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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

1	Title	: Kernel Probes (Kprobes)
2	Authors	: Jim Keniston <jkenisto@us.ibm.com>
3		: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
4		: Masami Hiramatsu <mhiramat@redhat.com>
5	
6	CONTENTS
7	
8	1. Concepts: Kprobes, Jprobes, Return Probes
9	2. Architectures Supported
10	3. Configuring Kprobes
11	4. API Reference
12	5. Kprobes Features and Limitations
13	6. Probe Overhead
14	7. TODO
15	8. Kprobes Example
16	9. Jprobes Example
17	10. Kretprobes Example
18	Appendix A: The kprobes debugfs interface
19	Appendix B: The kprobes sysctl interface
20	
21	1. Concepts: Kprobes, Jprobes, Return Probes
22	
23	Kprobes enables you to dynamically break into any kernel routine and
24	collect debugging and performance information non-disruptively. You
25	can trap at almost any kernel code address(*), specifying a handler
26	routine to be invoked when the breakpoint is hit.
27	(*: some parts of the kernel code can not be trapped, see 1.5 Blacklist)
28	
29	There are currently three types of probes: kprobes, jprobes, and
30	kretprobes (also called return probes).  A kprobe can be inserted
31	on virtually any instruction in the kernel.  A jprobe is inserted at
32	the entry to a kernel function, and provides convenient access to the
33	function's arguments.  A return probe fires when a specified function
34	returns.
35	
36	In the typical case, Kprobes-based instrumentation is packaged as
37	a kernel module.  The module's init function installs ("registers")
38	one or more probes, and the exit function unregisters them.  A
39	registration function such as register_kprobe() specifies where
40	the probe is to be inserted and what handler is to be called when
41	the probe is hit.
42	
43	There are also register_/unregister_*probes() functions for batch
44	registration/unregistration of a group of *probes. These functions
45	can speed up unregistration process when you have to unregister
46	a lot of probes at once.
47	
48	The next four subsections explain how the different types of
49	probes work and how jump optimization works.  They explain certain
50	things that you'll need to know in order to make the best use of
51	Kprobes -- e.g., the difference between a pre_handler and
52	a post_handler, and how to use the maxactive and nmissed fields of
53	a kretprobe.  But if you're in a hurry to start using Kprobes, you
54	can skip ahead to section 2.
55	
56	1.1 How Does a Kprobe Work?
57	
58	When a kprobe is registered, Kprobes makes a copy of the probed
59	instruction and replaces the first byte(s) of the probed instruction
60	with a breakpoint instruction (e.g., int3 on i386 and x86_64).
61	
62	When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
63	registers are saved, and control passes to Kprobes via the
64	notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
65	associated with the kprobe, passing the handler the addresses of the
66	kprobe struct and the saved registers.
67	
68	Next, Kprobes single-steps its copy of the probed instruction.
69	(It would be simpler to single-step the actual instruction in place,
70	but then Kprobes would have to temporarily remove the breakpoint
71	instruction.  This would open a small time window when another CPU
72	could sail right past the probepoint.)
73	
74	After the instruction is single-stepped, Kprobes executes the
75	"post_handler," if any, that is associated with the kprobe.
76	Execution then continues with the instruction following the probepoint.
77	
78	1.2 How Does a Jprobe Work?
79	
80	A jprobe is implemented using a kprobe that is placed on a function's
81	entry point.  It employs a simple mirroring principle to allow
82	seamless access to the probed function's arguments.  The jprobe
83	handler routine should have the same signature (arg list and return
84	type) as the function being probed, and must always end by calling
85	the Kprobes function jprobe_return().
86	
87	Here's how it works.  When the probe is hit, Kprobes makes a copy of
88	the saved registers and a generous portion of the stack (see below).
89	Kprobes then points the saved instruction pointer at the jprobe's
90	handler routine, and returns from the trap.  As a result, control
91	passes to the handler, which is presented with the same register and
92	stack contents as the probed function.  When it is done, the handler
93	calls jprobe_return(), which traps again to restore the original stack
94	contents and processor state and switch to the probed function.
95	
96	By convention, the callee owns its arguments, so gcc may produce code
97	that unexpectedly modifies that portion of the stack.  This is why
98	Kprobes saves a copy of the stack and restores it after the jprobe
99	handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
100	64 bytes on i386.
101	
102	Note that the probed function's args may be passed on the stack
103	or in registers.  The jprobe will work in either case, so long as the
104	handler's prototype matches that of the probed function.
105	
106	Note that in some architectures (e.g.: arm64 and sparc64) the stack
107	copy is not done, as the actual location of stacked parameters may be
108	outside of a reasonable MAX_STACK_SIZE value and because that location
109	cannot be determined by the jprobes code. In this case the jprobes
110	user must be careful to make certain the calling signature of the
111	function does not cause parameters to be passed on the stack (e.g.:
112	more than eight function arguments, an argument of more than sixteen
113	bytes, or more than 64 bytes of argument data, depending on
114	architecture).
115	
116	1.3 Return Probes
117	
118	1.3.1 How Does a Return Probe Work?
119	
120	When you call register_kretprobe(), Kprobes establishes a kprobe at
121	the entry to the function.  When the probed function is called and this
122	probe is hit, Kprobes saves a copy of the return address, and replaces
123	the return address with the address of a "trampoline."  The trampoline
124	is an arbitrary piece of code -- typically just a nop instruction.
125	At boot time, Kprobes registers a kprobe at the trampoline.
126	
127	When the probed function executes its return instruction, control
128	passes to the trampoline and that probe is hit.  Kprobes' trampoline
129	handler calls the user-specified return handler associated with the
130	kretprobe, then sets the saved instruction pointer to the saved return
131	address, and that's where execution resumes upon return from the trap.
132	
133	While the probed function is executing, its return address is
134	stored in an object of type kretprobe_instance.  Before calling
135	register_kretprobe(), the user sets the maxactive field of the
136	kretprobe struct to specify how many instances of the specified
137	function can be probed simultaneously.  register_kretprobe()
138	pre-allocates the indicated number of kretprobe_instance objects.
139	
140	For example, if the function is non-recursive and is called with a
141	spinlock held, maxactive = 1 should be enough.  If the function is
142	non-recursive and can never relinquish the CPU (e.g., via a semaphore
143	or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
144	set to a default value.  If CONFIG_PREEMPT is enabled, the default
145	is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
146	
147	It's not a disaster if you set maxactive too low; you'll just miss
148	some probes.  In the kretprobe struct, the nmissed field is set to
149	zero when the return probe is registered, and is incremented every
150	time the probed function is entered but there is no kretprobe_instance
151	object available for establishing the return probe.
152	
153	1.3.2 Kretprobe entry-handler
154	
155	Kretprobes also provides an optional user-specified handler which runs
156	on function entry. This handler is specified by setting the entry_handler
157	field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
158	function entry is hit, the user-defined entry_handler, if any, is invoked.
159	If the entry_handler returns 0 (success) then a corresponding return handler
160	is guaranteed to be called upon function return. If the entry_handler
161	returns a non-zero error then Kprobes leaves the return address as is, and
162	the kretprobe has no further effect for that particular function instance.
163	
164	Multiple entry and return handler invocations are matched using the unique
165	kretprobe_instance object associated with them. Additionally, a user
166	may also specify per return-instance private data to be part of each
167	kretprobe_instance object. This is especially useful when sharing private
168	data between corresponding user entry and return handlers. The size of each
169	private data object can be specified at kretprobe registration time by
170	setting the data_size field of the kretprobe struct. This data can be
171	accessed through the data field of each kretprobe_instance object.
172	
173	In case probed function is entered but there is no kretprobe_instance
174	object available, then in addition to incrementing the nmissed count,
175	the user entry_handler invocation is also skipped.
176	
177	1.4 How Does Jump Optimization Work?
178	
179	If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
180	is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
181	the "debug.kprobes_optimization" kernel parameter is set to 1 (see
182	sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
183	instruction instead of a breakpoint instruction at each probepoint.
184	
185	1.4.1 Init a Kprobe
186	
187	When a probe is registered, before attempting this optimization,
188	Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
189	address. So, even if it's not possible to optimize this particular
190	probepoint, there'll be a probe there.
191	
192	1.4.2 Safety Check
193	
194	Before optimizing a probe, Kprobes performs the following safety checks:
195	
196	- Kprobes verifies that the region that will be replaced by the jump
197	instruction (the "optimized region") lies entirely within one function.
198	(A jump instruction is multiple bytes, and so may overlay multiple
199	instructions.)
200	
201	- Kprobes analyzes the entire function and verifies that there is no
202	jump into the optimized region.  Specifically:
203	  - the function contains no indirect jump;
204	  - the function contains no instruction that causes an exception (since
205	  the fixup code triggered by the exception could jump back into the
206	  optimized region -- Kprobes checks the exception tables to verify this);
207	  and
208	  - there is no near jump to the optimized region (other than to the first
209	  byte).
210	
211	- For each instruction in the optimized region, Kprobes verifies that
212	the instruction can be executed out of line.
213	
214	1.4.3 Preparing Detour Buffer
215	
216	Next, Kprobes prepares a "detour" buffer, which contains the following
217	instruction sequence:
218	- code to push the CPU's registers (emulating a breakpoint trap)
219	- a call to the trampoline code which calls user's probe handlers.
220	- code to restore registers
221	- the instructions from the optimized region
222	- a jump back to the original execution path.
223	
224	1.4.4 Pre-optimization
225	
226	After preparing the detour buffer, Kprobes verifies that none of the
227	following situations exist:
228	- The probe has either a break_handler (i.e., it's a jprobe) or a
229	post_handler.
230	- Other instructions in the optimized region are probed.
231	- The probe is disabled.
232	In any of the above cases, Kprobes won't start optimizing the probe.
233	Since these are temporary situations, Kprobes tries to start
234	optimizing it again if the situation is changed.
235	
236	If the kprobe can be optimized, Kprobes enqueues the kprobe to an
237	optimizing list, and kicks the kprobe-optimizer workqueue to optimize
238	it.  If the to-be-optimized probepoint is hit before being optimized,
239	Kprobes returns control to the original instruction path by setting
240	the CPU's instruction pointer to the copied code in the detour buffer
241	-- thus at least avoiding the single-step.
242	
243	1.4.5 Optimization
244	
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(*).  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.(**)
252	
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().
256	
257	1.4.6 Unoptimization
258	
259	When an optimized kprobe is unregistered, disabled, or blocked by
260	another kprobe, it will be unoptimized.  If this happens before
261	the optimization is complete, the kprobe is just dequeued from the
262	optimized list.  If the optimization has been done, the jump is
263	replaced with the original code (except for an int3 breakpoint in
264	the first byte) by using text_poke_smp().
265	
266	(*)Please imagine that the 2nd instruction is interrupted and then
267	the optimizer replaces the 2nd instruction with the jump *address*
268	while the interrupt handler is running. When the interrupt
269	returns to original address, there is no valid instruction,
270	and it causes an unexpected result.
271	
272	(**)This optimization-safety checking may be replaced with the
273	stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
274	kernel.
275	
276	NOTE for geeks:
277	The jump optimization changes the kprobe's pre_handler behavior.
278	Without optimization, the pre_handler can change the kernel's execution
279	path by changing regs->ip and returning 1.  However, when the probe
280	is optimized, that modification is ignored.  Thus, if you want to
281	tweak the kernel's execution path, you need to suppress optimization,
282	using one of the following techniques:
283	- Specify an empty function for the kprobe's post_handler or break_handler.
284	 or
285	- Execute 'sysctl -w debug.kprobes_optimization=n'
286	
287	1.5 Blacklist
288	
289	Kprobes can probe most of the kernel except itself. This means
290	that there are some functions where kprobes cannot probe. Probing
291	(trapping) such functions can cause a recursive trap (e.g. double
292	fault) or the nested probe handler may never be called.
293	Kprobes manages such functions as a blacklist.
294	If you want to add a function into the blacklist, you just need
295	to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
296	to specify a blacklisted function.
297	Kprobes checks the given probe address against the blacklist and
298	rejects registering it, if the given address is in the blacklist.
299	
300	2. Architectures Supported
301	
302	Kprobes, jprobes, and return probes are implemented on the following
303	architectures:
304	
305	- i386 (Supports jump optimization)
306	- x86_64 (AMD-64, EM64T) (Supports jump optimization)
307	- ppc64
308	- ia64 (Does not support probes on instruction slot1.)
309	- sparc64 (Return probes not yet implemented.)
310	- arm
311	- ppc
312	- mips
313	- s390
314	
315	3. Configuring Kprobes
316	
317	When configuring the kernel using make menuconfig/xconfig/oldconfig,
318	ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
319	for "Kprobes".
320	
321	So that you can load and unload Kprobes-based instrumentation modules,
322	make sure "Loadable module support" (CONFIG_MODULES) and "Module
323	unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
324	
325	Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
326	are set to "y", since kallsyms_lookup_name() is used by the in-kernel
327	kprobe address resolution code.
328	
329	If you need to insert a probe in the middle of a function, you may find
330	it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
331	so you can use "objdump -d -l vmlinux" to see the source-to-object
332	code mapping.
333	
334	4. API Reference
335	
336	The Kprobes API includes a "register" function and an "unregister"
337	function for each type of probe. The API also includes "register_*probes"
338	and "unregister_*probes" functions for (un)registering arrays of probes.
339	Here are terse, mini-man-page specifications for these functions and
340	the associated probe handlers that you'll write. See the files in the
341	samples/kprobes/ sub-directory for examples.
342	
343	4.1 register_kprobe
344	
345	#include <linux/kprobes.h>
346	int register_kprobe(struct kprobe *kp);
347	
348	Sets a breakpoint at the address kp->addr.  When the breakpoint is
349	hit, Kprobes calls kp->pre_handler.  After the probed instruction
350	is single-stepped, Kprobe calls kp->post_handler.  If a fault
351	occurs during execution of kp->pre_handler or kp->post_handler,
352	or during single-stepping of the probed instruction, Kprobes calls
353	kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
354	is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
355	so, its handlers aren't hit until calling enable_kprobe(kp).
356	
357	NOTE:
358	1. With the introduction of the "symbol_name" field to struct kprobe,
359	the probepoint address resolution will now be taken care of by the kernel.
360	The following will now work:
361	
362		kp.symbol_name = "symbol_name";
363	
364	(64-bit powerpc intricacies such as function descriptors are handled
365	transparently)
366	
367	2. Use the "offset" field of struct kprobe if the offset into the symbol
368	to install a probepoint is known. This field is used to calculate the
369	probepoint.
370	
371	3. Specify either the kprobe "symbol_name" OR the "addr". If both are
372	specified, kprobe registration will fail with -EINVAL.
373	
374	4. With CISC architectures (such as i386 and x86_64), the kprobes code
375	does not validate if the kprobe.addr is at an instruction boundary.
376	Use "offset" with caution.
377	
378	register_kprobe() returns 0 on success, or a negative errno otherwise.
379	
380	User's pre-handler (kp->pre_handler):
381	#include <linux/kprobes.h>
382	#include <linux/ptrace.h>
383	int pre_handler(struct kprobe *p, struct pt_regs *regs);
384	
385	Called with p pointing to the kprobe associated with the breakpoint,
386	and regs pointing to the struct containing the registers saved when
387	the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
388	
389	User's post-handler (kp->post_handler):
390	#include <linux/kprobes.h>
391	#include <linux/ptrace.h>
392	void post_handler(struct kprobe *p, struct pt_regs *regs,
393		unsigned long flags);
394	
395	p and regs are as described for the pre_handler.  flags always seems
396	to be zero.
397	
398	User's fault-handler (kp->fault_handler):
399	#include <linux/kprobes.h>
400	#include <linux/ptrace.h>
401	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
402	
403	p and regs are as described for the pre_handler.  trapnr is the
404	architecture-specific trap number associated with the fault (e.g.,
405	on i386, 13 for a general protection fault or 14 for a page fault).
406	Returns 1 if it successfully handled the exception.
407	
408	4.2 register_jprobe
409	
410	#include <linux/kprobes.h>
411	int register_jprobe(struct jprobe *jp)
412	
413	Sets a breakpoint at the address jp->kp.addr, which must be the address
414	of the first instruction of a function.  When the breakpoint is hit,
415	Kprobes runs the handler whose address is jp->entry.
416	
417	The handler should have the same arg list and return type as the probed
418	function; and just before it returns, it must call jprobe_return().
419	(The handler never actually returns, since jprobe_return() returns
420	control to Kprobes.)  If the probed function is declared asmlinkage
421	or anything else that affects how args are passed, the handler's
422	declaration must match.
423	
424	register_jprobe() returns 0 on success, or a negative errno otherwise.
425	
426	4.3 register_kretprobe
427	
428	#include <linux/kprobes.h>
429	int register_kretprobe(struct kretprobe *rp);
430	
431	Establishes a return probe for the function whose address is
432	rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
433	You must set rp->maxactive appropriately before you call
434	register_kretprobe(); see "How Does a Return Probe Work?" for details.
435	
436	register_kretprobe() returns 0 on success, or a negative errno
437	otherwise.
438	
439	User's return-probe handler (rp->handler):
440	#include <linux/kprobes.h>
441	#include <linux/ptrace.h>
442	int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
443	
444	regs is as described for kprobe.pre_handler.  ri points to the
445	kretprobe_instance object, of which the following fields may be
446	of interest:
447	- ret_addr: the return address
448	- rp: points to the corresponding kretprobe object
449	- task: points to the corresponding task struct
450	- data: points to per return-instance private data; see "Kretprobe
451		entry-handler" for details.
452	
453	The regs_return_value(regs) macro provides a simple abstraction to
454	extract the return value from the appropriate register as defined by
455	the architecture's ABI.
456	
457	The handler's return value is currently ignored.
458	
459	4.4 unregister_*probe
460	
461	#include <linux/kprobes.h>
462	void unregister_kprobe(struct kprobe *kp);
463	void unregister_jprobe(struct jprobe *jp);
464	void unregister_kretprobe(struct kretprobe *rp);
465	
466	Removes the specified probe.  The unregister function can be called
467	at any time after the probe has been registered.
468	
469	NOTE:
470	If the functions find an incorrect probe (ex. an unregistered probe),
471	they clear the addr field of the probe.
472	
473	4.5 register_*probes
474	
475	#include <linux/kprobes.h>
476	int register_kprobes(struct kprobe **kps, int num);
477	int register_kretprobes(struct kretprobe **rps, int num);
478	int register_jprobes(struct jprobe **jps, int num);
479	
480	Registers each of the num probes in the specified array.  If any
481	error occurs during registration, all probes in the array, up to
482	the bad probe, are safely unregistered before the register_*probes
483	function returns.
484	- kps/rps/jps: an array of pointers to *probe data structures
485	- num: the number of the array entries.
486	
487	NOTE:
488	You have to allocate(or define) an array of pointers and set all
489	of the array entries before using these functions.
490	
491	4.6 unregister_*probes
492	
493	#include <linux/kprobes.h>
494	void unregister_kprobes(struct kprobe **kps, int num);
495	void unregister_kretprobes(struct kretprobe **rps, int num);
496	void unregister_jprobes(struct jprobe **jps, int num);
497	
498	Removes each of the num probes in the specified array at once.
499	
500	NOTE:
501	If the functions find some incorrect probes (ex. unregistered
502	probes) in the specified array, they clear the addr field of those
503	incorrect probes. However, other probes in the array are
504	unregistered correctly.
505	
506	4.7 disable_*probe
507	
508	#include <linux/kprobes.h>
509	int disable_kprobe(struct kprobe *kp);
510	int disable_kretprobe(struct kretprobe *rp);
511	int disable_jprobe(struct jprobe *jp);
512	
513	Temporarily disables the specified *probe. You can enable it again by using
514	enable_*probe(). You must specify the probe which has been registered.
515	
516	4.8 enable_*probe
517	
518	#include <linux/kprobes.h>
519	int enable_kprobe(struct kprobe *kp);
520	int enable_kretprobe(struct kretprobe *rp);
521	int enable_jprobe(struct jprobe *jp);
522	
523	Enables *probe which has been disabled by disable_*probe(). You must specify
524	the probe which has been registered.
525	
526	5. Kprobes Features and Limitations
527	
528	Kprobes allows multiple probes at the same address.  Currently,
529	however, there cannot be multiple jprobes on the same function at
530	the same time.  Also, a probepoint for which there is a jprobe or
531	a post_handler cannot be optimized.  So if you install a jprobe,
532	or a kprobe with a post_handler, at an optimized probepoint, the
533	probepoint will be unoptimized automatically.
534	
535	In general, you can install a probe anywhere in the kernel.
536	In particular, you can probe interrupt handlers.  Known exceptions
537	are discussed in this section.
538	
539	The register_*probe functions will return -EINVAL if you attempt
540	to install a probe in the code that implements Kprobes (mostly
541	kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
542	as do_page_fault and notifier_call_chain).
543	
544	If you install a probe in an inline-able function, Kprobes makes
545	no attempt to chase down all inline instances of the function and
546	install probes there.  gcc may inline a function without being asked,
547	so keep this in mind if you're not seeing the probe hits you expect.
548	
549	A probe handler can modify the environment of the probed function
550	-- e.g., by modifying kernel data structures, or by modifying the
551	contents of the pt_regs struct (which are restored to the registers
552	upon return from the breakpoint).  So Kprobes can be used, for example,
553	to install a bug fix or to inject faults for testing.  Kprobes, of
554	course, has no way to distinguish the deliberately injected faults
555	from the accidental ones.  Don't drink and probe.
556	
557	Kprobes makes no attempt to prevent probe handlers from stepping on
558	each other -- e.g., probing printk() and then calling printk() from a
559	probe handler.  If a probe handler hits a probe, that second probe's
560	handlers won't be run in that instance, and the kprobe.nmissed member
561	of the second probe will be incremented.
562	
563	As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
564	the same handler) may run concurrently on different CPUs.
565	
566	Kprobes does not use mutexes or allocate memory except during
567	registration and unregistration.
568	
569	Probe handlers are run with preemption disabled.  Depending on the
570	architecture and optimization state, handlers may also run with
571	interrupts disabled (e.g., kretprobe handlers and optimized kprobe
572	handlers run without interrupt disabled on x86/x86-64).  In any case,
573	your handler should not yield the CPU (e.g., by attempting to acquire
574	a semaphore).
575	
576	Since a return probe is implemented by replacing the return
577	address with the trampoline's address, stack backtraces and calls
578	to __builtin_return_address() will typically yield the trampoline's
579	address instead of the real return address for kretprobed functions.
580	(As far as we can tell, __builtin_return_address() is used only
581	for instrumentation and error reporting.)
582	
583	If the number of times a function is called does not match the number
584	of times it returns, registering a return probe on that function may
585	produce undesirable results. In such a case, a line:
586	kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
587	gets printed. With this information, one will be able to correlate the
588	exact instance of the kretprobe that caused the problem. We have the
589	do_exit() case covered. do_execve() and do_fork() are not an issue.
590	We're unaware of other specific cases where this could be a problem.
591	
592	If, upon entry to or exit from a function, the CPU is running on
593	a stack other than that of the current task, registering a return
594	probe on that function may produce undesirable results.  For this
595	reason, Kprobes doesn't support return probes (or kprobes or jprobes)
596	on the x86_64 version of __switch_to(); the registration functions
597	return -EINVAL.
598	
599	On x86/x86-64, since the Jump Optimization of Kprobes modifies
600	instructions widely, there are some limitations to optimization. To
601	explain it, we introduce some terminology. Imagine a 3-instruction
602	sequence consisting of a two 2-byte instructions and one 3-byte
603	instruction.
604	
605	        IA
606	         |
607	[-2][-1][0][1][2][3][4][5][6][7]
608	        [ins1][ins2][  ins3 ]
609		[<-     DCR       ->]
610		   [<- JTPR ->]
611	
612	ins1: 1st Instruction
613	ins2: 2nd Instruction
614	ins3: 3rd Instruction
615	IA:  Insertion Address
616	JTPR: Jump Target Prohibition Region
617	DCR: Detoured Code Region
618	
619	The instructions in DCR are copied to the out-of-line buffer
620	of the kprobe, because the bytes in DCR are replaced by
621	a 5-byte jump instruction. So there are several limitations.
622	
623	a) The instructions in DCR must be relocatable.
624	b) The instructions in DCR must not include a call instruction.
625	c) JTPR must not be targeted by any jump or call instruction.
626	d) DCR must not straddle the border between functions.
627	
628	Anyway, these limitations are checked by the in-kernel instruction
629	decoder, so you don't need to worry about that.
630	
631	6. Probe Overhead
632	
633	On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
634	microseconds to process.  Specifically, a benchmark that hits the same
635	probepoint repeatedly, firing a simple handler each time, reports 1-2
636	million hits per second, depending on the architecture.  A jprobe or
637	return-probe hit typically takes 50-75% longer than a kprobe hit.
638	When you have a return probe set on a function, adding a kprobe at
639	the entry to that function adds essentially no overhead.
640	
641	Here are sample overhead figures (in usec) for different architectures.
642	k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
643	on same function; jr = jprobe + return probe on same function
644	
645	i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
646	k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
647	
648	x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
649	k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
650	
651	ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
652	k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
653	
654	6.1 Optimized Probe Overhead
655	
656	Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
657	process. Here are sample overhead figures (in usec) for x86 architectures.
658	k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
659	r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
660	
661	i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
662	k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
663	
664	x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
665	k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
666	
667	7. TODO
668	
669	a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
670	programming interface for probe-based instrumentation.  Try it out.
671	b. Kernel return probes for sparc64.
672	c. Support for other architectures.
673	d. User-space probes.
674	e. Watchpoint probes (which fire on data references).
675	
676	8. Kprobes Example
677	
678	See samples/kprobes/kprobe_example.c
679	
680	9. Jprobes Example
681	
682	See samples/kprobes/jprobe_example.c
683	
684	10. Kretprobes Example
685	
686	See samples/kprobes/kretprobe_example.c
687	
688	For additional information on Kprobes, refer to the following URLs:
689	http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
690	http://www.redhat.com/magazine/005mar05/features/kprobes/
691	http://www-users.cs.umn.edu/~boutcher/kprobes/
692	http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
693	
694	
695	Appendix A: The kprobes debugfs interface
696	
697	With recent kernels (> 2.6.20) the list of registered kprobes is visible
698	under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
699	
700	/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
701	
702	c015d71a  k  vfs_read+0x0
703	c011a316  j  do_fork+0x0
704	c03dedc5  r  tcp_v4_rcv+0x0
705	
706	The first column provides the kernel address where the probe is inserted.
707	The second column identifies the type of probe (k - kprobe, r - kretprobe
708	and j - jprobe), while the third column specifies the symbol+offset of
709	the probe. If the probed function belongs to a module, the module name
710	is also specified. Following columns show probe status. If the probe is on
711	a virtual address that is no longer valid (module init sections, module
712	virtual addresses that correspond to modules that've been unloaded),
713	such probes are marked with [GONE]. If the probe is temporarily disabled,
714	such probes are marked with [DISABLED]. If the probe is optimized, it is
715	marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
716	[FTRACE].
717	
718	/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
719	
720	Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
721	By default, all kprobes are enabled. By echoing "0" to this file, all
722	registered probes will be disarmed, till such time a "1" is echoed to this
723	file. Note that this knob just disarms and arms all kprobes and doesn't
724	change each probe's disabling state. This means that disabled kprobes (marked
725	[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
726	
727	
728	Appendix B: The kprobes sysctl interface
729	
730	/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
731	
732	When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
733	a knob to globally and forcibly turn jump optimization (see section
734	1.4) ON or OFF. By default, jump optimization is allowed (ON).
735	If you echo "0" to this file or set "debug.kprobes_optimization" to
736	0 via sysctl, all optimized probes will be unoptimized, and any new
737	probes registered after that will not be optimized.  Note that this
738	knob *changes* the optimized state. This means that optimized probes
739	(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
740	removed). If the knob is turned on, they will be optimized again.
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