Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 17 May 2016 17:39:33 -0500 | Subject | Re: [PATCH v4 03/21] fs: Allow sysfs and cgroupfs to share super blocks between user namespaces |
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Seth Forshee <seth.forshee@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
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