kprobes.txt 29.4 KB
Newer Older
1 2 3 4 5 6 7 8 9 10
Kernel Probes (Kprobes)

:Author: Jim Keniston <>
:Author: Prasanna S Panchamukhi <>
:Author: Masami Hiramatsu <>


  1. Concepts: Kprobes, and Return Probes
12 13 14 15 16 17 18
  2. Architectures Supported
  3. Configuring Kprobes
  4. API Reference
  5. Kprobes Features and Limitations
  6. Probe Overhead
  7. TODO
  8. Kprobes Example
19 20
  9. Kretprobes Example
  10. Deprecated Features
21 22 23
  Appendix A: The kprobes debugfs interface
  Appendix B: The kprobes sysctl interface

Concepts: Kprobes and Return Probes
26 27 28

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 [1]_, specifying a handler
routine to be invoked when the breakpoint is hit.
31 32 33

.. [1] some parts of the kernel code can not be trapped, see

35 36 37 38
There are currently two types of probes: kprobes, and kretprobes
(also called return probes).  A kprobe can be inserted on virtually
any instruction in the kernel.  A return probe fires when a specified
function returns.
39 40 41 42 43 44 45 46

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.

47 48
There are also ``register_/unregister_*probes()`` functions for batch
registration/unregistration of a group of ``*probes``. These functions
49 50 51
can speed up unregistration process when you have to unregister
a lot of probes at once.

52 53 54 55 56 57
The next four subsections explain how the different types of
probes work and how jump optimization works.  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 :ref:`kprobes_archs_supported`.

60 61
How Does a Kprobe Work?
62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82

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.

83 84
Return Probes

86 87
How Does a Return Probe Work?
88 89 90 91 92 93 94 95 96 97

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
98 99 100
handler calls the user-specified return 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.
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121

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.

122 123
Kretprobe entry-handler
124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146

Kretprobes also provides an optional user-specified handler which runs
on function entry. This handler is specified by setting the entry_handler
field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
function entry is hit, the user-defined entry_handler, if any, is invoked.
If the entry_handler returns 0 (success) then a corresponding return handler
is guaranteed to be called upon function return. If the entry_handler
returns a non-zero error then Kprobes leaves the return address as is, and
the kretprobe has no further effect for that particular function instance.

Multiple entry and return handler invocations are matched using the unique
kretprobe_instance object associated with them. Additionally, a user
may also specify per return-instance private data to be part of each
kretprobe_instance object. This is especially useful when sharing private
data between corresponding user entry and return handlers. The size of each
private data object can be specified at kretprobe registration time by
setting the data_size field of the kretprobe struct. This data can be
accessed through the data field of each kretprobe_instance object.

In case probed function is entered but there is no kretprobe_instance
object available, then in addition to incrementing the nmissed count,
the user entry_handler invocation is also skipped.

147 148 149 150
.. _kprobes_jump_optimization:

How Does Jump Optimization Work?

152 153
If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
154 155 156 157
the "debug.kprobes_optimization" kernel parameter is set to 1 (see
sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
instruction instead of a breakpoint instruction at each probepoint.

158 159
Init a Kprobe
160 161 162 163 164 165

When a probe is registered, before attempting this optimization,
Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
address. So, even if it's not possible to optimize this particular
probepoint, there'll be a probe there.

166 167
Safety Check
168 169 170 171

Before optimizing a probe, Kprobes performs the following safety checks:

- Kprobes verifies that the region that will be replaced by the jump
172 173 174
  instruction (the "optimized region") lies entirely within one function.
  (A jump instruction is multiple bytes, and so may overlay multiple
175 176

- Kprobes analyzes the entire function and verifies that there is no
177 178
  jump into the optimized region.  Specifically:

179 180
  - the function contains no indirect jump;
  - the function contains no instruction that causes an exception (since
181 182
    the fixup code triggered by the exception could jump back into the
    optimized region -- Kprobes checks the exception tables to verify this);
  - there is no near jump to the optimized region (other than to the first
185 186

- For each instruction in the optimized region, Kprobes verifies that
  the instruction can be executed out of line.

189 190
Preparing Detour Buffer
191 192 193

Next, Kprobes prepares a "detour" buffer, which contains the following
instruction sequence:

195 196 197 198 199 200
- code to push the CPU's registers (emulating a breakpoint trap)
- a call to the trampoline code which calls user's probe handlers.
- code to restore registers
- the instructions from the optimized region
- a jump back to the original execution path.

201 202
203 204 205

After preparing the detour buffer, Kprobes verifies that none of the
following situations exist:

- The probe has a post_handler.
208 209
- Other instructions in the optimized region are probed.
- The probe is disabled.

211 212 213 214 215 216 217 218 219 220 221
In any of the above cases, Kprobes won't start optimizing the probe.
Since these are temporary situations, Kprobes tries to start
optimizing it again if the situation is changed.

If the kprobe can be optimized, Kprobes enqueues the kprobe to an
optimizing list, and kicks the kprobe-optimizer workqueue to optimize
it.  If the to-be-optimized probepoint is hit before being optimized,
Kprobes returns control to the original instruction path by setting
the CPU's instruction pointer to the copied code in the detour buffer
-- thus at least avoiding the single-step.

222 223
224 225 226 227

The Kprobe-optimizer doesn't insert the jump instruction immediately;
rather, it calls synchronize_sched() for safety first, because it's
possible for a CPU to be interrupted in the middle of executing the
optimized region [3]_.  As you know, synchronize_sched() can ensure
229 230
that all interruptions that were active when synchronize_sched()
was called are done, but only if CONFIG_PREEMPT=n.  So, this version
of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
232 233 234 235 236

After that, the Kprobe-optimizer calls stop_machine() to replace
the optimized region with a jump instruction to the detour buffer,
using text_poke_smp().

237 238
239 240 241 242 243 244 245 246

When an optimized kprobe is unregistered, disabled, or blocked by
another kprobe, it will be unoptimized.  If this happens before
the optimization is complete, the kprobe is just dequeued from the
optimized list.  If the optimization has been done, the jump is
replaced with the original code (except for an int3 breakpoint in
the first byte) by using text_poke_smp().

247 248 249 250 251
.. [3] Please imagine that the 2nd instruction is interrupted and then
   the optimizer replaces the 2nd instruction with the jump *address*
   while the interrupt handler is running. When the interrupt
   returns to original address, there is no valid instruction,
   and it causes an unexpected result.

253 254 255
.. [4] This optimization-safety checking may be replaced with the
   stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
256 257 258 259 260 261 262 263

NOTE for geeks:
The jump optimization changes the kprobe's pre_handler behavior.
Without optimization, the pre_handler can change the kernel's execution
path by changing regs->ip and returning 1.  However, when the probe
is optimized, that modification is ignored.  Thus, if you want to
tweak the kernel's execution path, you need to suppress optimization,
using one of the following techniques:

- Specify an empty function for the kprobe's post_handler or break_handler.
266 267 268


269 270
- Execute 'sysctl -w debug.kprobes_optimization=n'

271 272 273 274
.. _kprobes_blacklist:

275 276 277 278 279 280 281 282 283 284 285 286

Kprobes can probe most of the kernel except itself. This means
that there are some functions where kprobes cannot probe. Probing
(trapping) such functions can cause a recursive trap (e.g. double
fault) or the nested probe handler may never be called.
Kprobes manages such functions as a blacklist.
If you want to add a function into the blacklist, you just need
to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
to specify a blacklisted function.
Kprobes checks the given probe address against the blacklist and
rejects registering it, if the given address is in the blacklist.

287 288 289 290
.. _kprobes_archs_supported:

Architectures Supported

Kprobes and return probes are implemented on the following
293 294

295 296
- i386 (Supports jump optimization)
- x86_64 (AMD-64, EM64T) (Supports jump optimization)
- ppc64
- ia64 (Does not support probes on instruction slot1.)
- sparc64 (Return probes not yet implemented.)
Nicolas Pitre's avatar
Nicolas Pitre committed
- arm
- ppc
- mips
- s390

305 306
Configuring Kprobes
307 308

When configuring the kernel using make menuconfig/xconfig/oldconfig,
309 310
ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
for "Kprobes".
311 312 313 314

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".

316 317 318
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.
319 320 321 322 323 324

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.

325 326
API Reference
327 328

The Kprobes API includes a "register" function and an "unregister"
329 330 331 332 333
function for each type of probe. The API also includes "register_*probes"
and "unregister_*probes" functions for (un)registering arrays of probes.
Here are terse, mini-man-page specifications for these functions and
the associated probe handlers that you'll write. See the files in the
samples/kprobes/ sub-directory for examples.

335 336

338 339 340 341

	#include <linux/kprobes.h>
	int register_kprobe(struct kprobe *kp);
342 343 344 345 346 347

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
348 349
kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
so, its handlers aren't hit until calling enable_kprobe(kp).

352 353 354 355 356
.. 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::
357 358 359

	kp.symbol_name = "symbol_name";

360 361
      (64-bit powerpc intricacies such as function descriptors are handled

363 364 365
   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

367 368
   3. Specify either the kprobe "symbol_name" OR the "addr". If both are
      specified, kprobe registration will fail with -EINVAL.

370 371 372
   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.

374 375
register_kprobe() returns 0 on success, or a negative errno otherwise.

376 377 378 379 380
User's pre-handler (kp->pre_handler)::

	#include <linux/kprobes.h>
	#include <linux/ptrace.h>
	int pre_handler(struct kprobe *p, struct pt_regs *regs);
381 382 383 384 385

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.

386 387 388 389 390 391
User's post-handler (kp->post_handler)::

	#include <linux/kprobes.h>
	#include <linux/ptrace.h>
	void post_handler(struct kprobe *p, struct pt_regs *regs,
			  unsigned long flags);
392 393 394 395

p and regs are as described for the pre_handler.  flags always seems
to be zero.

396 397 398 399 400
User's fault-handler (kp->fault_handler)::

	#include <linux/kprobes.h>
	#include <linux/ptrace.h>
	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
401 402 403 404 405 406

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.

407 408

410 411 412 413

	#include <linux/kprobes.h>
	int register_kretprobe(struct kretprobe *rp);
414 415 416 417 418 419 420 421 422

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

423 424 425 426 427 428
User's return-probe handler (rp->handler)::

	#include <linux/kprobes.h>
	#include <linux/ptrace.h>
	int kretprobe_handler(struct kretprobe_instance *ri,
			      struct pt_regs *regs);
429 430 431 432

regs is as described for kprobe.pre_handler.  ri points to the
kretprobe_instance object, of which the following fields may be
of interest:

434 435 436
- ret_addr: the return address
- rp: points to the corresponding kretprobe object
- task: points to the corresponding task struct
437 438
- data: points to per return-instance private data; see "Kretprobe
	entry-handler" for details.
439 440 441 442 443

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.

444 445
The handler's return value is currently ignored.

446 447

449 450 451 452 453

	#include <linux/kprobes.h>
	void unregister_kprobe(struct kprobe *kp);
	void unregister_kretprobe(struct kretprobe *rp);
454 455 456 457

Removes the specified probe.  The unregister function can be called
at any time after the probe has been registered.

458 459 460 461
.. note::

   If the functions find an incorrect probe (ex. an unregistered probe),
   they clear the addr field of the probe.

463 464

466 467 468 469 470

	#include <linux/kprobes.h>
	int register_kprobes(struct kprobe **kps, int num);
	int register_kretprobes(struct kretprobe **rps, int num);
471 472 473 474 475

Registers each of the num probes in the specified array.  If any
error occurs during registration, all probes in the array, up to
the bad probe, are safely unregistered before the register_*probes
function returns.
476 477

- kps/rps/jps: an array of pointers to ``*probe`` data structures
478 479
- num: the number of the array entries.

480 481 482 483 484 485 486
.. note::

   You have to allocate(or define) an array of pointers and set all
   of the array entries before using these functions.



490 491 492
	#include <linux/kprobes.h>
	void unregister_kprobes(struct kprobe **kps, int num);
	void unregister_kretprobes(struct kretprobe **rps, int num);
493 494 495

Removes each of the num probes in the specified array at once.

.. note::

498 499 500 501
   If the functions find some incorrect probes (ex. unregistered
   probes) in the specified array, they clear the addr field of those
   incorrect probes. However, other probes in the array are
   unregistered correctly.

503 504

506 507 508 509 510 511 512

	#include <linux/kprobes.h>
	int disable_kprobe(struct kprobe *kp);
	int disable_kretprobe(struct kretprobe *rp);

Temporarily disables the specified ``*probe``. You can enable it again by using
enable_*probe(). You must specify the probe which has been registered.

515 516


520 521 522 523 524
	#include <linux/kprobes.h>
	int enable_kprobe(struct kprobe *kp);
	int enable_kretprobe(struct kretprobe *rp);

Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
the probe which has been registered.

527 528
Kprobes Features and Limitations

530 531 532 533
Kprobes allows multiple probes at the same address. Also,
a probepoint for which there is a post_handler cannot be optimized.
So if you install a kprobe with a post_handler, at an optimized
probepoint, the probepoint will be unoptimized automatically.
534 535 536 537 538

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.

539 540
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).
543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558

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
559 560 561 562 563 564 565 566
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
567 568 569
registration and unregistration.

Probe handlers are run with preemption disabled.  Depending on the
570 571 572 573 574
architecture and optimization state, handlers may also run with
interrupts disabled (e.g., kretprobe handlers and optimized kprobe
handlers run without interrupt disabled on x86/x86-64).  In any case,
your handler should not yield the CPU (e.g., by attempting to acquire
a semaphore).
575 576 577 578 579 580 581 582

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.)

583 584
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
585 586 587 588 589 590
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.
591 592 593 594

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)
596 597
on the x86_64 version of __switch_to(); the registration functions
return -EINVAL.

599 600 601 602 603 604
On x86/x86-64, since the Jump Optimization of Kprobes modifies
instructions widely, there are some limitations to optimization. To
explain it, we introduce some terminology. Imagine a 3-instruction
sequence consisting of a two 2-byte instructions and one 3-byte

605 606 607 608 609 610 611 612

		[ins1][ins2][  ins3 ]
		[<-     DCR       ->]
		[<- JTPR ->]

614 615 616 617 618 619
	ins1: 1st Instruction
	ins2: 2nd Instruction
	ins3: 3rd Instruction
	IA:  Insertion Address
	JTPR: Jump Target Prohibition Region
	DCR: Detoured Code Region
620 621 622 623 624 625 626 627

The instructions in DCR are copied to the out-of-line buffer
of the kprobe, because the bytes in DCR are replaced by
a 5-byte jump instruction. So there are several limitations.

a) The instructions in DCR must be relocatable.
b) The instructions in DCR must not include a call instruction.
c) JTPR must not be targeted by any jump or call instruction.
d) DCR must not straddle the border between functions.
629 630 631 632

Anyway, these limitations are checked by the in-kernel instruction
decoder, so you don't need to worry about that.

633 634
Probe Overhead
635 636 637 638

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
639 640
million hits per second, depending on the architecture.  A return-probe
hit typically takes 50-75% longer than a kprobe hit.
641 642 643
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::

646 647
  k = kprobe; r = return probe; kr = kprobe + return probe
  on same function

  i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
  k = 0.57 usec; r = 0.92; kr = 0.99

  x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
  k = 0.49 usec; r = 0.80; kr = 0.82

  ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
  k = 0.77 usec; r = 1.26; kr = 1.45
657 658 659

Optimized Probe Overhead
660 661

Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
662 663 664 665
process. Here are sample overhead figures (in usec) for x86 architectures::

  k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
  r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.

667 668
  i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
  k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33

670 671
  x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
  k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30

673 674

a. SystemTap ( Provides a simplified
   programming interface for probe-based instrumentation.  Try it out.
678 679 680 681
b. Kernel return probes for sparc64.
c. Support for other architectures.
d. User-space probes.
e. Watchpoint probes (which fire on data references).

683 684
Kprobes Example

See samples/kprobes/kprobe_example.c

688 689
Kretprobes Example

See samples/kprobes/kretprobe_example.c
692 693

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

695 696 697 698 699
- (pages 101-115)

700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730
Deprecated Features

Jprobes is now a deprecated feature. People who are depending on it should
migrate to other tracing features or use older kernels. Please consider to
migrate your tool to one of the following options:

- Use trace-event to trace target function with arguments.

  trace-event is a low-overhead (and almost no visible overhead if it
  is off) statically defined event interface. You can define new events
  and trace it via ftrace or any other tracing tools.

  See the following urls:


- Use ftrace dynamic events (kprobe event) with perf-probe.

  If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
  find which register/stack is assigned to which local variable or arguments
  by using perf-probe and set up new event to trace it.

  See following documents:

  - Documentation/trace/kprobetrace.txt
  - Documentation/trace/events.txt
  - tools/perf/Documentation/perf-probe.txt

731 732 733

The kprobes debugfs interface
734 735 736

With recent kernels (> 2.6.20) the list of registered kprobes is visible
under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).

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

741 742
	c015d71a  k  vfs_read+0x0
	c03dedc5  r  tcp_v4_rcv+0x0
743 744

The first column provides the kernel address where the probe is inserted.
745 746 747 748
The second column identifies the type of probe (k - kprobe and r - kretprobe)
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. Following columns show probe status. If the probe is on
749 750
a virtual address that is no longer valid (module init sections, module
virtual addresses that correspond to modules that've been unloaded),
such probes are marked with [GONE]. If the probe is temporarily disabled,
such probes are marked with [DISABLED]. If the probe is optimized, it is
753 754
marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with

/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.

758 759 760 761 762 763
Provides a knob to globally and forcibly 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. Note that this knob just disarms and arms all kprobes and doesn't
change each probe's disabling state. This means that disabled kprobes (marked
[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
764 765

766 767
The kprobes sysctl interface
768 769 770 771 772

/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.

When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
a knob to globally and forcibly turn jump optimization (see section
773 774 775 776
:ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
is allowed (ON). If you echo "0" to this file or set
"debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
unoptimized, and any new probes registered after that will not be optimized.
777 778

Note that this knob *changes* the optimized state. This means that optimized
probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
780 781
removed). If the knob is turned on, they will be optimized again.