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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 7 Mar 2018 11:44:35 -0800
Message-ID: <CA+55aFx+dcUPK4A19ewpjNKkes7AgmEuTcqY8wYopZ-31ujpjQ@mail.gmail.com>
Subject: Re: Invalid /proc/<pid>/fd/{0,1,2} symlinks with TIOCGPTPEER
To:     Christian Brauner <christian.brauner@canonical.com>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
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On Wed, Mar 7, 2018 at 8:17 AM, Christian Brauner
<christian.brauner@canonical.com> wrote:
>
> unshare --mount
> mount --bind /dev/pts/ptmx /dev/ptmx
> chmod 666 /dev/ptmx

Oh. Why are you using a bind mount in the first place?

Anyway, I guess we just have to add another special case for this.

Which doesn't look horrible. Right now path_pts() just does

        ret = path_parent_directory(path);

and that simply doesn't work for a bind mount file.

I think we could just change path_parent_directory() to go through
file bind mounts. The other user is follow_dotdot(), but that always
takes a directory, so it wouldn't be affected.

But it's probably safer to just teach path_pts to just walk up the
bind mount first, and then do the existing path_parent_directory.

Anybody want to just try that thing?

            Linus