How can I “mount” within my process space without being root?

I’m trying to mount overlayfs without being root in my C++ code; I would want to be able to have this mount happen and be visible just for my own process and its descendants:

if(-1 == mount("overlay", "./mnt", "overlay", MS_MGC_VAL, "lowerdir=/,upperdir=./upper,workdir=./work"))
    std::cerr << "e: " << errno << std::endl;
    ....

Unfortunately the error I get is Operation not permitted. How can I make this happen? Even a simple tmpfs mount fails not being root; I’m running on a kernel 4.4.0-53.

Answer

One can do programmatically via user namespaces; by invoking:

clone(child_func, ..., CLONE_NEWNS | CLONE_NEWUSER | SIGCHLD, ...);
...
int child_func(void* args) {
    mount("overlay", "./mnt", "overlay", MS_MGC_VAL, "lowerdir=/,upperdir=./upper,workdir=./work");
    ...
}

Ideally you would also want to setup the /proc/<pid>/uid_map and /proc/<id>/gid_map for better execution. Probably the best example can be found on the man pages.