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Title   : Kernel Probes (Kprobes)

Authors : Jim Keniston 

        : Prasanna S Panchamukhi 

CONTENTS

1. Concepts: Kprobes, Jprobes, Return Probes

2. Architectures Supported

3. Configuring Kprobes

4. API Reference

5. Kprobes Features and Limitations

6. Probe Overhead

7. TODO

8. Kprobes Example

9. Jprobes Example

10. Kretprobes Example

Appendix A: The kprobes debugfs interface

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

returns.

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 via the

notifier_call_chain mechanism.  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 signature (arg list and return

type) 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

the saved registers and a generous portion of the stack (see below).

Kprobes then 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 register and

stack contents as the probed function.  When it is done, the handler

calls jprobe_return(), which traps again to restore the original stack

contents and processor state and switch to the probed function.

By convention, the callee owns its arguments, so gcc may produce code

that unexpectedly modifies that portion of the stack.  This is why

Kprobes saves a copy of the stack and restores it after the jprobe

handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,

64 bytes on i386.

Note that the probed function's args may be passed on the stack

or in registers (e.g., for x86_64 or for an i386 fastcall function).

The jprobe will work in either case, so long as the handler's

prototype matches that of the probed function.

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

architectures:

- i386

- x86_64 (AMD-64, EM64T)

- ppc64

- ia64 (Does not support probes on instruction slot1.)

- sparc64 (Return probes not yet implemented.)

3. Configuring Kprobes

When configuring the kernel using make menuconfig/xconfig/oldconfig,

ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation

Support", look for "Kprobes".

So that you can load and unload Kprobes-based instrumentation modules,

make sure "Loadable module support" (CONFIG_MODULES) and "Module

unloading" (CONFIG_MODULE_UNLOAD) are set to "y".

Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL

are set to "y", since kallsyms_lookup_name() is used by the in-kernel

kprobe address resolution code.

If you need to insert a probe in the middle of a function, you may find

it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),

so you can use "objdump -d -l vmlinux" to see the source-to-object

code mapping.

4. API Reference

The Kprobes API includes a "register" function and an "unregister"

function for each type of probe.  Here are terse, mini-man-page

specifications for these functions and the associated probe handlers

that you'll write.  See the latter half of this document for examples.

4.1 register_kprobe

#include 

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.

NOTE:

1. With the introduction of the "symbol_name" field to struct kprobe,

the probepoint address resolution will now be taken care of by the kernel.

The following will now work:

        kp.symbol_name = "symbol_name";

(64-bit powerpc intricacies such as function descriptors are handled

transparently)

2. Use the "offset" field of struct kprobe if the offset into the symbol

to install a probepoint is known. This field is used to calculate the

probepoint.

3. Specify either the kprobe "symbol_name" OR the "addr". If both are

specified, kprobe registration will fail with -EINVAL.

4. With CISC architectures (such as i386 and x86_64), the kprobes code

does not validate if the kprobe.addr is at an instruction boundary.

Use "offset" with caution.

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

User's pre-handler (kp->pre_handler):

#include 

#include 

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 (kp->post_handler):

#include 

#include 

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 (kp->fault_handler):

#include 

#include 

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 

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 runs the handler whose address is jp->entry.

The handler should have the same arg list and return type as the probed

function; and just before it returns, it must call jprobe_return().

(The handler never actually returns, since jprobe_return() returns

control to Kprobes.)  If the probed function is declared asmlinkage,

fastcall, or anything else that affects how args are passed, the

handler's declaration must match.

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

4.3 register_kretprobe

#include 

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

otherwise.

User's return-probe handler (rp->handler):

#include 

#include 

int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *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

- task: points to the corresponding task struct

The regs_return_value(regs) macro provides a simple abstraction to

extract the return value from the appropriate register as defined by

the architecture's ABI.

The handler's return value is currently ignored.

4.4 unregister_*probe

#include 

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

Kprobes allows multiple 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.

The register_*probe functions will return -EINVAL if you attempt

to install a probe in the code that implements Kprobes (mostly

kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such

as do_page_fault and notifier_call_chain).

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.  If a probe handler hits a probe, that second probe's

handlers won't be run in that instance, and the kprobe.nmissed member

of the second probe will be incremented.

As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of

the same handler) may run concurrently on different CPUs.

Kprobes does not use mutexes 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. In such a case, a line:

kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c

gets printed. With this information, one will be able to correlate the

exact instance of the kretprobe that caused the problem. We have the

do_exit() case covered. do_execve() and do_fork() are not an issue.

We're unaware of other specific cases where this could be a problem.

If, upon entry to or exit from a function, the CPU is running on

a stack other than that of the current task, registering a return

probe on that function may produce undesirable results.  For this

reason, Kprobes doesn't support return probes (or kprobes or jprobes)

on the x86_64 version of __switch_to(); the registration functions

return -EINVAL.

6. Probe Overhead

On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0

microseconds to process.  Specifically, a benchmark that hits the same

probepoint repeatedly, firing a simple handler each time, reports 1-2

million hits per second, depending on the architecture.  A jprobe or

return-probe hit typically takes 50-75% longer than a kprobe hit.

When you have a return probe set on a function, adding a kprobe at

the entry to that function adds essentially no overhead.

Here are sample overhead figures (in usec) for different architectures.

k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe

on same function; jr = jprobe + return probe on same function

i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips

k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40

x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips

k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07

ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)

k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99

7. TODO

a. SystemTap (http://sourceware.org/systemtap): Provides a simplified

programming interface for probe-based instrumentation.  Try it out.

b. Kernel return probes for sparc64.

c. Support for other architectures.

d. User-space probes.

e. Watchpoint probes (which fire on data references).

8. 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 -----

/*kprobe_example.c*/

#include 

#include 

#include 

#include 

/*For each probe you need to allocate a kprobe structure*/

static struct kprobe kp;

/*kprobe pre_handler: called just before the probed instruction is executed*/

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);

        dump_stack();

        return 0;

}

/*kprobe post_handler: called after the probed instruction is executed*/

void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long flags)

{

        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;

}

static int __init kprobe_init(void)

{

        int ret;

        kp.pre_handler = handler_pre;

        kp.post_handler = handler_post;

        kp.fault_handler = handler_fault;

        kp.symbol_name = "do_fork";

        ret = register_kprobe(&kp);

        if (ret < 0) {

                printk("register_kprobe failed, returned %d\n", ret);

                return ret;

        }

        printk("kprobe registered\n");

        return 0;

}

static void __exit kprobe_exit(void)

{

        unregister_kprobe(&kp);

        printk("kprobe unregistered\n");

}

module_init(kprobe_init)

module_exit(kprobe_exit)

MODULE_LICENSE("GPL");

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

You can build the kernel module, kprobe-example.ko, using the following

Makefile:

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

obj-m := kprobe-example.o

KDIR := /lib/modules/$(shell uname -r)/build

PWD := $(shell pwd)

default:

        $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:

        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.

9. 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 

#include 

#include 

#include 

#include 

/*

 * 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(). */

        jprobe_return();

        /*NOTREACHED*/

        return 0;

}

static struct jprobe my_jprobe = {

        .entry = jdo_fork

};

static int __init jprobe_init(void)

{

        int ret;

        my_jprobe.kp.symbol_name = "do_fork";

        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.kp.addr, my_jprobe.entry);

        return 0;

}

static void __exit jprobe_exit(void)

{

        unregister_jprobe(&my_jprobe);

        printk("jprobe unregistered\n");

}

module_init(jprobe_init)

module_exit(jprobe_exit)

MODULE_LICENSE("GPL");

----- 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.)

10. Kretprobes Example

Here's a sample kernel module showing the use of return probes to

report failed calls to sys_open().

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

/*kretprobe-example.c*/

#include 

#include 

#include 

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 *regs)

{

        int retval = regs_return_value(regs);

        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

};

static int __init kretprobe_init(void)

{

        int ret;

        my_kretprobe.kp.symbol_name = (char *)probed_func;

        if ((ret = register_kretprobe(&my_kretprobe)) < 0) {

                printk("register_kretprobe failed, returned %d\n", ret);

                return -1;

        }

        printk("Planted return probe at %p\n", my_kretprobe.kp.addr);

        return 0;

}

static void __exit kretprobe_exit(void)

{

        unregister_kretprobe(&my_kretprobe);

        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);

}

module_init(kretprobe_init)

module_exit(kretprobe_exit)

MODULE_LICENSE("GPL");

----- 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

messages.)

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

http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe

http://www.redhat.com/magazine/005mar05/features/kprobes/

http://www-users.cs.umn.edu/~boutcher/kprobes/

http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)

Appendix A: The kprobes debugfs interface

With recent kernels (> 2.6.20) the list of registered kprobes is visible

under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).

/debug/kprobes/list: Lists all registered probes on the system

c015d71a  k  vfs_read+0x0

c011a316  j  do_fork+0x0

c03dedc5  r  tcp_v4_rcv+0x0

The first column provides the kernel address where the probe is inserted.

The second column identifies the type of probe (k - kprobe, r - kretprobe

and j - jprobe), while the third column specifies the symbol+offset of

the probe. If the probed function belongs to a module, the module name

is also specified.

/debug/kprobes/enabled: Turn kprobes ON/OFF

Provides a knob to globally turn registered kprobes ON or OFF. By default,

all kprobes are enabled. By echoing "0" to this file, all registered probes

will be disarmed, till such time a "1" is echoed to this file.