Date	2017-08-25 01:10 +0200
Message-ID	<ui9qa-4Bp-7@gated-at.bofh.it> (permalink)
References	(16 earlier) <ui5mx-1JS-9@gated-at.bofh.it> <ui5Zg-2bY-27@gated-at.bofh.it> <ui5Zg-2bY-25@gated-at.bofh.it> <ui7eG-2Ut-25@gated-at.bofh.it> <ui7y2-3iu-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Thu, Aug 24, 2017 at 1:43 PM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>>
>> There are just enough weird one off scripts like xen image builder (I
>> think that was the nasty test case that broke in debian) that I can't
>> imagine ever being able to responsibly remove the path based lookups in
>> /dev/ptmx.  I do dream of it sometimes.
>
> Not going to happen.

Which is what I said.

> The fact is, /dev/ptmx is the simply the standard location.
> /dev/pts/ptmx simply is *not*.

The standard is posix_openpt().  That is a syscall on the bsds.
Opening something called ptmx at this point is a Linuxism.

There are a lot of programs that are going to be calling posix_openpt()
simply because /dev/ptmx can not be counted on to exist.

> So pretty much every single user that ever uses pty's will use
> /dev/ptmx, it's just how it has always worked.
>
> Trying to change it to anything else is just stupid. There's no
> upside, there is only downsides - mainly the "we'll have to support
> the standard way anyway, that newfangled way doesn't add anything".

Except the new fangled way does add quite a bit.  Not everyone who
mounts devpts has permission to call mknod.  So /dev/ptmx frequently
winds up either being a bind mount or a symlink to /dev/pts/ptmx in
containers.

It is going to take a long time but device nodes like one of those
filesystem features thare are very slowly on their way out.

> Our "pts" lookup isn't expensive.
>
> So quite frankly, we should discourage people from using the
> non-standard place. It really has no real advantages, and it's simply
> not worth it.

The "pts" lookup admitted isn't runtime expensive.  I could propbably
measure a cost but anyone who is creating ptys fast enough to care
likely has other issues.

The "pts" lookup does have some real maintenance costs as it takes
someone with a pretty deep understanding of things to figure out what is
going on.  I hope things have finally been abstracted well enough, and
the code is used heavily enough we don't have to worry about a
regression there.   I still worry.

As for non-standard locations.  Anything that isn't /dev/ptmx and
/dev/pts/NNN simply won't work for anything isn't very specialized.
At which point I don't think there is any reason to skip using the ptmx
node on the devpts filesystem as you have already given up compatibility
with everything else.

But I agree it doesn't look worth it to change glibc to deal with an
alternate location for /dev/ptmx.  I see a huge point in changing glibc
to use the new TIOCGPTPEER ioctl when available as that is really the
functionality the glibc internals are after.

Eric

Thread

[PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@ubuntu.com> - 2017-08-16 19:20 +0200
  Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 20:30 +0200
    Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 20:50 +0200
      Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-16 21:50 +0200
        Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 22:00 +0200
          Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 22:20 +0200
            Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 22:40 +0200
              Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 23:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-16 23:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-16 23:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-17 00:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-17 00:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-17 00:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-23 17:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-23 23:20 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-17 00:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-17 01:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-17 02:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-17 02:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-17 03:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Stefan Lippers-Hollmann <s.l-h@gmx.de> - 2017-08-24 02:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 02:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 03:20 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 03:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 03:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 03:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 04:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 05:20 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 05:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 18:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Stefan Lippers-Hollmann <s.l-h@gmx.de> - 2017-08-24 06:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 18:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 20:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 20:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 20:20 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 20:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 20:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Stefan Lippers-Hollmann <s.l-h@gmx.de> - 2017-08-24 22:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 22:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 20:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 21:00 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 21:30 +0200
                [PATCH v3] pty: Repair TIOCGPTPEER ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 22:20 +0200
                Re: [PATCH v3] pty: Repair TIOCGPTPEER Stefan Lippers-Hollmann <s.l-h@gmx.de> - 2017-08-24 23:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 21:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 21:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 22:50 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-24 23:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-25 01:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-25 01:30 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Christian Brauner <christian.brauner@canonical.com> - 2017-08-25 01:40 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-26 03:10 +0200
                Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-24 21:50 +0200
            Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name ebiederm@xmission.com (Eric W. Biederman) - 2017-08-17 03:40 +0200

csiph-web