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eBPF Basic Concepts and Architecture

The architecture of an extended Berkeley Packet Filter includes the following elements.

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Predefined Hooks​

eBPF programs run according to events that trigger them. An application (or the kernel) passes a threshold known as a hook point. Hooks are predefined and can include events such as network events, system calls, function entry and exit, and kernel tracepoints. If there is no predefined hook for a certain requirement, you can create a user or kernel probe (uprobe or kprobe).

Program Verification​

Once a hook is identified, the BPF system call can be used to load the corresponding eBPF program into the Linux kernel. This usually involves using an eBPF library. When a program is loaded into the kernel, it has to be verified to ensure it is safe to run.

Validation takes into account conditions such as:

  • The process loading the eBPF program holds the required capabilities (privileges). Unless unprivileged eBPF is enabled, only privileged processes can load eBPF programs.
  • The program does not crash or otherwise harm the system.
  • The program always runs to completion (i.e. the program does not sit in a loop forever, holding up further processing).

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eBPF Maps​

An eBPF program must be able to store its state and share collected data. eBPF maps can help programs retrieve and store information according to a range of data structures. Users can access eBPF maps via system calls, from both eBPF programs and applications.

Map types include hash tables or arrays, ring buffer, stack trace, least-recently used, longest prefix match, and more.

Helper Calls​

An eBPF program cannot arbitrarily call into a kernel function. This is because eBPF programs need to maintain compatibility and avoid being bound to specific versions of the kernel. Thus, eBPF programs use helper functions to make function calls. Helper functions are APIs provided by the kernel, and can be easily adjusted.

Helper calls allow programs to generate random numbers, receive current time and date, access eBPF maps, manipulate forwarding logic and network packets, and more.

Tail and Function Calls​

These calls make eBPF programs composable. Function calls enable functions to be defined and called in a program. Tail calls enable the execution of other eBPF programs. They can also change the execution context.