1 =======================
2 Kernel Probes (Kprobes)
3 =======================
5 :Author: Jim Keniston <jkenisto@us.ibm.com>
6 :Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
7 :Author: Masami Hiramatsu <mhiramat@redhat.com>
11 1. Concepts: Kprobes, and Return Probes
12 2. Architectures Supported
13 3. Configuring Kprobes
15 5. Kprobes Features and Limitations
20 10. Deprecated Features
21 Appendix A: The kprobes debugfs interface
22 Appendix B: The kprobes sysctl interface
24 Concepts: Kprobes and Return Probes
25 =========================================
27 Kprobes enables you to dynamically break into any kernel routine and
28 collect debugging and performance information non-disruptively. You
29 can trap at almost any kernel code address [1]_, specifying a handler
30 routine to be invoked when the breakpoint is hit.
32 .. [1] some parts of the kernel code can not be trapped, see
33 :ref:`kprobes_blacklist`)
35 There are currently two types of probes: kprobes, and kretprobes
36 (also called return probes). A kprobe can be inserted on virtually
37 any instruction in the kernel. A return probe fires when a specified
40 In the typical case, Kprobes-based instrumentation is packaged as
41 a kernel module. The module's init function installs ("registers")
42 one or more probes, and the exit function unregisters them. A
43 registration function such as register_kprobe() specifies where
44 the probe is to be inserted and what handler is to be called when
47 There are also ``register_/unregister_*probes()`` functions for batch
48 registration/unregistration of a group of ``*probes``. These functions
49 can speed up unregistration process when you have to unregister
50 a lot of probes at once.
52 The next four subsections explain how the different types of
53 probes work and how jump optimization works. They explain certain
54 things that you'll need to know in order to make the best use of
55 Kprobes -- e.g., the difference between a pre_handler and
56 a post_handler, and how to use the maxactive and nmissed fields of
57 a kretprobe. But if you're in a hurry to start using Kprobes, you
58 can skip ahead to :ref:`kprobes_archs_supported`.
60 How Does a Kprobe Work?
61 -----------------------
63 When a kprobe is registered, Kprobes makes a copy of the probed
64 instruction and replaces the first byte(s) of the probed instruction
65 with a breakpoint instruction (e.g., int3 on i386 and x86_64).
67 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
68 registers are saved, and control passes to Kprobes via the
69 notifier_call_chain mechanism. Kprobes executes the "pre_handler"
70 associated with the kprobe, passing the handler the addresses of the
71 kprobe struct and the saved registers.
73 Next, Kprobes single-steps its copy of the probed instruction.
74 (It would be simpler to single-step the actual instruction in place,
75 but then Kprobes would have to temporarily remove the breakpoint
76 instruction. This would open a small time window when another CPU
77 could sail right past the probepoint.)
79 After the instruction is single-stepped, Kprobes executes the
80 "post_handler," if any, that is associated with the kprobe.
81 Execution then continues with the instruction following the probepoint.
83 Changing Execution Path
84 -----------------------
86 Since kprobes can probe into a running kernel code, it can change the
87 register set, including instruction pointer. This operation requires
88 maximum care, such as keeping the stack frame, recovering the execution
89 path etc. Since it operates on a running kernel and needs deep knowledge
90 of computer architecture and concurrent computing, you can easily shoot
93 If you change the instruction pointer (and set up other related
94 registers) in pre_handler, you must return !0 so that kprobes stops
95 single stepping and just returns to the given address.
96 This also means post_handler should not be called anymore.
98 Note that this operation may be harder on some architectures which use
99 TOC (Table of Contents) for function call, since you have to setup a new
100 TOC for your function in your module, and recover the old one after
106 How Does a Return Probe Work?
107 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
109 When you call register_kretprobe(), Kprobes establishes a kprobe at
110 the entry to the function. When the probed function is called and this
111 probe is hit, Kprobes saves a copy of the return address, and replaces
112 the return address with the address of a "trampoline." The trampoline
113 is an arbitrary piece of code -- typically just a nop instruction.
114 At boot time, Kprobes registers a kprobe at the trampoline.
116 When the probed function executes its return instruction, control
117 passes to the trampoline and that probe is hit. Kprobes' trampoline
118 handler calls the user-specified return handler associated with the
119 kretprobe, then sets the saved instruction pointer to the saved return
120 address, and that's where execution resumes upon return from the trap.
122 While the probed function is executing, its return address is
123 stored in an object of type kretprobe_instance. Before calling
124 register_kretprobe(), the user sets the maxactive field of the
125 kretprobe struct to specify how many instances of the specified
126 function can be probed simultaneously. register_kretprobe()
127 pre-allocates the indicated number of kretprobe_instance objects.
129 For example, if the function is non-recursive and is called with a
130 spinlock held, maxactive = 1 should be enough. If the function is
131 non-recursive and can never relinquish the CPU (e.g., via a semaphore
132 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
133 set to a default value. If CONFIG_PREEMPT is enabled, the default
134 is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
136 It's not a disaster if you set maxactive too low; you'll just miss
137 some probes. In the kretprobe struct, the nmissed field is set to
138 zero when the return probe is registered, and is incremented every
139 time the probed function is entered but there is no kretprobe_instance
140 object available for establishing the return probe.
142 Kretprobe entry-handler
143 ^^^^^^^^^^^^^^^^^^^^^^^
145 Kretprobes also provides an optional user-specified handler which runs
146 on function entry. This handler is specified by setting the entry_handler
147 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
148 function entry is hit, the user-defined entry_handler, if any, is invoked.
149 If the entry_handler returns 0 (success) then a corresponding return handler
150 is guaranteed to be called upon function return. If the entry_handler
151 returns a non-zero error then Kprobes leaves the return address as is, and
152 the kretprobe has no further effect for that particular function instance.
154 Multiple entry and return handler invocations are matched using the unique
155 kretprobe_instance object associated with them. Additionally, a user
156 may also specify per return-instance private data to be part of each
157 kretprobe_instance object. This is especially useful when sharing private
158 data between corresponding user entry and return handlers. The size of each
159 private data object can be specified at kretprobe registration time by
160 setting the data_size field of the kretprobe struct. This data can be
161 accessed through the data field of each kretprobe_instance object.
163 In case probed function is entered but there is no kretprobe_instance
164 object available, then in addition to incrementing the nmissed count,
165 the user entry_handler invocation is also skipped.
167 .. _kprobes_jump_optimization:
169 How Does Jump Optimization Work?
170 --------------------------------
172 If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
173 is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
174 the "debug.kprobes_optimization" kernel parameter is set to 1 (see
175 sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
176 instruction instead of a breakpoint instruction at each probepoint.
181 When a probe is registered, before attempting this optimization,
182 Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
183 address. So, even if it's not possible to optimize this particular
184 probepoint, there'll be a probe there.
189 Before optimizing a probe, Kprobes performs the following safety checks:
191 - Kprobes verifies that the region that will be replaced by the jump
192 instruction (the "optimized region") lies entirely within one function.
193 (A jump instruction is multiple bytes, and so may overlay multiple
196 - Kprobes analyzes the entire function and verifies that there is no
197 jump into the optimized region. Specifically:
199 - the function contains no indirect jump;
200 - the function contains no instruction that causes an exception (since
201 the fixup code triggered by the exception could jump back into the
202 optimized region -- Kprobes checks the exception tables to verify this);
203 - there is no near jump to the optimized region (other than to the first
206 - For each instruction in the optimized region, Kprobes verifies that
207 the instruction can be executed out of line.
209 Preparing Detour Buffer
210 ^^^^^^^^^^^^^^^^^^^^^^^
212 Next, Kprobes prepares a "detour" buffer, which contains the following
213 instruction sequence:
215 - code to push the CPU's registers (emulating a breakpoint trap)
216 - a call to the trampoline code which calls user's probe handlers.
217 - code to restore registers
218 - the instructions from the optimized region
219 - a jump back to the original execution path.
224 After preparing the detour buffer, Kprobes verifies that none of the
225 following situations exist:
227 - The probe has a post_handler.
228 - Other instructions in the optimized region are probed.
229 - The probe is disabled.
231 In any of the above cases, Kprobes won't start optimizing the probe.
232 Since these are temporary situations, Kprobes tries to start
233 optimizing it again if the situation is changed.
235 If the kprobe can be optimized, Kprobes enqueues the kprobe to an
236 optimizing list, and kicks the kprobe-optimizer workqueue to optimize
237 it. If the to-be-optimized probepoint is hit before being optimized,
238 Kprobes returns control to the original instruction path by setting
239 the CPU's instruction pointer to the copied code in the detour buffer
240 -- thus at least avoiding the single-step.
245 The Kprobe-optimizer doesn't insert the jump instruction immediately;
246 rather, it calls synchronize_sched() for safety first, because it's
247 possible for a CPU to be interrupted in the middle of executing the
248 optimized region [3]_. As you know, synchronize_sched() can ensure
249 that all interruptions that were active when synchronize_sched()
250 was called are done, but only if CONFIG_PREEMPT=n. So, this version
251 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
253 After that, the Kprobe-optimizer calls stop_machine() to replace
254 the optimized region with a jump instruction to the detour buffer,
255 using text_poke_smp().
260 When an optimized kprobe is unregistered, disabled, or blocked by
261 another kprobe, it will be unoptimized. If this happens before
262 the optimization is complete, the kprobe is just dequeued from the
263 optimized list. If the optimization has been done, the jump is
264 replaced with the original code (except for an int3 breakpoint in
265 the first byte) by using text_poke_smp().
267 .. [3] Please imagine that the 2nd instruction is interrupted and then
268 the optimizer replaces the 2nd instruction with the jump *address*
269 while the interrupt handler is running. When the interrupt
270 returns to original address, there is no valid instruction,
271 and it causes an unexpected result.
273 .. [4] This optimization-safety checking may be replaced with the
274 stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
278 The jump optimization changes the kprobe's pre_handler behavior.
279 Without optimization, the pre_handler can change the kernel's execution
280 path by changing regs->ip and returning 1. However, when the probe
281 is optimized, that modification is ignored. Thus, if you want to
282 tweak the kernel's execution path, you need to suppress optimization,
283 using one of the following techniques:
285 - Specify an empty function for the kprobe's post_handler.
289 - Execute 'sysctl -w debug.kprobes_optimization=n'
291 .. _kprobes_blacklist:
296 Kprobes can probe most of the kernel except itself. This means
297 that there are some functions where kprobes cannot probe. Probing
298 (trapping) such functions can cause a recursive trap (e.g. double
299 fault) or the nested probe handler may never be called.
300 Kprobes manages such functions as a blacklist.
301 If you want to add a function into the blacklist, you just need
302 to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
303 to specify a blacklisted function.
304 Kprobes checks the given probe address against the blacklist and
305 rejects registering it, if the given address is in the blacklist.
307 .. _kprobes_archs_supported:
309 Architectures Supported
310 =======================
312 Kprobes and return probes are implemented on the following
315 - i386 (Supports jump optimization)
316 - x86_64 (AMD-64, EM64T) (Supports jump optimization)
318 - ia64 (Does not support probes on instruction slot1.)
319 - sparc64 (Return probes not yet implemented.)
328 When configuring the kernel using make menuconfig/xconfig/oldconfig,
329 ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
332 So that you can load and unload Kprobes-based instrumentation modules,
333 make sure "Loadable module support" (CONFIG_MODULES) and "Module
334 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
336 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
337 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
338 kprobe address resolution code.
340 If you need to insert a probe in the middle of a function, you may find
341 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
342 so you can use "objdump -d -l vmlinux" to see the source-to-object
348 The Kprobes API includes a "register" function and an "unregister"
349 function for each type of probe. The API also includes "register_*probes"
350 and "unregister_*probes" functions for (un)registering arrays of probes.
351 Here are terse, mini-man-page specifications for these functions and
352 the associated probe handlers that you'll write. See the files in the
353 samples/kprobes/ sub-directory for examples.
360 #include <linux/kprobes.h>
361 int register_kprobe(struct kprobe *kp);
363 Sets a breakpoint at the address kp->addr. When the breakpoint is
364 hit, Kprobes calls kp->pre_handler. After the probed instruction
365 is single-stepped, Kprobe calls kp->post_handler. If a fault
366 occurs during execution of kp->pre_handler or kp->post_handler,
367 or during single-stepping of the probed instruction, Kprobes calls
368 kp->fault_handler. Any or all handlers can be NULL. If kp->flags
369 is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
370 so, its handlers aren't hit until calling enable_kprobe(kp).
374 1. With the introduction of the "symbol_name" field to struct kprobe,
375 the probepoint address resolution will now be taken care of by the kernel.
376 The following will now work::
378 kp.symbol_name = "symbol_name";
380 (64-bit powerpc intricacies such as function descriptors are handled
383 2. Use the "offset" field of struct kprobe if the offset into the symbol
384 to install a probepoint is known. This field is used to calculate the
387 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
388 specified, kprobe registration will fail with -EINVAL.
390 4. With CISC architectures (such as i386 and x86_64), the kprobes code
391 does not validate if the kprobe.addr is at an instruction boundary.
392 Use "offset" with caution.
394 register_kprobe() returns 0 on success, or a negative errno otherwise.
396 User's pre-handler (kp->pre_handler)::
398 #include <linux/kprobes.h>
399 #include <linux/ptrace.h>
400 int pre_handler(struct kprobe *p, struct pt_regs *regs);
402 Called with p pointing to the kprobe associated with the breakpoint,
403 and regs pointing to the struct containing the registers saved when
404 the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
406 User's post-handler (kp->post_handler)::
408 #include <linux/kprobes.h>
409 #include <linux/ptrace.h>
410 void post_handler(struct kprobe *p, struct pt_regs *regs,
411 unsigned long flags);
413 p and regs are as described for the pre_handler. flags always seems
416 User's fault-handler (kp->fault_handler)::
418 #include <linux/kprobes.h>
419 #include <linux/ptrace.h>
420 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
422 p and regs are as described for the pre_handler. trapnr is the
423 architecture-specific trap number associated with the fault (e.g.,
424 on i386, 13 for a general protection fault or 14 for a page fault).
425 Returns 1 if it successfully handled the exception.
432 #include <linux/kprobes.h>
433 int register_kretprobe(struct kretprobe *rp);
435 Establishes a return probe for the function whose address is
436 rp->kp.addr. When that function returns, Kprobes calls rp->handler.
437 You must set rp->maxactive appropriately before you call
438 register_kretprobe(); see "How Does a Return Probe Work?" for details.
440 register_kretprobe() returns 0 on success, or a negative errno
443 User's return-probe handler (rp->handler)::
445 #include <linux/kprobes.h>
446 #include <linux/ptrace.h>
447 int kretprobe_handler(struct kretprobe_instance *ri,
448 struct pt_regs *regs);
450 regs is as described for kprobe.pre_handler. ri points to the
451 kretprobe_instance object, of which the following fields may be
454 - ret_addr: the return address
455 - rp: points to the corresponding kretprobe object
456 - task: points to the corresponding task struct
457 - data: points to per return-instance private data; see "Kretprobe
458 entry-handler" for details.
460 The regs_return_value(regs) macro provides a simple abstraction to
461 extract the return value from the appropriate register as defined by
462 the architecture's ABI.
464 The handler's return value is currently ignored.
471 #include <linux/kprobes.h>
472 void unregister_kprobe(struct kprobe *kp);
473 void unregister_kretprobe(struct kretprobe *rp);
475 Removes the specified probe. The unregister function can be called
476 at any time after the probe has been registered.
480 If the functions find an incorrect probe (ex. an unregistered probe),
481 they clear the addr field of the probe.
488 #include <linux/kprobes.h>
489 int register_kprobes(struct kprobe **kps, int num);
490 int register_kretprobes(struct kretprobe **rps, int num);
492 Registers each of the num probes in the specified array. If any
493 error occurs during registration, all probes in the array, up to
494 the bad probe, are safely unregistered before the register_*probes
497 - kps/rps: an array of pointers to ``*probe`` data structures
498 - num: the number of the array entries.
502 You have to allocate(or define) an array of pointers and set all
503 of the array entries before using these functions.
510 #include <linux/kprobes.h>
511 void unregister_kprobes(struct kprobe **kps, int num);
512 void unregister_kretprobes(struct kretprobe **rps, int num);
514 Removes each of the num probes in the specified array at once.
518 If the functions find some incorrect probes (ex. unregistered
519 probes) in the specified array, they clear the addr field of those
520 incorrect probes. However, other probes in the array are
521 unregistered correctly.
528 #include <linux/kprobes.h>
529 int disable_kprobe(struct kprobe *kp);
530 int disable_kretprobe(struct kretprobe *rp);
532 Temporarily disables the specified ``*probe``. You can enable it again by using
533 enable_*probe(). You must specify the probe which has been registered.
540 #include <linux/kprobes.h>
541 int enable_kprobe(struct kprobe *kp);
542 int enable_kretprobe(struct kretprobe *rp);
544 Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
545 the probe which has been registered.
547 Kprobes Features and Limitations
548 ================================
550 Kprobes allows multiple probes at the same address. Also,
551 a probepoint for which there is a post_handler cannot be optimized.
552 So if you install a kprobe with a post_handler, at an optimized
553 probepoint, the probepoint will be unoptimized automatically.
555 In general, you can install a probe anywhere in the kernel.
556 In particular, you can probe interrupt handlers. Known exceptions
557 are discussed in this section.
559 The register_*probe functions will return -EINVAL if you attempt
560 to install a probe in the code that implements Kprobes (mostly
561 kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
562 as do_page_fault and notifier_call_chain).
564 If you install a probe in an inline-able function, Kprobes makes
565 no attempt to chase down all inline instances of the function and
566 install probes there. gcc may inline a function without being asked,
567 so keep this in mind if you're not seeing the probe hits you expect.
569 A probe handler can modify the environment of the probed function
570 -- e.g., by modifying kernel data structures, or by modifying the
571 contents of the pt_regs struct (which are restored to the registers
572 upon return from the breakpoint). So Kprobes can be used, for example,
573 to install a bug fix or to inject faults for testing. Kprobes, of
574 course, has no way to distinguish the deliberately injected faults
575 from the accidental ones. Don't drink and probe.
577 Kprobes makes no attempt to prevent probe handlers from stepping on
578 each other -- e.g., probing printk() and then calling printk() from a
579 probe handler. If a probe handler hits a probe, that second probe's
580 handlers won't be run in that instance, and the kprobe.nmissed member
581 of the second probe will be incremented.
583 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
584 the same handler) may run concurrently on different CPUs.
586 Kprobes does not use mutexes or allocate memory except during
587 registration and unregistration.
589 Probe handlers are run with preemption disabled or interrupt disabled,
590 which depends on the architecture and optimization state. (e.g.,
591 kretprobe handlers and optimized kprobe handlers run without interrupt
592 disabled on x86/x86-64). In any case, your handler should not yield
593 the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
595 Since a return probe is implemented by replacing the return
596 address with the trampoline's address, stack backtraces and calls
597 to __builtin_return_address() will typically yield the trampoline's
598 address instead of the real return address for kretprobed functions.
599 (As far as we can tell, __builtin_return_address() is used only
600 for instrumentation and error reporting.)
602 If the number of times a function is called does not match the number
603 of times it returns, registering a return probe on that function may
604 produce undesirable results. In such a case, a line:
605 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
606 gets printed. With this information, one will be able to correlate the
607 exact instance of the kretprobe that caused the problem. We have the
608 do_exit() case covered. do_execve() and do_fork() are not an issue.
609 We're unaware of other specific cases where this could be a problem.
611 If, upon entry to or exit from a function, the CPU is running on
612 a stack other than that of the current task, registering a return
613 probe on that function may produce undesirable results. For this
614 reason, Kprobes doesn't support return probes (or kprobes)
615 on the x86_64 version of __switch_to(); the registration functions
618 On x86/x86-64, since the Jump Optimization of Kprobes modifies
619 instructions widely, there are some limitations to optimization. To
620 explain it, we introduce some terminology. Imagine a 3-instruction
621 sequence consisting of a two 2-byte instructions and one 3-byte
628 [-2][-1][0][1][2][3][4][5][6][7]
633 ins1: 1st Instruction
634 ins2: 2nd Instruction
635 ins3: 3rd Instruction
636 IA: Insertion Address
637 JTPR: Jump Target Prohibition Region
638 DCR: Detoured Code Region
640 The instructions in DCR are copied to the out-of-line buffer
641 of the kprobe, because the bytes in DCR are replaced by
642 a 5-byte jump instruction. So there are several limitations.
644 a) The instructions in DCR must be relocatable.
645 b) The instructions in DCR must not include a call instruction.
646 c) JTPR must not be targeted by any jump or call instruction.
647 d) DCR must not straddle the border between functions.
649 Anyway, these limitations are checked by the in-kernel instruction
650 decoder, so you don't need to worry about that.
655 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
656 microseconds to process. Specifically, a benchmark that hits the same
657 probepoint repeatedly, firing a simple handler each time, reports 1-2
658 million hits per second, depending on the architecture. A return-probe
659 hit typically takes 50-75% longer than a kprobe hit.
660 When you have a return probe set on a function, adding a kprobe at
661 the entry to that function adds essentially no overhead.
663 Here are sample overhead figures (in usec) for different architectures::
665 k = kprobe; r = return probe; kr = kprobe + return probe
668 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
669 k = 0.57 usec; r = 0.92; kr = 0.99
671 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
672 k = 0.49 usec; r = 0.80; kr = 0.82
674 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
675 k = 0.77 usec; r = 1.26; kr = 1.45
677 Optimized Probe Overhead
678 ------------------------
680 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
681 process. Here are sample overhead figures (in usec) for x86 architectures::
683 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
684 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
686 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
687 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
689 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
690 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
695 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
696 programming interface for probe-based instrumentation. Try it out.
697 b. Kernel return probes for sparc64.
698 c. Support for other architectures.
699 d. User-space probes.
700 e. Watchpoint probes (which fire on data references).
705 See samples/kprobes/kprobe_example.c
710 See samples/kprobes/kretprobe_example.c
712 For additional information on Kprobes, refer to the following URLs:
714 - http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
715 - http://www.redhat.com/magazine/005mar05/features/kprobes/
716 - http://www-users.cs.umn.edu/~boutcher/kprobes/
717 - http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
722 Jprobes is now a deprecated feature. People who are depending on it should
723 migrate to other tracing features or use older kernels. Please consider to
724 migrate your tool to one of the following options:
726 - Use trace-event to trace target function with arguments.
728 trace-event is a low-overhead (and almost no visible overhead if it
729 is off) statically defined event interface. You can define new events
730 and trace it via ftrace or any other tracing tools.
732 See the following urls:
734 - https://lwn.net/Articles/379903/
735 - https://lwn.net/Articles/381064/
736 - https://lwn.net/Articles/383362/
738 - Use ftrace dynamic events (kprobe event) with perf-probe.
740 If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
741 find which register/stack is assigned to which local variable or arguments
742 by using perf-probe and set up new event to trace it.
744 See following documents:
746 - Documentation/trace/kprobetrace.rst
747 - Documentation/trace/events.rst
748 - tools/perf/Documentation/perf-probe.txt
751 The kprobes debugfs interface
752 =============================
755 With recent kernels (> 2.6.20) the list of registered kprobes is visible
756 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
758 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
760 c015d71a k vfs_read+0x0
761 c03dedc5 r tcp_v4_rcv+0x0
763 The first column provides the kernel address where the probe is inserted.
764 The second column identifies the type of probe (k - kprobe and r - kretprobe)
765 while the third column specifies the symbol+offset of the probe.
766 If the probed function belongs to a module, the module name is also
767 specified. Following columns show probe status. If the probe is on
768 a virtual address that is no longer valid (module init sections, module
769 virtual addresses that correspond to modules that've been unloaded),
770 such probes are marked with [GONE]. If the probe is temporarily disabled,
771 such probes are marked with [DISABLED]. If the probe is optimized, it is
772 marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
775 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
777 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
778 By default, all kprobes are enabled. By echoing "0" to this file, all
779 registered probes will be disarmed, till such time a "1" is echoed to this
780 file. Note that this knob just disarms and arms all kprobes and doesn't
781 change each probe's disabling state. This means that disabled kprobes (marked
782 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
785 The kprobes sysctl interface
786 ============================
788 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
790 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
791 a knob to globally and forcibly turn jump optimization (see section
792 :ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
793 is allowed (ON). If you echo "0" to this file or set
794 "debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
795 unoptimized, and any new probes registered after that will not be optimized.
797 Note that this knob *changes* the optimized state. This means that optimized
798 probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
799 removed). If the knob is turned on, they will be optimized again.