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    SubjectRe: [RFC] fix devpts mount behavior
    Quoting Eric W. Biederman (ebiederm@xmission.com):
    > Al Viro <viro@ZenIV.linux.org.uk> writes:
    >
    > > On Mon, Jan 23, 2012 at 04:41:25PM -0800, Linus Torvalds wrote:
    > >
    > >> Right. I think the opportunity for problems should be pretty small.
    > >>
    > >> And it's not like the pty itself wouldn't continue to work - it's just
    > >> that programs like /usr/bin/tty wouldn't be able to *find* it.
    > >>
    > >> Although who knows - maybe there is some other subtle interaction.
    > >
    > > FWIW, the subtle and nasty part in all that is that you can mknod /dev/ptmx
    > > and it *will* work, refering to the "initial" instance. That's what
    > > concerns me about the chroot scenarios -
    > > mknod /jail/dev/ptmx c 5 2
    > > mkdir /jail/dev/pts
    > > mount -t devpts /jail/dev/pts
    > > chroot /jail
    > > works fine right now, but with that change behaviour will be all wrong -
    > > opening /dev/ptmx inside of jail will grab you a pts, all right, but
    > > it will *not* show up in (jail) /dev/pts/* as it does with the current
    > > kernel.
    > >
    > > Note that if you replace that mknod with symlink pts/ptmx /jail/dev/ptmx
    > > the things will keep working. However, that will _only_ work for kernels
    > > with DEVPTS_MULTIPLE_INSTANCES - without it you won't get ptmx inside
    > > devpts (which is arguably wrong, BTW)
    >
    > For testing I would recommend looking at the distro chroot build cases.

    Do you have a specific example in mind? I would expect build chroots
    generally don't mount a devpts.

    > It looks like relatively recent udev still creates /dev/ptmx and does

    Boy, it does, and it's stubborn about it. Removing the /lib/udev/rules.d
    entry doesn't stop it. (this is after I've had an init job replace the
    devtmpfs-created ptmx entry with a symlink)

    So current distros (well, Ubuntu and Fedora at least) would need to at least
    (a) fix udev, (b) change the default devpts mount (done from initramfs) to
    add ptmxmode=666, (c) (if not done in udev) create the /dev/ptmx symlink.

    For safety I'd recommend creating /dev/pts/ptmx with
    DEVPTS_MULTIPLE_INSTANCES=n (or dropping that support), and by default
    setting ptmxmode to 666 as that's what udev does.

    > not create the symlink. So we might get into the awkward situation of
    > /dev/ptmx not matching /dev/pts/ptmx with something as simple as
    > initramfs mounting /dev/pts and then initscripts mounting /dev/pts.

    That shouldn't matter with a symlink, though it is sloppy.

    -serge


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