kernel-debugging

Kernel Debugging

Purpose

Guide agents through debugging the Linux kernel: kgdb over serial or USB, kdb built-in debugger commands, ftrace and trace-cmd, kprobes and kretprobes, dynamic debug (dyndbg), crash dump analysis with crash and makedumpfile, dmesg log levels, and /proc/sys runtime tuning for debugging.

When to Use

• Kernel panic or oops with unclear stack trace
• Live debugging a kernel module or driver with kgdb
• Tracing function calls without recompiling (ftrace, kprobes)
• Enabling verbose driver logging with dyndbg
• Analyzing production crash dumps from kdump
• Tuning kernel runtime parameters for debug sessions

Workflow

1. dmesg and log levels

# Kernel ring buffer
dmesg -T -l err,warn
dmesg -w   # follow live

# Log level (0=emerg .. 7=debug)
cat /proc/sys/kernel/printk
# current  default  minimum  boot-default
# 4        4        1        7

# Temporarily increase verbosity
echo 8 > /proc/sys/kernel/printk

Kernel printk levels: KERN_EMERG through KERN_DEBUG. Driver dev_dbg() requires dyndbg or DEBUG define.

2. kgdb over serial

Kernel config: CONFIG_KGDB, CONFIG_KGDB_SERIAL_CONSOLE.

# Boot parameter (ttyS0, 115200)
kgdboc=ttyS0,115200 kgdbwait

# Or at runtime
echo ttyS0,115200 > /sys/module/kgdboc/parameters/kgdboc
echo g > /proc/sysrq-trigger   # break into kgdb

GDB host side:

# Connect to remote serial
gdb vmlinux
(gdb) set serial baud 115200
(gdb) target remote /dev/ttyUSB0
(gdb) bt
(gdb) list *panic+0x20

USB variant: CONFIG_KGDB_KDB with kgdboc=usb on supported hardware.

3. kdb commands

When CONFIG_KGDB_KDB enabled, kdb provides in-kernel shell:

kdb commands (at SysRq break)
├── bt          — stack backtrace
├── lsmod       — loaded modules
├── md <addr>   — memory display
├── ps          — process list
├── id <cpu>    — CPU registers
├── go          — continue execution
└── cpu <n>     — switch CPU context

# Enter kdb
echo k > /proc/sysrq-trigger

4. ftrace

# Enable function tracer
echo function > /sys/kernel/debug/tracing/current_tracer
echo 1 > /sys/kernel/debug/tracing/tracing_on

# Filter to specific function
echo schedule > /sys/kernel/debug/tracing/set_ftrace_filter
echo ':*probe_*' >> /sys/kernel/debug/tracing/set_ftrace_notrace

# Capture and read
cat /sys/kernel/debug/tracing/trace_pipe

# Disable
echo 0 > /sys/kernel/debug/tracing/tracing_on
echo nop > /sys/kernel/debug/tracing/current_tracer

# trace-cmd (userspace recorder)
trace-cmd record -p function -l schedule,do_page_fault
trace-cmd report
trace-cmd.dat  # kernelshark GUI

5. kprobes and kretprobes

# Dynamic kprobe via debugfs
echo 'p:myprobe do_sys_open %ax %si' > /sys/kernel/debug/tracing/kprobe_events
echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
cat /sys/kernel/debug/tracing/trace

# kretprobe
echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events

# Cleanup
echo 0 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
echo '-:myprobe' > /sys/kernel/debug/tracing/kprobe_events

In-kernel kprobe (module):

#include <linux/kprobes.h>

static struct kprobe kp = {
    .symbol_name = "do_sys_open",
    .pre_handler = my_pre_handler,
};

static int __init mod_init(void)
{
    return register_kprobe(&kp);
}

6. Dynamic debug (dyndbg)

# Enable all debug messages for a module
echo 'module mydriver +p' > /sys/kernel/debug/dynamic_debug/control

# Enable specific file:line
echo 'file drivers/i2c/i2c-core.c +p' > /sys/kernel/debug/dynamic_debug/control

# Query current settings
grep mydriver /sys/kernel/debug/dynamic_debug/control

Boot-time: dyndbg="module mydriver +p" on kernel command line.

7. kdump and crash analysis

# Setup kdump (crash kernel reserved memory)
# /etc/default/grub: crashkernel=256M
sudo kdump-config show

# After crash, vmcore in /var/crash/
ls /var/crash/*/vmcore

# Analyze with crash utility
crash /usr/lib/debug/boot/vmlinux-$(uname -r) /var/crash/*/vmcore

crash> bt
crash> log
crash> ps
crash> kmem -i
crash> mod

# Compress vmcore
makedumpfile -c -d /proc/vmcore /tmp/vmcore.lzo

8. /proc/sys runtime tuning for debugging

# Panic on oops (reproduce in VM)
sysctl kernel.panic_on_oops=1

# Soft lockup detection
sysctl kernel.softlockup_panic=1

# Watchdog
sysctl kernel.nmi_watchdog=1

# Slab debugging
# Boot: slub_debug=P,pagealloc

9. Debugging decision tree

Kernel issue type?
├── Panic/oops → dmesg; crash vmcore; CONFIG_DEBUG_INFO_BTF
├── Logic bug in driver → dyndbg; ftrace function_graph
├── Performance regression → perf record -g -a; trace-cmd
├── Intermittent → kprobes on suspect path
└── Module crash → kgdb; try_module_get audit

Common Problems

Symptom	Cause	Fix
kgdb won't connect	Wrong tty/baud	Match kgdboc to serial adapter
ftrace empty trace	tracing_off or nop tracer	echo function > current_tracer; enable tracing_on
kprobe registration fails	Missing CONFIG_KPROBES or inlined function	Use tracepoint or static_key
dyndbg has no effect	CONFIG_DYNAMIC_DEBUG disabled	Rebuild kernel with option
crash can't read vmcore	Wrong vmlinux debug symbols	Install linux-image-$(uname -r)-dbg
SysRq doesn't work	kernel.sysrq disabled	echo 1 > /proc/sys/kernel/sysrq

Related Skills

• skills/low-level-programming/linux-kernel-modules — module development being debugged
• skills/kernel/device-drivers — driver-specific probe/IRQ issues
• skills/kernel/kernel-internals — subsystem knowledge for interpreting traces
• skills/debuggers/gdb — GDB commands shared with kgdb
• skills/profilers/linux-perf — perf for kernel hotspots
• skills/observability/ebpf — non-invasive kernel tracing alternative