> No, the plan is only one namespace at a time.

Looking at this a bit more I am frustrated and relieved.

I was looking at what it would take to join an arbitrary mount
namespace and I realized it is completely non-obvious what fs->root
and fs->pwd should be set to.

If I leave them untouched the new mount namespace is useless,
as all path lookups will give results in a different mount namespace,
so not even mount or umount can be used.

I can not change fs->root to mnt_ns->root as that is rootfs and there
is always something mounted on top so I can not use that.

In comparison an unshare of the mount namespace doesn't have to move
fs->root or fs->pwd at all and only has to update their mounts to
the corresponding mounts in the new mount namespace.

I might be able to find the topmost root filesystem and put at least
root there, but I'm not particularly fond of that option.

Eric





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