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Re: Kprobes document

Here is the document that we propose to post to LKML for inclusion as
Documentation/kprobes.txt.  It's based on the document that Prasanna
posted to the SystemTap list on 6/24.  I've hacked it up quite a bit,
and Prasanna asked that I list myself as primary author.

Comments are welcome.  (My mailer tweaked a few lines that exceeded 72
chars.  That will be fixed.)


Title	: Kernel Probes (Kprobes)
Authors	: Jim Keniston <>
	: Prasanna S Panchamukhi <>


1. Concepts: Kprobes, Jprobes, Return Probes
2. Architectures Supported
3. Configuring Kprobes
4. API Reference
5. Kprobes Features and Limitations
7. Kprobes Example
8. Jprobes Example
9. Kretprobes Example

1. Concepts: Kprobes, Jprobes, Return Probes

Kprobes enables you to dynamically break into any kernel routine and
collect debugging and performance information non-disruptively. You
can trap at almost any kernel code address, specifying a handler
routine to be invoked when the breakpoint is hit.

There are currently three types of probes: kprobes, jprobes, and
kretprobes (also called return probes).  A kprobe can be inserted
on virtually any instruction in the kernel.  A jprobe is inserted at
the entry to a kernel function, and provides convenient access to the
function's arguments.  A return probe fires when a specified function

In the typical case, kprobes-based instrumentation is packaged as
a kernel module.  The module's init function installs ("registers")
one or more probes, and the exit function unregisters them.  A
registration function such as register_kprobe() specifies where
the probe is to be inserted and what handler is to be called when
the probe is hit.

The next three subsections explain how the different types of
probes work.  They explain certain things that you'll need to
know in order to make the best use of Kprobes -- e.g., the
difference between a pre_handler and a post_handler, and how
to use the maxactive and nmissed fields of a kretprobe.  But
if you're in a hurry to start using Kprobes, you can skip ahead
to section 2.

1.1 How Does a Kprobe Work?

When a kprobe is registered, Kprobes makes a copy of the probed
instruction and replaces the first byte(s) of the probed instruction
with a breakpoint instruction (e.g., int3 on i386 and x86_64).

When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
registers are saved, and control passes to Kprobes.  Kprobes executes
the "pre_handler" associated with the kprobe, passing the handler
the addresses of the kprobe struct and the saved registers.

Next, Kprobes single-steps its copy of the probed instruction.
(It would be simpler to single-step the actual instruction in place,
but then Kprobes would have to temporarily remove the breakpoint
instruction.  This would open a small time window when another CPU
could sail right past the probepoint.)

After the instruction is single-stepped, Kprobes executes the
"post_handler," if any, that is associated with the kprobe.
Execution then continues with the instruction following the probepoint.

1.2 How Does a Jprobe Work?

A jprobe is implemented using a kprobe that is placed on a function's
entry point.  It employs a simple mirroring principle to allow seamless
access to the probed function's arguments.  The jprobe handler routine
should have the same prototype (arg list) as the function being probed,
and must always end by calling the Kprobes function jprobe_return().

Here's how it works.  When the probe is hit, Kprobes makes a copy of
a generous portion of the stack (see below).  Kprobes then points the
saved stack pointer at the stack-copy, points the saved instruction
pointer at the jprobe's handler routine, and returns from the trap.
As a result, control passes to the handler, which is presented with
the same stack contents as the probed function.  When it is done,
the handler calls jprobe_return(), which traps again to restore
processor state and switch back to the probed function.

gcc assumes that the callee owns its arguments.  To prevent unexpected
modifications to the probed function's stack, Kprobes presents the
jprobe handler with a copy of the stack.  Up to MAX_STACK_SIZE bytes
are copied -- e.g., 64 bytes on i386.

1.3 How Does a Return Probe Work?

When you call register_kretprobe(), Kprobes establishes a kprobe at
the entry to the function.  When the probed function is called and this
probe is hit, Kprobes saves a copy of the return address, and replaces
the return address with the address of a "trampoline."  The trampoline
is an arbitrary piece of code -- typically just a nop instruction.
At boot time, Kprobes registers a kprobe at the trampoline.

When the probed function executes its return instruction, control
passes to the trampoline and that probe is hit.  Kprobes' trampoline
handler calls the user-specified handler associated with the kretprobe,
then sets the saved instruction pointer to the saved return address,
and that's where execution resumes upon return from the trap.

While the probed function is executing, its return address is
stored in an object of type kretprobe_instance.  Before calling
register_kretprobe(), the user sets the maxactive field of the
kretprobe struct to specify how many instances of the specified
function can be probed simultaneously.  register_kretprobe()
pre-allocates the indicated number of kretprobe_instance objects.

For example, if the function is non-recursive and is called with a
spinlock held, maxactive = 1 should be enough.  If the function is
non-recursive and can never relinquish the CPU (e.g., via a semaphore
or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
set to a default value.  If CONFIG_PREEMPT is enabled, the default
is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.

It's not a disaster if you set maxactive too low; you'll just miss
some probes.  In the kretprobe struct, the nmissed field is set to
zero when the return probe is registered, and is incremented every
time the probed function is entered but there is no kretprobe_instance
object available for establishing the return probe.

2. Architectures Supported

Kprobes, jprobes, and return probes are implemented on the following

- i386
- x86_64 (AMD-64, E64MT)
- ppc64
- ia64 (Support for probes on certain instruction types is still in
- sparc64 (Return probes not yet implemented.)

3. Configuring Kprobes

Ensure CONFIG_KPROBES is set to "y" while configuring the kernel using
make menuconfig/xconfig/oldconfig.  Under "Kernel hacking", look for

4. API Reference
Here are terse, mini-man-page specifications for the Kprobes API
and the associated probe handlers that you'll write.  See the latter
half of this document for examples.

4.1 register_kprobe

#include <linux/kprobes.h>
int register_kprobe(struct kprobe *kp);
Sets a breakpoint at the address kp->addr.  When the breakpoint is
hit, Kprobes calls kp->pre_handler.  After the probed instruction
is single-stepped, Kprobe calls kp->post_handler.  If a fault
occurs during execution of kp->pre_handler or kp->post_handler,
or during single-stepping of the probed instruction, Kprobes calls
kp->fault_handler.  Any or all handlers can be NULL.

register_kprobe() returns 0 on success, or a negative errno otherwise.

User's pre-handler:
#include <linux/kprobes.h>
#include <linux/ptrace.h>
int pre_handler(struct kprobe *p, struct pt_regs *regs);
Called with p pointing to the kprobe associated with the breakpoint,
and regs pointing to the struct containing the registers saved when
the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.

User's post-handler:
#include <linux/kprobes.h>
#include <linux/ptrace.h>
void post_handler(struct kprobe *p, struct pt_regs *regs,
	unsigned long flags);
p and regs are as described for the pre_handler.  flags always seems
to be zero.

User's fault-handler:
#include <linux/kprobes.h>
#include <linux/ptrace.h>
int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
p and regs are as described for the pre_handler.  trapnr is the
architecture-specific trap number associated with the fault (e.g.,
on i386, 13 for a general protection fault or 14 for a page fault).
Returns 1 if it successfully handled the exception.

4.2 register_jprobe

#include <linux/kprobes.h>
int register_jprobe(struct jprobe *jp)
Sets a breakpoint at the address jp->kp.addr, which must be the address
of the first instruction of a function.  When the breakpoint is hit,
Kprobes calls jp->entry.  jp->entry should have the same arg list
and return type as the probed function; and when it is ready to return,
it must call jprobe_return().

register_jprobe() returns 0 on success, or a negative errno otherwise.

4.3 register_kretprobe

#include <linux/kprobes.h>
int register_kretprobe(struct kretprobe *rp);
Establishes a return probe for the function whose address is
rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
You must set rp->maxactive appropriately before you call
register_kretprobe(); see "How Does a Return Probe Work?" for details.

register_kretprobe() returns 0 on success, or a negative errno

User's return-probe handler:
#include <linux/kprobes.h>
#include <linux/ptrace.h>
int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs
regs is as described for kprobe.pre_handler.  ri points to the
kretprobe_instance object, of which the following fields may be
of interest:
- ret_addr: the return address
- rp: points to the corresponding kretprobe object
The handler's return value is currently ignored.

4.4 unregister_*probe

#include <linux/kprobes.h>
void unregister_kprobe(struct kprobe *kp);
void unregister_jprobe(struct jprobe *jp);
void unregister_kretprobe(struct kretprobe *rp);
Removes the specified probe.  The unregister function can be called
at any time after the probe has been registered.

5. Kprobes Features and Limitations

As of Linux v2.6.12, Kprobes allows multiple concurrent probes at the
same address.  Currently, however, there cannot be multiple jprobes
on the same function at the same time.

In general, you can install a probe anywhere in the kernel.
In particular, you can probe interrupt handlers.  Known exceptions
are discussed in this section.

For obvious reasons, it's a bad idea to install a probe in the code
that implements Kprobes (kernel/kprobes.c and arch/*/kernel/kprobes.c).
Kprobes has not yet been trained to reject such requests.

If you install a probe in an inline-able function, Kprobes makes
no attempt to chase down all inline instances of the function and
install probes there.  gcc may inline a function without being asked,
so keep this in mind if you're not seeing the probe hits you expect.

A probe handler can modify the environment of the probed function
-- e.g., by modifying kernel data structures, or by modifying the
contents of the pt_regs struct (which are restored to the registers
upon return from the breakpoint).  So Kprobes can be used, for example,
to install a bug fix or to inject faults for testing.  Kprobes, of
course, has no way to distinguish the deliberately injected faults
from the accidental ones.  Don't drink and probe.

Kprobes makes no attempt to prevent probe handlers from stepping on
each other -- e.g., probing printk() and then calling printk() from a
probe handler.  As of Linux v2.6.12, if a probe handler hits a probe,
that second probe's handlers won't be run in that instance.

In Linux v2.6.12 and previous versions, Kprobes' data structures are
protected by a single lock that is held during probe registration and
unregistration and while handlers are run.  Thus, no two handlers
can run simultaneously.  To improve scalability on SMP systems,
this restriction will probably be removed soon, in which case
multiple handlers (or multiple instances of the same handler) may
run concurrently on different CPUs.  Code your handlers accordingly.

Kprobes does not use semaphores or allocate memory except during
registration and unregistration.

Probe handlers are run with preemption disabled.  Depending on the
architecture, handlers may also run with interrupts disabled.  In any
case, your handler should not yield the CPU (e.g., by attempting to
acquire a semaphore).

Since a return probe is implemented by replacing the return
address with the trampoline's address, stack backtraces and calls
to __builtin_return_address() will typically yield the trampoline's
address instead of the real return address for kretprobed functions.
(As far as we can tell, __builtin_return_address() is used only
for instrumentation and error reporting.)

If the number of times a function is called does not match the
number of times it returns, registering a return probe on that
function may produce undesirable results.  We have the do_exit()
and do_execve() cases covered.  do_fork() is not an issue.  We're
unaware of other specific cases where this could be a problem.


a. Systemtap: Work in progress to provide simple programming interface
to write
   probes handlers.
b. Scalability support: Currently work is in progress to run multiple
   in parallel.
c. Kernel return probes for sparc64.
d. Support for other architectures.

7. Kprobes Example

Here's a sample kernel module showing the use of kprobes to dump a
stack trace and selected i386 registers when do_fork() is called.
----- cut here -----
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/kallsyms.h>
#include <linux/sched.h>

/*For each probe you need to allocate a kprobe structure*/
static struct kprobe kp;

/*kprobe pre_handler: called just before the probed instruction is
int handler_pre(struct kprobe *p, struct pt_regs *regs)
	printk("pre_handler: p->addr=0x%p, eip=%lx, eflags=0x%lx\n",
		p->addr, regs->eip, regs->eflags);
	return 0;

/*kprobe post_handler: called after the probed instruction is executed*/
void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long
	printk("post_handler: p->addr=0x%p, eflags=0x%lx\n",
		p->addr, regs->eflags);

/* fault_handler: this is called if an exception is generated for any
 * instruction within the pre- or post-handler, or when Kprobes
 * single-steps the probed instruction.
int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
	printk("fault_handler: p->addr=0x%p, trap #%dn",
		p->addr, trapnr);
	/* Return 0 because we don't handle the fault. */
	return 0;

int init_module(void)
	int ret;
	kp.pre_handler = handler_pre;
	kp.post_handler = handler_post;
	kp.fault_handler = handler_fault;
	kp.addr = (kprobe_opcode_t*) kallsyms_lookup_name("do_fork");
	/* register the kprobe now */
	if (!kp.addr) {
		printk("Couldn't find %s to plant kprobe\n", "do_fork");
		return -1;
	if ((ret = register_kprobe(&kp) < 0)) {
		printk("register_kprobe failed, returned %d\n", ret);
		return -1;
	printk("kprobe registered\n");
	return 0;

void cleanup_module(void)
	printk("kprobe unregistered\n");

----- cut here -----

You can build the kernel module, kprobe-example.o, using the following
----- cut here -----
obj-m := kprobe-example.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
	rm -f *.mod.c *.ko *.o
----- cut here -----

$ make
$ su -
# insmod kprobe-example.ko

You will see the trace data in /var/log/messages and on the console
whenever do_fork() is invoked to create a new process.

8. Jprobes Example

Here's a sample kernel module showing the use of jprobes to dump
the arguments of do_fork().
----- cut here -----
/*jprobe-example.c */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uio.h>
#include <linux/kprobes.h>
#include <linux/kallsyms.h>

 * Jumper probe for do_fork. 
 * Mirror principle enables access to arguments of the probed routine 
 * from the probe handler.

/* Proxy routine having the same arguments as actual do_fork() routine
long jdo_fork(unsigned long clone_flags, unsigned long stack_start,
	      struct pt_regs *regs, unsigned long stack_size,
	      int __user * parent_tidptr, int __user * child_tidptr)
	printk("jprobe: clone_flags=0x%lx, stack_size=0x%lx, regs=0x%p\n",
	       clone_flags, stack_size, regs);
	/* Always end with a call to jprobe_return(). */
	return 0;

static struct jprobe my_jprobe = {
	.entry = (kprobe_opcode_t *) jdo_fork

int init_module(void)
	int ret; = (kprobe_opcode_t *)
	if (! {
		printk("Couldn't find %s to plant jprobe\n", "do_fork");
		return -1;

	if ((ret = register_jprobe(&my_jprobe)) <0) {
		printk("register_jprobe failed, returned %d\n", ret);
		return -1;
	printk("Planted jprobe at %p, handler addr %p\n",, my_jprobe.entry);
	return 0;

void cleanup_module(void)
	printk("jprobe unregistered\n");

----- cut here -----

Build and insert the kernel module as shown in the above kprobe
example.  You will see the trace data in /var/log/messages and on
the console whenever do_fork() is invoked to create a new process.
(Some messages may be suppressed if syslogd is configured to
eliminate duplicate messages.)

9. Kretprobes Example

Here's a sample kernel module showing the use of return probes to
report failed calls to sys_open().
----- cut here -----
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/kallsyms.h>

static const char *probed_func = "sys_open";

/* Return-probe handler: If the probed function fails, log the return
value. */
static int ret_handler(struct kretprobe_instance *ri, struct pt_regs
	int retval = (int) regs->eax;
	if (retval < 0) {
		printk("%s returns %d\n", probed_func, retval);
	return 0;

static struct kretprobe my_kretprobe = {
	.handler = ret_handler,
	/* Probe up to 20 instances concurrently. */
	.maxactive = 20

int init_module(void)
	int ret; =
		(kprobe_opcode_t *) kallsyms_lookup_name(probed_func);
	if (! {
		printk("Couldn't find %s to plant return probe\n", probed_func);
		return -1;
	if ((ret = register_kretprobe(&my_kretprobe)) < 0) {
		printk("register_kretprobe failed, returned %d\n", ret);
		return -1;
	printk("Planted return probe at %p\n",;
	return 0;

void cleanup_module(void)
	printk("kretprobe unregistered\n");
	/* nmissed > 0 suggests that maxactive was set too low. */
	printk("Missed probing %d instances of %s\n",
		my_kretprobe.nmissed, probed_func);

----- cut here -----

Build and insert the kernel module as shown in the above kprobe
example.  You will see the trace data in /var/log/messages and on the
console whenever sys_open() returns a negative value.  (Some messages
may be suppressed if syslogd is configured to eliminate duplicate

For additional information on Kprobes, refer to the following URLs:

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