The Linux kernel is a complex piece of software with many moving parts. As such, it can be difficult to debug when things go wrong. This article will explore some of the most useful techniques for debugging the Linux kernel.
One of the most basic and useful techniques for debugging the kernel is printing messages to the kernel log. The kernel log is a record of all kernel activity that is stored in the file /var/log/kern.log
. Messages can be printed to the kernel log using the printk
function.
The printk
function takes a format string and a variable number of arguments. The format string is similar to that of the printf
function in user-space, but with a few important differences. First, the printk
format string can contain format specifiers for %p
, %t
, and %i
, which are respectively the address of the current task, the current time in jiffies, and the current CPU ID. Second, the printk
format string can contain %ln
specifiers, which indicate the size of the following argument in bytes. Finally, the printk
format string can contain %s
specifiers, which print the string representation of the following argument.
Here is a simple example of printk
usage:
#include <linux/kernel.h>
void print_message(void)
{
printk("This is a test message.\n");
}
The kernel debugger, or kdb
, is a powerful tool for debugging the kernel. kdb
can be used to examine and change kernel data structures, set breakpoints, and single-step kernel code. kdb
is invoked by adding the kdb
command-line option to the kernel boot parameters. When the kernel is booted with this option, kdb
will be entered automatically when a kernel panic occurs.
kdb
can also be entered manually by pressing the Ctrl-Alt-SysRq-K
key combination. Once kdb
is entered, a kdb>
prompt will be displayed. kdb
commands can be entered at this prompt. Type help
at the kdb>
prompt to see a list of available commands.
One of the most useful kdb
commands is bt
. This command prints a backtrace of the current call stack. This can be helpful for understanding where a kernel panic occurred and why.
Another useful kdb
command is md
. This command can be used to examine the contents of memory. For example, the command md 0xffffffff80000000 0x200
will print the contents of the first 512 bytes of kernel memory.
The kernel probe, or kprobe
, is a Linux kernel feature that allows the execution of user-space code when a specific kernel function is called. kprobe
can be used to instrument the kernel for debugging purposes.
kprobe
is invoked using the kprobe
command-line option. The kprobe
option takes a kernel function name and a user-space handler function. The handler function is executed every time the kernel function is called. The handler function has access to the kernel function's arguments and return value.
Here is a simple example of kprobe
usage:
#include <linux/kprobes.h>
static int handler_kprobe(struct kprobe *p, void *data)
{
printk("kprobe hit!\n");
return 0;
}
static struct kprobe kp = {
.symbol_name = "do_fork",
.handler = handler_kprobe,
};
static int __init kprobe_init(void)
{
register_kprobe(&kp);
return 0;
}
static void __exit kprobe_exit(void)
{
unregister_kprobe(&kp);
}
module_init(kprobe_init)
module_exit(kprobe_exit)
In this example, a kprobe
is registered for the do_fork
kernel function. Every time do_fork
is called, the handler_kprobe
function is executed. The handler_kprobe
function simply prints a message to the kernel log.
The kernel trace, or ktrace
, is a Linux kernel feature that allows the tracing of kernel activity. ktrace
can be used to instrument the kernel for debugging purposes.
ktrace
is invoked using the ktrace
command-line option. The ktrace
option takes a trace buffer size and a trace file name. The trace buffer is used to store trace events. The trace file is used to store a human-readable representation of the trace events.
Here is a simple example of ktrace
usage:
#include <linux/ktrace.h>
static int __init ktrace_init(void)
{
ktrace("/tmp/ktrace.out", 1024);
return 0;
}
static void __exit ktrace_exit(void)
{
ktrace_stop();
}
module_init(ktrace_init)
module_exit(ktrace_exit)
In this example, ktrace
is configured to trace kernel activity to the file /tmp/ktrace.out
. The trace buffer is set to 1024 KB.
The kernel profiler, or kprof
, is a Linux kernel feature that allows the profiling of kernel code. kprof
can be used to collect performance information about the kernel.
kprof
is invoked using the kprof
command-line option. The kprof
option takes a profiling interval and a trace file name. The profiling interval is the time, in nanoseconds, between samples. The trace file is used to store the profiling information.
Here is a simple example of kprof
usage:
#include <linux/kprof.h>
static int __init kprof_init(void)
{
kprof("/tmp/kprof.out", 10000000);
return 0;
}
static void __exit kprof_exit(void)
{
kprof_stop();
}
module_init(kprof_init)
module_exit(kprof_exit)
In this example, kprof
is configured to profile kernel code every 10 milliseconds. The profiling information is stored in the file /tmp/kprof.out
.
This article has explored some of the most useful techniques for debugging the Linux kernel. These techniques can be used to debug kernel panics, performance issues, and other problems.