linux-kernel - Re: [BREAKAGE] Since 4.18, kernel sets SB_I_NODEV implicitly on userns mounts, breaking systemd-nspawn

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Message-ID: <CAPP7u0UhPyC=eewx4dYu+AU5tK_URE8_5PW9cNkK6Ppp7vkqHg@mail.gmail.com>
Date:   Sun, 23 Dec 2018 00:07:15 +0100
From:   Christian Brauner <christian.brauner@...onical.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     nix.or.die@...il.com, "Eric W. Biederman" <ebiederm@...ssion.com>,
        ellierevves@...il.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Seth Forshee <seth.forshee@...onical.com>
Subject: Re: [BREAKAGE] Since 4.18, kernel sets SB_I_NODEV implicitly on
 userns mounts, breaking systemd-nspawn

On Sun, Dec 23, 2018 at 12:02 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sat, Dec 22, 2018 at 2:49 PM Christian Brauner
> <christian.brauner@...onical.com> wrote:
> >
> > To be fair, no one apart from me was pointing out that it actually
> > breaks people including systemd folks
> > even though I was bringing it up with them. I even tried to fix all of
> > userspace after this got NACKED
>
> Seriously, the "we don't break user space" is the #1 rule in the
> kernel, and people should _know_ it's the #1 rule.
>
> If somebody ignores that rule, it needs to be escalated to me.
> Immediately. Because I need to know.

Fair enough. I usually try to be very conservative when sending patches
directly your way and Eric is otherwise very much on top of not regressing
userspace and I trust him.

However, for this case should I resend the revert?

Christian

>
> I need to know so that I can override the bogus NAK, and so that we
> can fix the breakage ASAP. The absolute last thing we need is some
> other user space then starting to rely on the new behavior, which just
> compounds the problem and makes it a *much* bigger problem.
>
> But I also need to know so that I can then make sure I know not to
> trust the person who broke rule #1.
>
> This is not some odd corner case for the kernel. This is literally the
> rule we have lived with for *decades*.
>
> So please escalate to me whenever you feel a kernel developer doesn't
> follow the first rule. Because the code that broke things *will* be
> reverted (*).
>
>                     Linus
>
> (*) Yes, there are exceptions. We have had situations where some
> interface was simply just a huge security issue or had some other
> fundamental issue. And we've had cases where the breakage was just so
> old that it was no longer fixable. So even rule #1 can sometimes have
> things that hold it back. But it is *very* rare. Certainly nothing
> like this.