[$] Calling kernel functions from BPF

The kernel's BPF virtual machine allows
programs loaded from user space to be safely run in the kernel's context.
That functionality would be of limited use, however, without the ability
for those programs to interact with the rest of the kernel. The interface
between BPF and the kernel has been kept narrow for a number of good
reasons, including safety and keeping the kernel in control of the system.
The 5.13 kernel, though, contains a feature that could, over
time, widen that interface considerably: the ability to directly call
kernel functions from BPF programs.