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SubjectRe: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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On 11/10/2018 10:17, David Howells wrote:
> 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

Very clearly written, thank you.  Hum, your solution would mean
open_tree(OPEN_TREE_CLONE) + move_mount() is not equivalent to the
current `mount --rbind` :-(.  That does not fit the current patch
description.

It sounds like you're under-estimating how we can use mnt_ns->seq (as is
currently used in mnt_ns_loop()).  Or maybe I am over-estimating it :).

In principle, it should suffice for attach_recursive_mount() to check
the NS sequence numbers of the NS files which are mounted. You can't
hide the loop at a deeper level inside the NS, because of the existing
mnt_ns_loop() check.

I think mnt_ns_loop() works 100% correctly upstream, and there is no
memory leak bug there.  You can pass a mount NS fd between processes in
arbitrary namespaces, and you can mount it with "mount --no-canonicalize
--bind /proc/self/fd/3 /other_ns".  But mnt_ns_loop() will only allow
the mount when the other NS is newer than your own mount namespace.

Upstream also covers mount propagation (and CLONE_NEWNS), by simply not
propagating mounts of mount NS files.  ( See commit 4ce5d2b1a8fd "vfs:
Don't copy mount bind mounts of /proc/<pid>/ns/mnt between namespaces" /
https://unix.stackexchange.com/questions/473717/what-code-prevents-mount-namespace-loops-in-a-more-complex-case-involving-mount-propagation
)

I think it is more a question of taste :-).  Would it be acceptable to
prune the tree (or fail?) in move_mount() (and also `mount --move`, if
you [ab]use it like I did) ?

I suspect we should prefer your solution.  It is clearly simpler, and I
don't know that anyone really uses `mount --rbind` to clone trees of
mount NS files.

Either way, I suggest we take care to say whether `mount --rbind` and
`mount --bind` can be implemented using open_tree() + move_mount(), or
whether we think it might be undesirable.  (E.g. because someone might
read the current commit message, and desire to implement `mount
--bind,ro` atomically, if/when we also have mount_setattr() ).

Regards

Alan


> ---
>
> 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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