# Kernel Debugging

## 1. Preparation

### Set up a development environment:
- Install a Linux distribution (e.g., Ubuntu, Fedora)
- Install essential development tools: gcc, make, binutils, etc.
- Download kernel source code from kernel.org

### Configure kernel for debugging:
- Enable CONFIG_DEBUG_INFO in kernel configuration
- Enable CONFIG_KGDB for remote debugging
- Enable other relevant debug options (e.g., CONFIG_DEBUG_KERNEL)

### Build the kernel with debug symbols:
```bash
make menuconfig
make -j$(nproc)
```

## 2. Kernel Debugging Techniques

### Printk Debugging:
- Use printk() function to add log messages
- Set appropriate log levels (KERN_INFO, KERN_DEBUG, etc.)
- View logs using dmesg or /var/log/kern.log

### Kernel Oops and Panic Analysis:
- Analyze oops messages in kernel logs
- Use addr2line to translate addresses to source code lines
- Examine call traces to identify the source of the problem

### KGDB (Kernel GNU Debugger):
- Configure a serial connection or network for remote debugging
- Set up a GDB client on the host machine
- Connect to the target machine running the kernel
- Set breakpoints, step through code, and examine variables

### Ftrace (Function Tracer):
- Enable Ftrace in kernel configuration
- Use trace-cmd tool to collect and analyze function call data
- Examine function graph to understand kernel execution flow

### Systemtap:
- Write custom scripts to instrument the kernel
- Collect detailed performance data and debug information
- Analyze complex scenarios without modifying kernel source

### Kernel Probes (Kprobes):
- Insert dynamic breakpoints in the kernel
- Gather debugging and performance information
- Use jprobes for function entry probing and kretprobes for return probes

## 3. Advanced Debugging Techniques

### Memory Debugging:
- Use KASAN (Kernel Address Sanitizer) to detect memory errors
- Enable CONFIG_DEBUG_PAGEALLOC to catch use-after-free bugs
- Utilize SLUB debug features for slab allocator issues

### Lockdep (Lock Dependency) Checker:
- Enable CONFIG_PROVE_LOCKING and CONFIG_DEBUG_LOCKDEP
- Detect potential deadlocks and incorrect locking behavior
- Analyze lock dependency graphs

### RCU (Read-Copy-Update) Debugging:
- Enable CONFIG_RCU_TRACE for RCU tracing
- Use rcutorture module to stress-test RCU implementations
- Analyze RCU grace period and callback execution

### Kernel Live Patching:
- Use kpatch or livepatch to apply runtime fixes
- Debug issues related to live patching mechanisms
- Verify patch consistency and function replacement

## 4. Tools and Utilities

### Crash utility:
- Analyze kernel core dumps
- Examine kernel data structures and memory
- Useful for post-mortem analysis of kernel panics

### Perf:
- Profile kernel and user space performance
- Identify hotspots and performance bottlenecks
- Analyze hardware events and software tracepoints

### eBPF (extended Berkeley Packet Filter):
- Write eBPF programs for advanced tracing and debugging
- Use bpftrace for one-liners and short scripts
- Develop custom eBPF tools for specific debugging needs

### Kdump:
- Configure and enable kernel crash dump mechanism
- Capture memory dumps on kernel panics
- Analyze dumps using crash utility or GDB

## 5. Best Practices

### Use git for version control:
- Maintain a clean development history
- Utilize git bisect for identifying problematic commits

### Maintain a test environment:
- Use virtual machines or dedicated hardware for testing
- Set up automated testing frameworks (e.g., KernelCI)

### Collaborate with the community:
- Subscribe to relevant mailing lists (e.g., LKML)
- Share and discuss issues on appropriate forums
- Submit well-formatted patch series for review

### Document your findings:
- Maintain detailed notes of debugging sessions
- Create reproducible test cases for bugs
- Write clear bug reports and patch descriptions

## 6. Debugging Real-world Scenarios

### Boot issues:
- Use early printk for early boot problems
- Analyze initcall debugging output
- Utilize kernel command line parameters for troubleshooting

### Device driver debugging:
- Use debugfs to expose driver information
- Implement IOCTL commands for debugging
- Utilize kernel's tracepoints in the driver code

### File system issues:
- Enable verbose mount options
- Use debugfs to examine file system internals
- Analyze block layer tracing for I/O related problems

### Network stack debugging:
- Use network sniffer tools (e.g., tcpdump, Wireshark)
- Enable netconsole for remote kernel debugging
- Analyze netfilter and netlink debugging information