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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

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, Jprobes, Return Probes
12	  2. Architectures Supported
13	  3. Configuring Kprobes
14	  4. API Reference
15	  5. Kprobes Features and Limitations
16	  6. Probe Overhead
17	  7. TODO
18	  8. Kprobes Example
19	  9. Jprobes Example
20	  10. Kretprobes Example
21	  Appendix A: The kprobes debugfs interface
22	  Appendix B: The kprobes sysctl interface
24	Concepts: Kprobes, Jprobes, 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 three types of probes: kprobes, jprobes, and
36	kretprobes (also called return probes).  A kprobe can be inserted
37	on virtually any instruction in the kernel.  A jprobe is inserted at
38	the entry to a kernel function, and provides convenient access to the
39	function's arguments.  A return probe fires when a specified function
40	returns.
42	In the typical case, Kprobes-based instrumentation is packaged as
43	a kernel module.  The module's init function installs ("registers")
44	one or more probes, and the exit function unregisters them.  A
45	registration function such as register_kprobe() specifies where
46	the probe is to be inserted and what handler is to be called when
47	the probe is hit.
49	There are also ``register_/unregister_*probes()`` functions for batch
50	registration/unregistration of a group of ``*probes``. These functions
51	can speed up unregistration process when you have to unregister
52	a lot of probes at once.
54	The next four subsections explain how the different types of
55	probes work and how jump optimization works.  They explain certain
56	things that you'll need to know in order to make the best use of
57	Kprobes -- e.g., the difference between a pre_handler and
58	a post_handler, and how to use the maxactive and nmissed fields of
59	a kretprobe.  But if you're in a hurry to start using Kprobes, you
60	can skip ahead to :ref:`kprobes_archs_supported`.
62	How Does a Kprobe Work?
63	-----------------------
65	When a kprobe is registered, Kprobes makes a copy of the probed
66	instruction and replaces the first byte(s) of the probed instruction
67	with a breakpoint instruction (e.g., int3 on i386 and x86_64).
69	When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
70	registers are saved, and control passes to Kprobes via the
71	notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
72	associated with the kprobe, passing the handler the addresses of the
73	kprobe struct and the saved registers.
75	Next, Kprobes single-steps its copy of the probed instruction.
76	(It would be simpler to single-step the actual instruction in place,
77	but then Kprobes would have to temporarily remove the breakpoint
78	instruction.  This would open a small time window when another CPU
79	could sail right past the probepoint.)
81	After the instruction is single-stepped, Kprobes executes the
82	"post_handler," if any, that is associated with the kprobe.
83	Execution then continues with the instruction following the probepoint.
85	How Does a Jprobe Work?
86	-----------------------
88	A jprobe is implemented using a kprobe that is placed on a function's
89	entry point.  It employs a simple mirroring principle to allow
90	seamless access to the probed function's arguments.  The jprobe
91	handler routine should have the same signature (arg list and return
92	type) as the function being probed, and must always end by calling
93	the Kprobes function jprobe_return().
95	Here's how it works.  When the probe is hit, Kprobes makes a copy of
96	the saved registers and a generous portion of the stack (see below).
97	Kprobes then points the saved instruction pointer at the jprobe's
98	handler routine, and returns from the trap.  As a result, control
99	passes to the handler, which is presented with the same register and
100	stack contents as the probed function.  When it is done, the handler
101	calls jprobe_return(), which traps again to restore the original stack
102	contents and processor state and switch to the probed function.
104	By convention, the callee owns its arguments, so gcc may produce code
105	that unexpectedly modifies that portion of the stack.  This is why
106	Kprobes saves a copy of the stack and restores it after the jprobe
107	handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
108	64 bytes on i386.
110	Note that the probed function's args may be passed on the stack
111	or in registers.  The jprobe will work in either case, so long as the
112	handler's prototype matches that of the probed function.
114	Note that in some architectures (e.g.: arm64 and sparc64) the stack
115	copy is not done, as the actual location of stacked parameters may be
116	outside of a reasonable MAX_STACK_SIZE value and because that location
117	cannot be determined by the jprobes code. In this case the jprobes
118	user must be careful to make certain the calling signature of the
119	function does not cause parameters to be passed on the stack (e.g.:
120	more than eight function arguments, an argument of more than sixteen
121	bytes, or more than 64 bytes of argument data, depending on
122	architecture).
124	Return Probes
125	-------------
127	How Does a Return Probe Work?
128	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
130	When you call register_kretprobe(), Kprobes establishes a kprobe at
131	the entry to the function.  When the probed function is called and this
132	probe is hit, Kprobes saves a copy of the return address, and replaces
133	the return address with the address of a "trampoline."  The trampoline
134	is an arbitrary piece of code -- typically just a nop instruction.
135	At boot time, Kprobes registers a kprobe at the trampoline.
137	When the probed function executes its return instruction, control
138	passes to the trampoline and that probe is hit.  Kprobes' trampoline
139	handler calls the user-specified return handler associated with the
140	kretprobe, then sets the saved instruction pointer to the saved return
141	address, and that's where execution resumes upon return from the trap.
143	While the probed function is executing, its return address is
144	stored in an object of type kretprobe_instance.  Before calling
145	register_kretprobe(), the user sets the maxactive field of the
146	kretprobe struct to specify how many instances of the specified
147	function can be probed simultaneously.  register_kretprobe()
148	pre-allocates the indicated number of kretprobe_instance objects.
150	For example, if the function is non-recursive and is called with a
151	spinlock held, maxactive = 1 should be enough.  If the function is
152	non-recursive and can never relinquish the CPU (e.g., via a semaphore
153	or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
154	set to a default value.  If CONFIG_PREEMPT is enabled, the default
155	is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
157	It's not a disaster if you set maxactive too low; you'll just miss
158	some probes.  In the kretprobe struct, the nmissed field is set to
159	zero when the return probe is registered, and is incremented every
160	time the probed function is entered but there is no kretprobe_instance
161	object available for establishing the return probe.
163	Kretprobe entry-handler
164	^^^^^^^^^^^^^^^^^^^^^^^
166	Kretprobes also provides an optional user-specified handler which runs
167	on function entry. This handler is specified by setting the entry_handler
168	field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
169	function entry is hit, the user-defined entry_handler, if any, is invoked.
170	If the entry_handler returns 0 (success) then a corresponding return handler
171	is guaranteed to be called upon function return. If the entry_handler
172	returns a non-zero error then Kprobes leaves the return address as is, and
173	the kretprobe has no further effect for that particular function instance.
175	Multiple entry and return handler invocations are matched using the unique
176	kretprobe_instance object associated with them. Additionally, a user
177	may also specify per return-instance private data to be part of each
178	kretprobe_instance object. This is especially useful when sharing private
179	data between corresponding user entry and return handlers. The size of each
180	private data object can be specified at kretprobe registration time by
181	setting the data_size field of the kretprobe struct. This data can be
182	accessed through the data field of each kretprobe_instance object.
184	In case probed function is entered but there is no kretprobe_instance
185	object available, then in addition to incrementing the nmissed count,
186	the user entry_handler invocation is also skipped.
188	.. _kprobes_jump_optimization:
190	How Does Jump Optimization Work?
191	--------------------------------
193	If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
194	is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
195	the "debug.kprobes_optimization" kernel parameter is set to 1 (see
196	sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
197	instruction instead of a breakpoint instruction at each probepoint.
199	Init a Kprobe
200	^^^^^^^^^^^^^
202	When a probe is registered, before attempting this optimization,
203	Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
204	address. So, even if it's not possible to optimize this particular
205	probepoint, there'll be a probe there.
207	Safety Check
208	^^^^^^^^^^^^
210	Before optimizing a probe, Kprobes performs the following safety checks:
212	- Kprobes verifies that the region that will be replaced by the jump
213	  instruction (the "optimized region") lies entirely within one function.
214	  (A jump instruction is multiple bytes, and so may overlay multiple
215	  instructions.)
217	- Kprobes analyzes the entire function and verifies that there is no
218	  jump into the optimized region.  Specifically:
220	  - the function contains no indirect jump;
221	  - the function contains no instruction that causes an exception (since
222	    the fixup code triggered by the exception could jump back into the
223	    optimized region -- Kprobes checks the exception tables to verify this);
224	  - there is no near jump to the optimized region (other than to the first
225	    byte).
227	- For each instruction in the optimized region, Kprobes verifies that
228	  the instruction can be executed out of line.
230	Preparing Detour Buffer
231	^^^^^^^^^^^^^^^^^^^^^^^
233	Next, Kprobes prepares a "detour" buffer, which contains the following
234	instruction sequence:
236	- code to push the CPU's registers (emulating a breakpoint trap)
237	- a call to the trampoline code which calls user's probe handlers.
238	- code to restore registers
239	- the instructions from the optimized region
240	- a jump back to the original execution path.
242	Pre-optimization
243	^^^^^^^^^^^^^^^^
245	After preparing the detour buffer, Kprobes verifies that none of the
246	following situations exist:
248	- The probe has either a break_handler (i.e., it's a jprobe) or a
249	  post_handler.
250	- Other instructions in the optimized region are probed.
251	- The probe is disabled.
253	In any of the above cases, Kprobes won't start optimizing the probe.
254	Since these are temporary situations, Kprobes tries to start
255	optimizing it again if the situation is changed.
257	If the kprobe can be optimized, Kprobes enqueues the kprobe to an
258	optimizing list, and kicks the kprobe-optimizer workqueue to optimize
259	it.  If the to-be-optimized probepoint is hit before being optimized,
260	Kprobes returns control to the original instruction path by setting
261	the CPU's instruction pointer to the copied code in the detour buffer
262	-- thus at least avoiding the single-step.
264	Optimization
265	^^^^^^^^^^^^
267	The Kprobe-optimizer doesn't insert the jump instruction immediately;
268	rather, it calls synchronize_sched() for safety first, because it's
269	possible for a CPU to be interrupted in the middle of executing the
270	optimized region [3]_.  As you know, synchronize_sched() can ensure
271	that all interruptions that were active when synchronize_sched()
272	was called are done, but only if CONFIG_PREEMPT=n.  So, this version
273	of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
275	After that, the Kprobe-optimizer calls stop_machine() to replace
276	the optimized region with a jump instruction to the detour buffer,
277	using text_poke_smp().
279	Unoptimization
280	^^^^^^^^^^^^^^
282	When an optimized kprobe is unregistered, disabled, or blocked by
283	another kprobe, it will be unoptimized.  If this happens before
284	the optimization is complete, the kprobe is just dequeued from the
285	optimized list.  If the optimization has been done, the jump is
286	replaced with the original code (except for an int3 breakpoint in
287	the first byte) by using text_poke_smp().
289	.. [3] Please imagine that the 2nd instruction is interrupted and then
290	   the optimizer replaces the 2nd instruction with the jump *address*
291	   while the interrupt handler is running. When the interrupt
292	   returns to original address, there is no valid instruction,
293	   and it causes an unexpected result.
295	.. [4] This optimization-safety checking may be replaced with the
296	   stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
297	   kernel.
299	NOTE for geeks:
300	The jump optimization changes the kprobe's pre_handler behavior.
301	Without optimization, the pre_handler can change the kernel's execution
302	path by changing regs->ip and returning 1.  However, when the probe
303	is optimized, that modification is ignored.  Thus, if you want to
304	tweak the kernel's execution path, you need to suppress optimization,
305	using one of the following techniques:
307	- Specify an empty function for the kprobe's post_handler or break_handler.
309	or
311	- Execute 'sysctl -w debug.kprobes_optimization=n'
313	.. _kprobes_blacklist:
315	Blacklist
316	---------
318	Kprobes can probe most of the kernel except itself. This means
319	that there are some functions where kprobes cannot probe. Probing
320	(trapping) such functions can cause a recursive trap (e.g. double
321	fault) or the nested probe handler may never be called.
322	Kprobes manages such functions as a blacklist.
323	If you want to add a function into the blacklist, you just need
324	to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
325	to specify a blacklisted function.
326	Kprobes checks the given probe address against the blacklist and
327	rejects registering it, if the given address is in the blacklist.
329	.. _kprobes_archs_supported:
331	Architectures Supported
332	=======================
334	Kprobes, jprobes, and return probes are implemented on the following
335	architectures:
337	- i386 (Supports jump optimization)
338	- x86_64 (AMD-64, EM64T) (Supports jump optimization)
339	- ppc64
340	- ia64 (Does not support probes on instruction slot1.)
341	- sparc64 (Return probes not yet implemented.)
342	- arm
343	- ppc
344	- mips
345	- s390
347	Configuring Kprobes
348	===================
350	When configuring the kernel using make menuconfig/xconfig/oldconfig,
351	ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
352	for "Kprobes".
354	So that you can load and unload Kprobes-based instrumentation modules,
355	make sure "Loadable module support" (CONFIG_MODULES) and "Module
356	unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
358	Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
359	are set to "y", since kallsyms_lookup_name() is used by the in-kernel
360	kprobe address resolution code.
362	If you need to insert a probe in the middle of a function, you may find
363	it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
364	so you can use "objdump -d -l vmlinux" to see the source-to-object
365	code mapping.
367	API Reference
368	=============
370	The Kprobes API includes a "register" function and an "unregister"
371	function for each type of probe. The API also includes "register_*probes"
372	and "unregister_*probes" functions for (un)registering arrays of probes.
373	Here are terse, mini-man-page specifications for these functions and
374	the associated probe handlers that you'll write. See the files in the
375	samples/kprobes/ sub-directory for examples.
377	register_kprobe
378	---------------
380	::
382		#include <linux/kprobes.h>
383		int register_kprobe(struct kprobe *kp);
385	Sets a breakpoint at the address kp->addr.  When the breakpoint is
386	hit, Kprobes calls kp->pre_handler.  After the probed instruction
387	is single-stepped, Kprobe calls kp->post_handler.  If a fault
388	occurs during execution of kp->pre_handler or kp->post_handler,
389	or during single-stepping of the probed instruction, Kprobes calls
390	kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
391	is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
392	so, its handlers aren't hit until calling enable_kprobe(kp).
394	.. note::
396	   1. With the introduction of the "symbol_name" field to struct kprobe,
397	      the probepoint address resolution will now be taken care of by the kernel.
398	      The following will now work::
400		kp.symbol_name = "symbol_name";
402	      (64-bit powerpc intricacies such as function descriptors are handled
403	      transparently)
405	   2. Use the "offset" field of struct kprobe if the offset into the symbol
406	      to install a probepoint is known. This field is used to calculate the
407	      probepoint.
409	   3. Specify either the kprobe "symbol_name" OR the "addr". If both are
410	      specified, kprobe registration will fail with -EINVAL.
412	   4. With CISC architectures (such as i386 and x86_64), the kprobes code
413	      does not validate if the kprobe.addr is at an instruction boundary.
414	      Use "offset" with caution.
416	register_kprobe() returns 0 on success, or a negative errno otherwise.
418	User's pre-handler (kp->pre_handler)::
420		#include <linux/kprobes.h>
421		#include <linux/ptrace.h>
422		int pre_handler(struct kprobe *p, struct pt_regs *regs);
424	Called with p pointing to the kprobe associated with the breakpoint,
425	and regs pointing to the struct containing the registers saved when
426	the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
428	User's post-handler (kp->post_handler)::
430		#include <linux/kprobes.h>
431		#include <linux/ptrace.h>
432		void post_handler(struct kprobe *p, struct pt_regs *regs,
433				  unsigned long flags);
435	p and regs are as described for the pre_handler.  flags always seems
436	to be zero.
438	User's fault-handler (kp->fault_handler)::
440		#include <linux/kprobes.h>
441		#include <linux/ptrace.h>
442		int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
444	p and regs are as described for the pre_handler.  trapnr is the
445	architecture-specific trap number associated with the fault (e.g.,
446	on i386, 13 for a general protection fault or 14 for a page fault).
447	Returns 1 if it successfully handled the exception.
449	register_jprobe
450	---------------
452	::
454		#include <linux/kprobes.h>
455		int register_jprobe(struct jprobe *jp)
457	Sets a breakpoint at the address jp->kp.addr, which must be the address
458	of the first instruction of a function.  When the breakpoint is hit,
459	Kprobes runs the handler whose address is jp->entry.
461	The handler should have the same arg list and return type as the probed
462	function; and just before it returns, it must call jprobe_return().
463	(The handler never actually returns, since jprobe_return() returns
464	control to Kprobes.)  If the probed function is declared asmlinkage
465	or anything else that affects how args are passed, the handler's
466	declaration must match.
468	register_jprobe() returns 0 on success, or a negative errno otherwise.
470	register_kretprobe
471	------------------
473	::
475		#include <linux/kprobes.h>
476		int register_kretprobe(struct kretprobe *rp);
478	Establishes a return probe for the function whose address is
479	rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
480	You must set rp->maxactive appropriately before you call
481	register_kretprobe(); see "How Does a Return Probe Work?" for details.
483	register_kretprobe() returns 0 on success, or a negative errno
484	otherwise.
486	User's return-probe handler (rp->handler)::
488		#include <linux/kprobes.h>
489		#include <linux/ptrace.h>
490		int kretprobe_handler(struct kretprobe_instance *ri,
491				      struct pt_regs *regs);
493	regs is as described for kprobe.pre_handler.  ri points to the
494	kretprobe_instance object, of which the following fields may be
495	of interest:
497	- ret_addr: the return address
498	- rp: points to the corresponding kretprobe object
499	- task: points to the corresponding task struct
500	- data: points to per return-instance private data; see "Kretprobe
501		entry-handler" for details.
503	The regs_return_value(regs) macro provides a simple abstraction to
504	extract the return value from the appropriate register as defined by
505	the architecture's ABI.
507	The handler's return value is currently ignored.
509	unregister_*probe
510	------------------
512	::
514		#include <linux/kprobes.h>
515		void unregister_kprobe(struct kprobe *kp);
516		void unregister_jprobe(struct jprobe *jp);
517		void unregister_kretprobe(struct kretprobe *rp);
519	Removes the specified probe.  The unregister function can be called
520	at any time after the probe has been registered.
522	.. note::
524	   If the functions find an incorrect probe (ex. an unregistered probe),
525	   they clear the addr field of the probe.
527	register_*probes
528	----------------
530	::
532		#include <linux/kprobes.h>
533		int register_kprobes(struct kprobe **kps, int num);
534		int register_kretprobes(struct kretprobe **rps, int num);
535		int register_jprobes(struct jprobe **jps, int num);
537	Registers each of the num probes in the specified array.  If any
538	error occurs during registration, all probes in the array, up to
539	the bad probe, are safely unregistered before the register_*probes
540	function returns.
542	- kps/rps/jps: an array of pointers to ``*probe`` data structures
543	- num: the number of the array entries.
545	.. note::
547	   You have to allocate(or define) an array of pointers and set all
548	   of the array entries before using these functions.
550	unregister_*probes
551	------------------
553	::
555		#include <linux/kprobes.h>
556		void unregister_kprobes(struct kprobe **kps, int num);
557		void unregister_kretprobes(struct kretprobe **rps, int num);
558		void unregister_jprobes(struct jprobe **jps, int num);
560	Removes each of the num probes in the specified array at once.
562	.. note::
564	   If the functions find some incorrect probes (ex. unregistered
565	   probes) in the specified array, they clear the addr field of those
566	   incorrect probes. However, other probes in the array are
567	   unregistered correctly.
569	disable_*probe
570	--------------
572	::
574		#include <linux/kprobes.h>
575		int disable_kprobe(struct kprobe *kp);
576		int disable_kretprobe(struct kretprobe *rp);
577		int disable_jprobe(struct jprobe *jp);
579	Temporarily disables the specified ``*probe``. You can enable it again by using
580	enable_*probe(). You must specify the probe which has been registered.
582	enable_*probe
583	-------------
585	::
587		#include <linux/kprobes.h>
588		int enable_kprobe(struct kprobe *kp);
589		int enable_kretprobe(struct kretprobe *rp);
590		int enable_jprobe(struct jprobe *jp);
592	Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
593	the probe which has been registered.
595	Kprobes Features and Limitations
596	================================
598	Kprobes allows multiple probes at the same address.  Currently,
599	however, there cannot be multiple jprobes on the same function at
600	the same time.  Also, a probepoint for which there is a jprobe or
601	a post_handler cannot be optimized.  So if you install a jprobe,
602	or a kprobe with a post_handler, at an optimized probepoint, the
603	probepoint will be unoptimized automatically.
605	In general, you can install a probe anywhere in the kernel.
606	In particular, you can probe interrupt handlers.  Known exceptions
607	are discussed in this section.
609	The register_*probe functions will return -EINVAL if you attempt
610	to install a probe in the code that implements Kprobes (mostly
611	kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
612	as do_page_fault and notifier_call_chain).
614	If you install a probe in an inline-able function, Kprobes makes
615	no attempt to chase down all inline instances of the function and
616	install probes there.  gcc may inline a function without being asked,
617	so keep this in mind if you're not seeing the probe hits you expect.
619	A probe handler can modify the environment of the probed function
620	-- e.g., by modifying kernel data structures, or by modifying the
621	contents of the pt_regs struct (which are restored to the registers
622	upon return from the breakpoint).  So Kprobes can be used, for example,
623	to install a bug fix or to inject faults for testing.  Kprobes, of
624	course, has no way to distinguish the deliberately injected faults
625	from the accidental ones.  Don't drink and probe.
627	Kprobes makes no attempt to prevent probe handlers from stepping on
628	each other -- e.g., probing printk() and then calling printk() from a
629	probe handler.  If a probe handler hits a probe, that second probe's
630	handlers won't be run in that instance, and the kprobe.nmissed member
631	of the second probe will be incremented.
633	As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
634	the same handler) may run concurrently on different CPUs.
636	Kprobes does not use mutexes or allocate memory except during
637	registration and unregistration.
639	Probe handlers are run with preemption disabled.  Depending on the
640	architecture and optimization state, handlers may also run with
641	interrupts disabled (e.g., kretprobe handlers and optimized kprobe
642	handlers run without interrupt disabled on x86/x86-64).  In any case,
643	your handler should not yield the CPU (e.g., by attempting to acquire
644	a semaphore).
646	Since a return probe is implemented by replacing the return
647	address with the trampoline's address, stack backtraces and calls
648	to __builtin_return_address() will typically yield the trampoline's
649	address instead of the real return address for kretprobed functions.
650	(As far as we can tell, __builtin_return_address() is used only
651	for instrumentation and error reporting.)
653	If the number of times a function is called does not match the number
654	of times it returns, registering a return probe on that function may
655	produce undesirable results. In such a case, a line:
656	kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
657	gets printed. With this information, one will be able to correlate the
658	exact instance of the kretprobe that caused the problem. We have the
659	do_exit() case covered. do_execve() and do_fork() are not an issue.
660	We're unaware of other specific cases where this could be a problem.
662	If, upon entry to or exit from a function, the CPU is running on
663	a stack other than that of the current task, registering a return
664	probe on that function may produce undesirable results.  For this
665	reason, Kprobes doesn't support return probes (or kprobes or jprobes)
666	on the x86_64 version of __switch_to(); the registration functions
667	return -EINVAL.
669	On x86/x86-64, since the Jump Optimization of Kprobes modifies
670	instructions widely, there are some limitations to optimization. To
671	explain it, we introduce some terminology. Imagine a 3-instruction
672	sequence consisting of a two 2-byte instructions and one 3-byte
673	instruction.
675	::
677			IA
678			|
679		[-2][-1][0][1][2][3][4][5][6][7]
680			[ins1][ins2][  ins3 ]
681			[<-     DCR       ->]
682			[<- JTPR ->]
684		ins1: 1st Instruction
685		ins2: 2nd Instruction
686		ins3: 3rd Instruction
687		IA:  Insertion Address
688		JTPR: Jump Target Prohibition Region
689		DCR: Detoured Code Region
691	The instructions in DCR are copied to the out-of-line buffer
692	of the kprobe, because the bytes in DCR are replaced by
693	a 5-byte jump instruction. So there are several limitations.
695	a) The instructions in DCR must be relocatable.
696	b) The instructions in DCR must not include a call instruction.
697	c) JTPR must not be targeted by any jump or call instruction.
698	d) DCR must not straddle the border between functions.
700	Anyway, these limitations are checked by the in-kernel instruction
701	decoder, so you don't need to worry about that.
703	Probe Overhead
704	==============
706	On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
707	microseconds to process.  Specifically, a benchmark that hits the same
708	probepoint repeatedly, firing a simple handler each time, reports 1-2
709	million hits per second, depending on the architecture.  A jprobe or
710	return-probe hit typically takes 50-75% longer than a kprobe hit.
711	When you have a return probe set on a function, adding a kprobe at
712	the entry to that function adds essentially no overhead.
714	Here are sample overhead figures (in usec) for different architectures::
716	  k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
717	  on same function; jr = jprobe + return probe on same function::
719	  i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
720	  k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
722	  x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
723	  k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
725	  ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
726	  k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
728	Optimized Probe Overhead
729	------------------------
731	Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
732	process. Here are sample overhead figures (in usec) for x86 architectures::
734	  k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
735	  r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
737	  i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
738	  k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
740	  x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
741	  k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
743	TODO
744	====
746	a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
747	   programming interface for probe-based instrumentation.  Try it out.
748	b. Kernel return probes for sparc64.
749	c. Support for other architectures.
750	d. User-space probes.
751	e. Watchpoint probes (which fire on data references).
753	Kprobes Example
754	===============
756	See samples/kprobes/kprobe_example.c
758	Jprobes Example
759	===============
761	See samples/kprobes/jprobe_example.c
763	Kretprobes Example
764	==================
766	See samples/kprobes/kretprobe_example.c
768	For additional information on Kprobes, refer to the following URLs:
770	- http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
771	- http://www.redhat.com/magazine/005mar05/features/kprobes/
772	- http://www-users.cs.umn.edu/~boutcher/kprobes/
773	- http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
776	The kprobes debugfs interface
777	=============================
780	With recent kernels (> 2.6.20) the list of registered kprobes is visible
781	under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
783	/sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
785		c015d71a  k  vfs_read+0x0
786		c011a316  j  do_fork+0x0
787		c03dedc5  r  tcp_v4_rcv+0x0
789	The first column provides the kernel address where the probe is inserted.
790	The second column identifies the type of probe (k - kprobe, r - kretprobe
791	and j - jprobe), while the third column specifies the symbol+offset of
792	the probe. If the probed function belongs to a module, the module name
793	is also specified. Following columns show probe status. If the probe is on
794	a virtual address that is no longer valid (module init sections, module
795	virtual addresses that correspond to modules that've been unloaded),
796	such probes are marked with [GONE]. If the probe is temporarily disabled,
797	such probes are marked with [DISABLED]. If the probe is optimized, it is
798	marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
799	[FTRACE].
801	/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
803	Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
804	By default, all kprobes are enabled. By echoing "0" to this file, all
805	registered probes will be disarmed, till such time a "1" is echoed to this
806	file. Note that this knob just disarms and arms all kprobes and doesn't
807	change each probe's disabling state. This means that disabled kprobes (marked
808	[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
811	The kprobes sysctl interface
812	============================
814	/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
816	When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
817	a knob to globally and forcibly turn jump optimization (see section
818	:ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
819	is allowed (ON). If you echo "0" to this file or set
820	"debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
821	unoptimized, and any new probes registered after that will not be optimized.
823	Note that this knob *changes* the optimized state. This means that optimized
824	probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
825	removed). If the knob is turned on, they will be optimized again.
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