BiteSizedChunks.comLearn one small thing at a time.
Course contentsShow
← Linux
Lesson 1199 of 1,81910. Network Administration and Kernel StacksFree lesson

Troubleshooting BPF Program Failures

Use kernel logs and crash dumps to debug failed network eBPF programs.

In this lesson, you will learn how to leverage system logs and kernel crash dumps to diagnose why your network eBPF programs fail at runtime.

When an eBPF program fails—whether due to an invalid memory access or a logic error—the kernel doesn't always provide a friendly error message in your terminal. Because these programs run in the kernel context, a failure often results in a "silent" drop or, in severe cases, a kernel oops.

Using Kernel Logs (dmesg)

The first place to look is the kernel ring buffer. When the BPF verifier rejects a program or the runtime detects an illegal operation (like an out-of-bounds map access), it logs details to dmesg. Always look for logs starting with bpf::

# Check the last 50 lines for BPF-related errors
dmesg | grep -i "bpf" | tail -n 50

If your program triggers a helper function error, the kernel will often log the exact instruction pointer where the failure occurred. You can correlate this with the output of bpftool prog dump xlated to find the specific line of BPF bytecode that caused the issue.

Analyzing Crash Dumps

If a BPF program causes a kernel panic (a hard crash), you won't have a live shell to query. In these cases, you must analyze the vmcore (the memory snapshot created after the crash) using the crash utility.

  1. Load the debug symbols: Point crash to your vmlinux image.
  2. Examine the stack trace: Use the bt command to see if the execution was trapped within a BPF helper or the JIT-compiled code block.
  3. Inspect BPF state: Use the rd (read memory) command to inspect map values at the time of the crash. Often, a program fails because it expects a packet buffer to have a certain length, but it received a malformed or truncated packet that violates the program's safety assumptions.

By mapping the kernel's stack trace back to your eBPF bytecode, you can determine if the failure was a result of an unsafe pointer dereference or a race condition while accessing a shared map.

Key Takeaway: Use dmesg to catch runtime errors reported by the kernel and crash to inspect memory snapshots if your program triggers a kernel panic.