Alan Jenkins <alan.christopher.jenk...@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
