Modifying an existing mount namespace where user_ns != mnt_ns->user_ns

Hi! If I create a process with CLONE_NEWNS|CLONE_NEWUSER set, how can I modify the mount namespace later? I thought I simply can run setns(/proc//ns/mnt) within my privileged process which resists in the initial userns and then mount whatever I want into the child's mount namespace. But this

Modifying an existing mount namespace where user_ns != mnt_ns-user_ns

Hi! If I create a process with CLONE_NEWNS|CLONE_NEWUSER set, how can I modify the mount namespace later? I thought I simply can run setns(/proc/child/ns/mnt) within my privileged process which resists in the initial userns and then mount whatever I want into the child's mount namespace. But