Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12] | Date | Thu, 11 Oct 2018 10:17:31 +0100 |
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Alan Jenkins <alan.christopher.jenkins@gmail.com> wrote:
> # unshare --mount=private_mnt/child_ns --propagation=shared ls -l /proc/self/ns/mnt
I think the problem is that the mount of the nsfs object done by unshare here pins the new mount namespace - but doesn't add the namespace's contents into the mount tree, so the mount struct cycle-detection code is bypassed.
I think it's fine for all other namespaces, just not the mount namespace.
It looks like this bug might theoretically exist upstream also, though I don't think there's any way to actually effect it given that mount() doesn't take a dirfd argument.
The reason that you can do this with open_tree()/move_mount() is that it allows you to create a mount tree (OPEN_TREE_CLONE) that has no namespace assignment, pass it through the namespace switch and then attach it inside the child namespace. The cross-namespace checks in do_move_mount() are bypassed because the root of the newly-cloned mount tree doesn't have one.
Unfortunately, just searching the newly-cloned mount tree for a conflicting nsfs mount doesn't help because the potential loop could be hidden several levels deep.
I think the simplest solution is to either reject a request for open_tree(OPEN_TREE_CLONE) if there are any nsfs objects in the source tree, or to just not copy said objects.
David ---
Test script:
mount -t tmpfs none /a mount --make-shared /a cd /a mkdir private_mnt mount -t tmpfs xxx private_mnt mount --make-private private_mnt touch private_mnt/child_ns unshare --mount=private_mnt/child_ns --propagation=shared \ ls -l /proc/self/ns/mnt findmnt
~/open_tree 3</a/private_mnt 3 \ nsenter --mount=/a/private_mnt/child_ns \ sh -c '~/move_mount 4</mnt'
grep Shmem: /proc/meminfo dd if=/dev/zero of=/a/private_mnt/bigfile bs=1M count=10
umount -l /a/private_mnt/ grep Shmem: /proc/meminfo
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