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Date: Tue, 17 May 2016 17:39:33 -0500 From: ebiederm@...ssion.com (Eric W. Biederman) To: Seth Forshee <seth.forshee@...onical.com> Cc: Alexander Viro <viro@...iv.linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jeff Layton <jlayton@...chiereds.net>, "J. Bruce Fields" <bfields@...ldses.org>, Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>, Johannes Weiner <hannes@...xchg.org>, Serge Hallyn <serge.hallyn@...onical.com>, Richard Weinberger <richard.weinberger@...il.com>, Austin S Hemmelgarn <ahferroin7@...il.com>, Miklos Szeredi <mszeredi@...hat.com>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org, dm-devel@...hat.com, linux-raid@...r.kernel.org, linux-mtd@...ts.infradead.org, linux-fsdevel@...r.kernel.org, fuse-devel@...ts.sourceforge.net, linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov, cgroups@...r.kernel.org Subject: Re: [PATCH v4 03/21] fs: Allow sysfs and cgroupfs to share super blocks between user namespaces Seth Forshee <seth.forshee@...onical.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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