[$] Calling kernel functions from BPF

The kernel's BPF virtual machine allowsprograms loaded from user space to be safely run in the kernel's context.That functionality would be of limited use, however, without the abilityfor those programs to interact with the rest of the kernel. The interfacebetween BPF and the kernel has been kept narrow for a number of goodreasons, 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 callkernel functions from BPF programs.