[PATCH v2] devpts: Make ptmx be owned by the userns owner as a fallback

[Andy Lutomirski]

On Tue, Mar 15, 2016 at 1:05 PM, Andy Lutomirski <luto at kernel.org> wrote:
> New devpts instances have ptmx owned by the inner uid and gid 0.
>
> For container-style namespaces (LXC, etc), this should have no
> effect, this is fine.
>
> For sandbox-style namespaces (xdg-app and similar), this is
> problematic -- there may not be an inner 0:0.  If that happens,
> devpts mounts will fail.
>
> Fix it by adding a fallback: if 0:0 is not mapped but the userns
> owner and group are mapped, then ptmx will be owned by the namespace
> owner.
>
> This won't change behavior except in cases where mount would
> currently return -EINVAL.

Eric, this patch is straightforward, fixes a real problem, and is
mostly orthogonal to the devpts stuff you're working on.  Could you
apply it for 4.7?

--Andy