[PATCH v4 03/21] fs: Allow sysfs and cgroupfs to share super blocks between user namespaces

[Eric W. Biederman]

Seth Forshee <seth.forshee at canonical.com> writes:

> Both of these filesystems already have use cases for mounting the
> same super block from multiple user namespaces. For sysfs this
> happens when using criu for snapshotting a container, where sysfs
> is mnounted in the containers network ns but the hosts user ns.
> The cgroup filesystem shares the same super block for all mounts
> of the same hierarchy regardless of the namespace.
>
> As a result, the restriction on mounting a super block from a
> single user namespace creates regressions for existing uses of
> these filesystems. For these specific filesystems this
> restriction isn't really necessary since the backing store is
> objects in kernel memory and thus the ids assigned from inodes
> is not subject to translation relative to s_user_ns.
>
> Add a new filesystem flag, FS_USERNS_SHARE_SB, which when set
> causes sget_userns() to skip the check of s_user_ns. Set this
> flag for the sysfs and cgroup filesystems to fix the
> regressions.

So this one needs to be sget_userns(..., &init_user_ns, ...).
And not a new special case.

Apologies for not catching this earlier.

I am looking at folding all of this into the patch that introduces
sget_userns so that even bisects won't have regresssions.

We loose the ability to call mount -o remount and actually affect
these filesystems (which we don't have without s_user_ns) but we gain a
whole lot of simplicity, and we don't break the xattr and security label
on sysfs code.

Eric