How does the syscall instruction work in User Mode Linux?

Question

linux

User Mode Linux runs as a normal process on the operating system (Linux) of the host.
In this case, programs that run inside User Mode Linux perform system calls in the usual way (syscall/sysenter/int 0x80).
But at the same time, such a system call should be executed (by the logic of things) on the main core, and not inside the UML.
How exactly does the UML intercept system calls to programs within it?

Answer 1

Via ptrace, see section 3
https://lwn.net/2001/features/OLS/pdf/pdf/uml.pdf