Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 07 Oct 2014 14:50:47 -0700 | Subject | Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root |
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Andy Lutomirski <luto@amacapital.net> writes:
> On Tue, Oct 7, 2014 at 2:26 PM, Eric W. Biederman <ebiederm@xmission.com> wrote: >> Andy Lutomirski <luto@amacapital.net> writes: >> >>> Why should MNT_LOCKED on submounts be enforced? >>> >>> Is it because, if you retain a reference to the detached tree, then >>> you can see under the submounts? >> >> Yes. MNT_DETACH is a recursive operation that detaches all of the mount >> and all of it's submounts. Which means you can see under the submounts >> if you have a reference to a detached mount. >> >>> If so, let's fix *that*. Because >>> otherwise the whole model of pivot_root + detach will break. >> >> I am not certain what you are referring to. pivot_root doesn't >> manipulate the mount tree so you can see under anything. >> >> What I believe is the appropriate fix is to fail umount2(...,MNT_DETACH) >> if there are any referenced mount points being detached that have a >> locked submount. > > Most of the container-using things do, roughly: > > Unshare userns and mountns > Mount some new stuff > pivot_root to the new stuff > MNT_DETACH the old. > > That last step will almost always fail if you make this change.
I don't think so.
I expect I could add full busy detection of normal umounts and those applications would not fail.
What I am proposing is a more targeted version of busy detection that looks at each mount in the set that detach will unmount. For each mount if it is busy with non-submount references and it has at least one locked submount fail the detach with -EBUSY.
Do you really think we have userspace references to the one or more of the mounts under old?
Eric
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