ebpf
# Idea
- Compiling eBPF programs into bytecode
- Verifying programs execute safely in a VM before being being loaded at the hook point
- Attaching programs to hook points within the kernel that are triggered by specified events
- Compiling at runtime for maximum efficiency
- Calling helper functions to manipulate data when a program is triggered
- Using maps (key-value pairs) to share data between the user space and kernel space and for keeping state.
# Maps
- allow eBPF program to store and retrieve data in a wide set of data structures
- Example
- Hash tables, Arrays
- LRU (Least Recently Used)
- Ring Buffer
- Stack Trace
- LPM (Longest Prefix match)
# Verification
- Programs are validated to ensure they always run to completion
- e.g. an eBPF program may never block or sit in a loop forever. eBPF programs may contain so called bounded loops but the program is only accepted if the verifier can ensure that the loop contains an exit condition which is guaranteed to become true.
- Programs may not use any uninitialized variables or access memory out of bounds.
- Programs must fit within the size requirements of the system. It is not possible to load arbitrarily large eBPF programs.
- Program must have a finite complexity. The verifier will evaluate all possible execution paths and must be capable of completing the analysis within the limits of the configured upper complexity limit.
- eBPF verifier — The Linux Kernel documentation
# Hooks
If a predefined hook does not exist for a particular need, it is possible to create a kernel probe (kprobe) or user probe (uprobe) to attach eBPF programs almost anywhere in kernel or user applications.