In this lesson, you will learn how to diagnose and resolve eBPF verification failures by leveraging kernel log output and diagnostic register states.
When your BPF program fails to load, it is usually because the kernel’s Verifier has determined your code is unsafe—perhaps due to out-of-bounds memory access, uninitialized registers, or potentially infinite loops. Since the verifier is a static analysis engine, it doesn't just reject your code; it explains why through the kernel log.
Accessing the Verifier Log
The most powerful tool at your disposal is the verifier’s internal log. When loading a BPF program via bpf() system calls, you can provide a buffer to receive this log. If the load fails, the kernel populates this buffer with a step-by-step trace of its decision-making process.
If you are using libbpf or bpftool, this output is often surfaced automatically. You can also view the last verification attempt via dmesg:
# Check the tail of the kernel log after a failed load
sudo dmesg | tail -n 50
Interpreting Diagnostic Registers
The log output reveals the state of the BPF virtual machine's registers (R0 through R10) at the specific instruction where the verification failed.
Consider a scenario where you attempt to access a map value without checking for a null pointer:
struct my_data *val = bpf_map_lookup_elem(&my_map, &key);
val->count += 1; // Verification failure here
The log will show something like:
R1=map_value_or_null(id=...,off=0,imm=0) ... 1: (79) r1 = *(u64 *)(r1 + 0) R1 invalid mem access 'inv'
The verifier is telling you that R1 (which holds your val pointer) is marked as map_value_or_null. Because you didn't check if the pointer is null, the verifier assumes it could be, and rejects the instruction (79) because dereferencing a null pointer is unsafe. By inspecting the registers provided in the log, you can pinpoint the exact instruction that violated safety constraints and identify the missing validation logic needed to satisfy the verifier's strict security requirements.
Key Takeaway: The verifier log acts as your static analysis debugger, providing a instruction-by-instruction breakdown of register states that pinpoint exactly why your code was deemed unsafe.