The System Call API is generally perceived as the language used by processes to ask the kernel for services. The same language can be used to forward the System Call requests to processes running in user space. In this way hypervisors running in user space (without any specific privilege or capability) can provide processes with different views. SCUSE is a general methodology enabling the implementation of many features in user-space like namespaces, chroot, file system and networking support... Projects like FUSE support the user-space delegation of specific services implementation, SCUSE implements a similar concept but working at system call level it provides a general purpose solution, not limited to a specific domain (as file systems for FUSE).
FUSE allows users to implement their own file systems in user space. A kernel module forwards requests for each FUSE mounted file system to its corresponding user process. The communication between the kernel module and the user process takes place through /dev/fuse. Although FUSE implemented file systems are less efficient than the file systems implemented in the kernel, FUSE is very appreciated for its flexibility.
VUOS forwards all the system call requests to a hypervisor running in user-space. At each system request the hypervisor can use its own implementation of the service or let the process forward the request to the actual Linux Kernel. In some sense it is a generalized approach: FUSE is only for file systems, the VUOS approach to forward System Calls in USErspace (SCUSE) is for "everything".
VUOS hypervisor (umvu) runs in user-space and does not require any specific user privilege or capability. Using specific plug-in modules VUOS provides implementations of file systems (fuse), networking stacks, devices, user/groups, overlay file systems, chroot.
For example the fuse module of VUOS is compatible with FUSE but it needs no code in the kernel: mount and /dev/fuse are virtualized by VUOS.
VUOS can be seen as a way to implement namespaces at user level, a Virtual Operating System in user space.
SCUSE is a general methodology to extend the system services at user level
by forwarding the system call request in the user-space.
The current implementation of the VUOS hypervisor (umvu) is based on ptrace.
umvu shows the effectiveness and the fields of application of SCUSE.
More efficient supports for SCUSE can be developed in the future.