About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / kprobes.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:03 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	
28	There are currently three types of probes: kprobes, jprobes, and
29	kretprobes (also called return probes).  A kprobe can be inserted
30	on virtually any instruction in the kernel.  A jprobe is inserted at
31	the entry to a kernel function, and provides convenient access to the
32	function's arguments.  A return probe fires when a specified function
33	returns.
34	
35	In the typical case, Kprobes-based instrumentation is packaged as
36	a kernel module.  The module's init function installs ("registers")
37	one or more probes, and the exit function unregisters them.  A
38	registration function such as register_kprobe() specifies where
39	the probe is to be inserted and what handler is to be called when
40	the probe is hit.
41	
42	There are also register_/unregister_*probes() functions for batch
43	registration/unregistration of a group of *probes. These functions
44	can speed up unregistration process when you have to unregister
45	a lot of probes at once.
46	
47	The next four subsections explain how the different types of
48	probes work and how jump optimization works.  They explain certain
49	things that you'll need to know in order to make the best use of
50	Kprobes -- e.g., the difference between a pre_handler and
51	a post_handler, and how to use the maxactive and nmissed fields of
52	a kretprobe.  But if you're in a hurry to start using Kprobes, you
53	can skip ahead to section 2.
54	
55	1.1 How Does a Kprobe Work?
56	
57	When a kprobe is registered, Kprobes makes a copy of the probed
58	instruction and replaces the first byte(s) of the probed instruction
59	with a breakpoint instruction (e.g., int3 on i386 and x86_64).
60	
61	When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
62	registers are saved, and control passes to Kprobes via the
63	notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
64	associated with the kprobe, passing the handler the addresses of the
65	kprobe struct and the saved registers.
66	
67	Next, Kprobes single-steps its copy of the probed instruction.
68	(It would be simpler to single-step the actual instruction in place,
69	but then Kprobes would have to temporarily remove the breakpoint
70	instruction.  This would open a small time window when another CPU
71	could sail right past the probepoint.)
72	
73	After the instruction is single-stepped, Kprobes executes the
74	"post_handler," if any, that is associated with the kprobe.
75	Execution then continues with the instruction following the probepoint.
76	
77	1.2 How Does a Jprobe Work?
78	
79	A jprobe is implemented using a kprobe that is placed on a function's
80	entry point.  It employs a simple mirroring principle to allow
81	seamless access to the probed function's arguments.  The jprobe
82	handler routine should have the same signature (arg list and return
83	type) as the function being probed, and must always end by calling
84	the Kprobes function jprobe_return().
85	
86	Here's how it works.  When the probe is hit, Kprobes makes a copy of
87	the saved registers and a generous portion of the stack (see below).
88	Kprobes then points the saved instruction pointer at the jprobe's
89	handler routine, and returns from the trap.  As a result, control
90	passes to the handler, which is presented with the same register and
91	stack contents as the probed function.  When it is done, the handler
92	calls jprobe_return(), which traps again to restore the original stack
93	contents and processor state and switch to the probed function.
94	
95	By convention, the callee owns its arguments, so gcc may produce code
96	that unexpectedly modifies that portion of the stack.  This is why
97	Kprobes saves a copy of the stack and restores it after the jprobe
98	handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
99	64 bytes on i386.
100	
101	Note that the probed function's args may be passed on the stack
102	or in registers.  The jprobe will work in either case, so long as the
103	handler's prototype matches that of the probed function.
104	
105	1.3 Return Probes
106	
107	1.3.1 How Does a Return Probe Work?
108	
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.
115	
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.
121	
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.
128	
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.
135	
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.
141	
142	1.3.2 Kretprobe entry-handler
143	
144	Kretprobes also provides an optional user-specified handler which runs
145	on function entry. This handler is specified by setting the entry_handler
146	field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
147	function entry is hit, the user-defined entry_handler, if any, is invoked.
148	If the entry_handler returns 0 (success) then a corresponding return handler
149	is guaranteed to be called upon function return. If the entry_handler
150	returns a non-zero error then Kprobes leaves the return address as is, and
151	the kretprobe has no further effect for that particular function instance.
152	
153	Multiple entry and return handler invocations are matched using the unique
154	kretprobe_instance object associated with them. Additionally, a user
155	may also specify per return-instance private data to be part of each
156	kretprobe_instance object. This is especially useful when sharing private
157	data between corresponding user entry and return handlers. The size of each
158	private data object can be specified at kretprobe registration time by
159	setting the data_size field of the kretprobe struct. This data can be
160	accessed through the data field of each kretprobe_instance object.
161	
162	In case probed function is entered but there is no kretprobe_instance
163	object available, then in addition to incrementing the nmissed count,
164	the user entry_handler invocation is also skipped.
165	
166	1.4 How Does Jump Optimization Work?
167	
168	If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
169	is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
170	the "debug.kprobes_optimization" kernel parameter is set to 1 (see
171	sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
172	instruction instead of a breakpoint instruction at each probepoint.
173	
174	1.4.1 Init a Kprobe
175	
176	When a probe is registered, before attempting this optimization,
177	Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
178	address. So, even if it's not possible to optimize this particular
179	probepoint, there'll be a probe there.
180	
181	1.4.2 Safety Check
182	
183	Before optimizing a probe, Kprobes performs the following safety checks:
184	
185	- Kprobes verifies that the region that will be replaced by the jump
186	instruction (the "optimized region") lies entirely within one function.
187	(A jump instruction is multiple bytes, and so may overlay multiple
188	instructions.)
189	
190	- Kprobes analyzes the entire function and verifies that there is no
191	jump into the optimized region.  Specifically:
192	  - the function contains no indirect jump;
193	  - the function contains no instruction that causes an exception (since
194	  the fixup code triggered by the exception could jump back into the
195	  optimized region -- Kprobes checks the exception tables to verify this);
196	  and
197	  - there is no near jump to the optimized region (other than to the first
198	  byte).
199	
200	- For each instruction in the optimized region, Kprobes verifies that
201	the instruction can be executed out of line.
202	
203	1.4.3 Preparing Detour Buffer
204	
205	Next, Kprobes prepares a "detour" buffer, which contains the following
206	instruction sequence:
207	- code to push the CPU's registers (emulating a breakpoint trap)
208	- a call to the trampoline code which calls user's probe handlers.
209	- code to restore registers
210	- the instructions from the optimized region
211	- a jump back to the original execution path.
212	
213	1.4.4 Pre-optimization
214	
215	After preparing the detour buffer, Kprobes verifies that none of the
216	following situations exist:
217	- The probe has either a break_handler (i.e., it's a jprobe) or a
218	post_handler.
219	- Other instructions in the optimized region are probed.
220	- The probe is disabled.
221	In any of the above cases, Kprobes won't start optimizing the probe.
222	Since these are temporary situations, Kprobes tries to start
223	optimizing it again if the situation is changed.
224	
225	If the kprobe can be optimized, Kprobes enqueues the kprobe to an
226	optimizing list, and kicks the kprobe-optimizer workqueue to optimize
227	it.  If the to-be-optimized probepoint is hit before being optimized,
228	Kprobes returns control to the original instruction path by setting
229	the CPU's instruction pointer to the copied code in the detour buffer
230	-- thus at least avoiding the single-step.
231	
232	1.4.5 Optimization
233	
234	The Kprobe-optimizer doesn't insert the jump instruction immediately;
235	rather, it calls synchronize_sched() for safety first, because it's
236	possible for a CPU to be interrupted in the middle of executing the
237	optimized region(*).  As you know, synchronize_sched() can ensure
238	that all interruptions that were active when synchronize_sched()
239	was called are done, but only if CONFIG_PREEMPT=n.  So, this version
240	of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
241	
242	After that, the Kprobe-optimizer calls stop_machine() to replace
243	the optimized region with a jump instruction to the detour buffer,
244	using text_poke_smp().
245	
246	1.4.6 Unoptimization
247	
248	When an optimized kprobe is unregistered, disabled, or blocked by
249	another kprobe, it will be unoptimized.  If this happens before
250	the optimization is complete, the kprobe is just dequeued from the
251	optimized list.  If the optimization has been done, the jump is
252	replaced with the original code (except for an int3 breakpoint in
253	the first byte) by using text_poke_smp().
254	
255	(*)Please imagine that the 2nd instruction is interrupted and then
256	the optimizer replaces the 2nd instruction with the jump *address*
257	while the interrupt handler is running. When the interrupt
258	returns to original address, there is no valid instruction,
259	and it causes an unexpected result.
260	
261	(**)This optimization-safety checking may be replaced with the
262	stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
263	kernel.
264	
265	NOTE for geeks:
266	The jump optimization changes the kprobe's pre_handler behavior.
267	Without optimization, the pre_handler can change the kernel's execution
268	path by changing regs->ip and returning 1.  However, when the probe
269	is optimized, that modification is ignored.  Thus, if you want to
270	tweak the kernel's execution path, you need to suppress optimization,
271	using one of the following techniques:
272	- Specify an empty function for the kprobe's post_handler or break_handler.
273	 or
274	- Execute 'sysctl -w debug.kprobes_optimization=n'
275	
276	2. Architectures Supported
277	
278	Kprobes, jprobes, and return probes are implemented on the following
279	architectures:
280	
281	- i386 (Supports jump optimization)
282	- x86_64 (AMD-64, EM64T) (Supports jump optimization)
283	- ppc64
284	- ia64 (Does not support probes on instruction slot1.)
285	- sparc64 (Return probes not yet implemented.)
286	- arm
287	- ppc
288	- mips
289	
290	3. Configuring Kprobes
291	
292	When configuring the kernel using make menuconfig/xconfig/oldconfig,
293	ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
294	Support", look for "Kprobes".
295	
296	So that you can load and unload Kprobes-based instrumentation modules,
297	make sure "Loadable module support" (CONFIG_MODULES) and "Module
298	unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
299	
300	Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
301	are set to "y", since kallsyms_lookup_name() is used by the in-kernel
302	kprobe address resolution code.
303	
304	If you need to insert a probe in the middle of a function, you may find
305	it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
306	so you can use "objdump -d -l vmlinux" to see the source-to-object
307	code mapping.
308	
309	4. API Reference
310	
311	The Kprobes API includes a "register" function and an "unregister"
312	function for each type of probe. The API also includes "register_*probes"
313	and "unregister_*probes" functions for (un)registering arrays of probes.
314	Here are terse, mini-man-page specifications for these functions and
315	the associated probe handlers that you'll write. See the files in the
316	samples/kprobes/ sub-directory for examples.
317	
318	4.1 register_kprobe
319	
320	#include <linux/kprobes.h>
321	int register_kprobe(struct kprobe *kp);
322	
323	Sets a breakpoint at the address kp->addr.  When the breakpoint is
324	hit, Kprobes calls kp->pre_handler.  After the probed instruction
325	is single-stepped, Kprobe calls kp->post_handler.  If a fault
326	occurs during execution of kp->pre_handler or kp->post_handler,
327	or during single-stepping of the probed instruction, Kprobes calls
328	kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
329	is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
330	so, its handlers aren't hit until calling enable_kprobe(kp).
331	
332	NOTE:
333	1. With the introduction of the "symbol_name" field to struct kprobe,
334	the probepoint address resolution will now be taken care of by the kernel.
335	The following will now work:
336	
337		kp.symbol_name = "symbol_name";
338	
339	(64-bit powerpc intricacies such as function descriptors are handled
340	transparently)
341	
342	2. Use the "offset" field of struct kprobe if the offset into the symbol
343	to install a probepoint is known. This field is used to calculate the
344	probepoint.
345	
346	3. Specify either the kprobe "symbol_name" OR the "addr". If both are
347	specified, kprobe registration will fail with -EINVAL.
348	
349	4. With CISC architectures (such as i386 and x86_64), the kprobes code
350	does not validate if the kprobe.addr is at an instruction boundary.
351	Use "offset" with caution.
352	
353	register_kprobe() returns 0 on success, or a negative errno otherwise.
354	
355	User's pre-handler (kp->pre_handler):
356	#include <linux/kprobes.h>
357	#include <linux/ptrace.h>
358	int pre_handler(struct kprobe *p, struct pt_regs *regs);
359	
360	Called with p pointing to the kprobe associated with the breakpoint,
361	and regs pointing to the struct containing the registers saved when
362	the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
363	
364	User's post-handler (kp->post_handler):
365	#include <linux/kprobes.h>
366	#include <linux/ptrace.h>
367	void post_handler(struct kprobe *p, struct pt_regs *regs,
368		unsigned long flags);
369	
370	p and regs are as described for the pre_handler.  flags always seems
371	to be zero.
372	
373	User's fault-handler (kp->fault_handler):
374	#include <linux/kprobes.h>
375	#include <linux/ptrace.h>
376	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
377	
378	p and regs are as described for the pre_handler.  trapnr is the
379	architecture-specific trap number associated with the fault (e.g.,
380	on i386, 13 for a general protection fault or 14 for a page fault).
381	Returns 1 if it successfully handled the exception.
382	
383	4.2 register_jprobe
384	
385	#include <linux/kprobes.h>
386	int register_jprobe(struct jprobe *jp)
387	
388	Sets a breakpoint at the address jp->kp.addr, which must be the address
389	of the first instruction of a function.  When the breakpoint is hit,
390	Kprobes runs the handler whose address is jp->entry.
391	
392	The handler should have the same arg list and return type as the probed
393	function; and just before it returns, it must call jprobe_return().
394	(The handler never actually returns, since jprobe_return() returns
395	control to Kprobes.)  If the probed function is declared asmlinkage
396	or anything else that affects how args are passed, the handler's
397	declaration must match.
398	
399	register_jprobe() returns 0 on success, or a negative errno otherwise.
400	
401	4.3 register_kretprobe
402	
403	#include <linux/kprobes.h>
404	int register_kretprobe(struct kretprobe *rp);
405	
406	Establishes a return probe for the function whose address is
407	rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
408	You must set rp->maxactive appropriately before you call
409	register_kretprobe(); see "How Does a Return Probe Work?" for details.
410	
411	register_kretprobe() returns 0 on success, or a negative errno
412	otherwise.
413	
414	User's return-probe handler (rp->handler):
415	#include <linux/kprobes.h>
416	#include <linux/ptrace.h>
417	int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
418	
419	regs is as described for kprobe.pre_handler.  ri points to the
420	kretprobe_instance object, of which the following fields may be
421	of interest:
422	- ret_addr: the return address
423	- rp: points to the corresponding kretprobe object
424	- task: points to the corresponding task struct
425	- data: points to per return-instance private data; see "Kretprobe
426		entry-handler" for details.
427	
428	The regs_return_value(regs) macro provides a simple abstraction to
429	extract the return value from the appropriate register as defined by
430	the architecture's ABI.
431	
432	The handler's return value is currently ignored.
433	
434	4.4 unregister_*probe
435	
436	#include <linux/kprobes.h>
437	void unregister_kprobe(struct kprobe *kp);
438	void unregister_jprobe(struct jprobe *jp);
439	void unregister_kretprobe(struct kretprobe *rp);
440	
441	Removes the specified probe.  The unregister function can be called
442	at any time after the probe has been registered.
443	
444	NOTE:
445	If the functions find an incorrect probe (ex. an unregistered probe),
446	they clear the addr field of the probe.
447	
448	4.5 register_*probes
449	
450	#include <linux/kprobes.h>
451	int register_kprobes(struct kprobe **kps, int num);
452	int register_kretprobes(struct kretprobe **rps, int num);
453	int register_jprobes(struct jprobe **jps, int num);
454	
455	Registers each of the num probes in the specified array.  If any
456	error occurs during registration, all probes in the array, up to
457	the bad probe, are safely unregistered before the register_*probes
458	function returns.
459	- kps/rps/jps: an array of pointers to *probe data structures
460	- num: the number of the array entries.
461	
462	NOTE:
463	You have to allocate(or define) an array of pointers and set all
464	of the array entries before using these functions.
465	
466	4.6 unregister_*probes
467	
468	#include <linux/kprobes.h>
469	void unregister_kprobes(struct kprobe **kps, int num);
470	void unregister_kretprobes(struct kretprobe **rps, int num);
471	void unregister_jprobes(struct jprobe **jps, int num);
472	
473	Removes each of the num probes in the specified array at once.
474	
475	NOTE:
476	If the functions find some incorrect probes (ex. unregistered
477	probes) in the specified array, they clear the addr field of those
478	incorrect probes. However, other probes in the array are
479	unregistered correctly.
480	
481	4.7 disable_*probe
482	
483	#include <linux/kprobes.h>
484	int disable_kprobe(struct kprobe *kp);
485	int disable_kretprobe(struct kretprobe *rp);
486	int disable_jprobe(struct jprobe *jp);
487	
488	Temporarily disables the specified *probe. You can enable it again by using
489	enable_*probe(). You must specify the probe which has been registered.
490	
491	4.8 enable_*probe
492	
493	#include <linux/kprobes.h>
494	int enable_kprobe(struct kprobe *kp);
495	int enable_kretprobe(struct kretprobe *rp);
496	int enable_jprobe(struct jprobe *jp);
497	
498	Enables *probe which has been disabled by disable_*probe(). You must specify
499	the probe which has been registered.
500	
501	5. Kprobes Features and Limitations
502	
503	Kprobes allows multiple probes at the same address.  Currently,
504	however, there cannot be multiple jprobes on the same function at
505	the same time.  Also, a probepoint for which there is a jprobe or
506	a post_handler cannot be optimized.  So if you install a jprobe,
507	or a kprobe with a post_handler, at an optimized probepoint, the
508	probepoint will be unoptimized automatically.
509	
510	In general, you can install a probe anywhere in the kernel.
511	In particular, you can probe interrupt handlers.  Known exceptions
512	are discussed in this section.
513	
514	The register_*probe functions will return -EINVAL if you attempt
515	to install a probe in the code that implements Kprobes (mostly
516	kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
517	as do_page_fault and notifier_call_chain).
518	
519	If you install a probe in an inline-able function, Kprobes makes
520	no attempt to chase down all inline instances of the function and
521	install probes there.  gcc may inline a function without being asked,
522	so keep this in mind if you're not seeing the probe hits you expect.
523	
524	A probe handler can modify the environment of the probed function
525	-- e.g., by modifying kernel data structures, or by modifying the
526	contents of the pt_regs struct (which are restored to the registers
527	upon return from the breakpoint).  So Kprobes can be used, for example,
528	to install a bug fix or to inject faults for testing.  Kprobes, of
529	course, has no way to distinguish the deliberately injected faults
530	from the accidental ones.  Don't drink and probe.
531	
532	Kprobes makes no attempt to prevent probe handlers from stepping on
533	each other -- e.g., probing printk() and then calling printk() from a
534	probe handler.  If a probe handler hits a probe, that second probe's
535	handlers won't be run in that instance, and the kprobe.nmissed member
536	of the second probe will be incremented.
537	
538	As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
539	the same handler) may run concurrently on different CPUs.
540	
541	Kprobes does not use mutexes or allocate memory except during
542	registration and unregistration.
543	
544	Probe handlers are run with preemption disabled.  Depending on the
545	architecture and optimization state, handlers may also run with
546	interrupts disabled (e.g., kretprobe handlers and optimized kprobe
547	handlers run without interrupt disabled on x86/x86-64).  In any case,
548	your handler should not yield the CPU (e.g., by attempting to acquire
549	a semaphore).
550	
551	Since a return probe is implemented by replacing the return
552	address with the trampoline's address, stack backtraces and calls
553	to __builtin_return_address() will typically yield the trampoline's
554	address instead of the real return address for kretprobed functions.
555	(As far as we can tell, __builtin_return_address() is used only
556	for instrumentation and error reporting.)
557	
558	If the number of times a function is called does not match the number
559	of times it returns, registering a return probe on that function may
560	produce undesirable results. In such a case, a line:
561	kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
562	gets printed. With this information, one will be able to correlate the
563	exact instance of the kretprobe that caused the problem. We have the
564	do_exit() case covered. do_execve() and do_fork() are not an issue.
565	We're unaware of other specific cases where this could be a problem.
566	
567	If, upon entry to or exit from a function, the CPU is running on
568	a stack other than that of the current task, registering a return
569	probe on that function may produce undesirable results.  For this
570	reason, Kprobes doesn't support return probes (or kprobes or jprobes)
571	on the x86_64 version of __switch_to(); the registration functions
572	return -EINVAL.
573	
574	On x86/x86-64, since the Jump Optimization of Kprobes modifies
575	instructions widely, there are some limitations to optimization. To
576	explain it, we introduce some terminology. Imagine a 3-instruction
577	sequence consisting of a two 2-byte instructions and one 3-byte
578	instruction.
579	
580	        IA
581	         |
582	[-2][-1][0][1][2][3][4][5][6][7]
583	        [ins1][ins2][  ins3 ]
584		[<-     DCR       ->]
585		   [<- JTPR ->]
586	
587	ins1: 1st Instruction
588	ins2: 2nd Instruction
589	ins3: 3rd Instruction
590	IA:  Insertion Address
591	JTPR: Jump Target Prohibition Region
592	DCR: Detoured Code Region
593	
594	The instructions in DCR are copied to the out-of-line buffer
595	of the kprobe, because the bytes in DCR are replaced by
596	a 5-byte jump instruction. So there are several limitations.
597	
598	a) The instructions in DCR must be relocatable.
599	b) The instructions in DCR must not include a call instruction.
600	c) JTPR must not be targeted by any jump or call instruction.
601	d) DCR must not straddle the border between functions.
602	
603	Anyway, these limitations are checked by the in-kernel instruction
604	decoder, so you don't need to worry about that.
605	
606	6. Probe Overhead
607	
608	On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
609	microseconds to process.  Specifically, a benchmark that hits the same
610	probepoint repeatedly, firing a simple handler each time, reports 1-2
611	million hits per second, depending on the architecture.  A jprobe or
612	return-probe hit typically takes 50-75% longer than a kprobe hit.
613	When you have a return probe set on a function, adding a kprobe at
614	the entry to that function adds essentially no overhead.
615	
616	Here are sample overhead figures (in usec) for different architectures.
617	k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
618	on same function; jr = jprobe + return probe on same function
619	
620	i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
621	k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
622	
623	x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
624	k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
625	
626	ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
627	k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
628	
629	6.1 Optimized Probe Overhead
630	
631	Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
632	process. Here are sample overhead figures (in usec) for x86 architectures.
633	k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
634	r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
635	
636	i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
637	k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
638	
639	x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
640	k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
641	
642	7. TODO
643	
644	a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
645	programming interface for probe-based instrumentation.  Try it out.
646	b. Kernel return probes for sparc64.
647	c. Support for other architectures.
648	d. User-space probes.
649	e. Watchpoint probes (which fire on data references).
650	
651	8. Kprobes Example
652	
653	See samples/kprobes/kprobe_example.c
654	
655	9. Jprobes Example
656	
657	See samples/kprobes/jprobe_example.c
658	
659	10. Kretprobes Example
660	
661	See samples/kprobes/kretprobe_example.c
662	
663	For additional information on Kprobes, refer to the following URLs:
664	http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
665	http://www.redhat.com/magazine/005mar05/features/kprobes/
666	http://www-users.cs.umn.edu/~boutcher/kprobes/
667	http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
668	
669	
670	Appendix A: The kprobes debugfs interface
671	
672	With recent kernels (> 2.6.20) the list of registered kprobes is visible
673	under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
674	
675	/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
676	
677	c015d71a  k  vfs_read+0x0
678	c011a316  j  do_fork+0x0
679	c03dedc5  r  tcp_v4_rcv+0x0
680	
681	The first column provides the kernel address where the probe is inserted.
682	The second column identifies the type of probe (k - kprobe, r - kretprobe
683	and j - jprobe), while the third column specifies the symbol+offset of
684	the probe. If the probed function belongs to a module, the module name
685	is also specified. Following columns show probe status. If the probe is on
686	a virtual address that is no longer valid (module init sections, module
687	virtual addresses that correspond to modules that've been unloaded),
688	such probes are marked with [GONE]. If the probe is temporarily disabled,
689	such probes are marked with [DISABLED]. If the probe is optimized, it is
690	marked with [OPTIMIZED].
691	
692	/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
693	
694	Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
695	By default, all kprobes are enabled. By echoing "0" to this file, all
696	registered probes will be disarmed, till such time a "1" is echoed to this
697	file. Note that this knob just disarms and arms all kprobes and doesn't
698	change each probe's disabling state. This means that disabled kprobes (marked
699	[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
700	
701	
702	Appendix B: The kprobes sysctl interface
703	
704	/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
705	
706	When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
707	a knob to globally and forcibly turn jump optimization (see section
708	1.4) ON or OFF. By default, jump optimization is allowed (ON).
709	If you echo "0" to this file or set "debug.kprobes_optimization" to
710	0 via sysctl, all optimized probes will be unoptimized, and any new
711	probes registered after that will not be optimized.  Note that this
712	knob *changes* the optimized state. This means that optimized probes
713	(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
714	removed). If the knob is turned on, they will be optimized again.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.