linux-kernel - Re: [PATCH 0/1] devpts: use dynamic

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Date:   Wed, 16 Aug 2017 13:19:01 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian Brauner <christian.brauner@...onical.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/1] devpts: use dynamic_dname() to generate proc name

On Wed, Aug 16, 2017 at 12:56 PM, Linus Torvalds
<torvalds@...ux-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