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

From Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups linux.kernel
Subject Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name
Date 2017-08-16 22:20 +0200
Message-ID <ufcXg-3dc-13@gated-at.bofh.it> (permalink)
References <ufa94-1so-11@gated-at.bofh.it> <ufbeO-25d-31@gated-at.bofh.it> <ufby9-2bt-5@gated-at.bofh.it> <ufcue-2Ob-3@gated-at.bofh.it> <ufcDT-2Ry-1@gated-at.bofh.it>
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On Wed, Aug 16, 2017 at 12:56 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So the fact that we _don't_ get the right pathname for the pts entry
> here means that something got screwed up in setting filp->f_path to
> the right thing. We have all the code in place that _tries_ to do it,
> but it clearly has a bug somewhere.

Ok, I think I see what the bug is, although I don't have a fix for it yet.

We generate the path largely correctly: the path has a nice dentry
that contains the right pts number, and has the right parent pointer
that points to the root of the pts mount.

And we also fill in the path 'mnt' field. Everything should be fine.

Except when we actually hit that root dentry of the pts mount, the
code in prepend_path() hits this condition:

                if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
                        struct mount *parent = ACCESS_ONCE(mnt->mnt_parent);
                        /* Escaped? */
                        if (dentry != vfsmnt->mnt_root) {

and we break out, and reset the path to '/' because we think it
somehow escaped out of the user namespace.

So it looks like we filled in the path with the *wrong* mount information.

And THAT in turn is because we fill the path with the mount
information for the "/dev/ptmx" field - which is *not* in the
/dev/pts/ mount - that's the mount for '/dev'.

So we have a dentry and a mnt, but they simply aren't paired up correctly.

And you can see this with your test program: if you open /dev/pts/ptmx
for the master, it actually works correctly (but you need to make sure
the permissions for that ptmx node allow that).

Anyway, I know what's wrong, next step is to figure out what the fix is.

                   Linus

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[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 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

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