About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / kprobes.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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	1.3 Return Probes
107	
108	1.3.1 How Does a Return Probe Work?
109	
110	When you call register_kretprobe(), Kprobes establishes a kprobe at
111	the entry to the function.  When the probed function is called and this
112	probe is hit, Kprobes saves a copy of the return address, and replaces
113	the return address with the address of a "trampoline."  The trampoline
114	is an arbitrary piece of code -- typically just a nop instruction.
115	At boot time, Kprobes registers a kprobe at the trampoline.
116	
117	When the probed function executes its return instruction, control
118	passes to the trampoline and that probe is hit.  Kprobes' trampoline
119	handler calls the user-specified return handler associated with the
120	kretprobe, then sets the saved instruction pointer to the saved return
121	address, and that's where execution resumes upon return from the trap.
122	
123	While the probed function is executing, its return address is
124	stored in an object of type kretprobe_instance.  Before calling
125	register_kretprobe(), the user sets the maxactive field of the
126	kretprobe struct to specify how many instances of the specified
127	function can be probed simultaneously.  register_kretprobe()
128	pre-allocates the indicated number of kretprobe_instance objects.
129	
130	For example, if the function is non-recursive and is called with a
131	spinlock held, maxactive = 1 should be enough.  If the function is
132	non-recursive and can never relinquish the CPU (e.g., via a semaphore
133	or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
134	set to a default value.  If CONFIG_PREEMPT is enabled, the default
135	is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
136	
137	It's not a disaster if you set maxactive too low; you'll just miss
138	some probes.  In the kretprobe struct, the nmissed field is set to
139	zero when the return probe is registered, and is incremented every
140	time the probed function is entered but there is no kretprobe_instance
141	object available for establishing the return probe.
142	
143	1.3.2 Kretprobe entry-handler
144	
145	Kretprobes also provides an optional user-specified handler which runs
146	on function entry. This handler is specified by setting the entry_handler
147	field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
148	function entry is hit, the user-defined entry_handler, if any, is invoked.
149	If the entry_handler returns 0 (success) then a corresponding return handler
150	is guaranteed to be called upon function return. If the entry_handler
151	returns a non-zero error then Kprobes leaves the return address as is, and
152	the kretprobe has no further effect for that particular function instance.
153	
154	Multiple entry and return handler invocations are matched using the unique
155	kretprobe_instance object associated with them. Additionally, a user
156	may also specify per return-instance private data to be part of each
157	kretprobe_instance object. This is especially useful when sharing private
158	data between corresponding user entry and return handlers. The size of each
159	private data object can be specified at kretprobe registration time by
160	setting the data_size field of the kretprobe struct. This data can be
161	accessed through the data field of each kretprobe_instance object.
162	
163	In case probed function is entered but there is no kretprobe_instance
164	object available, then in addition to incrementing the nmissed count,
165	the user entry_handler invocation is also skipped.
166	
167	1.4 How Does Jump Optimization Work?
168	
169	If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
170	is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
171	the "debug.kprobes_optimization" kernel parameter is set to 1 (see
172	sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
173	instruction instead of a breakpoint instruction at each probepoint.
174	
175	1.4.1 Init a Kprobe
176	
177	When a probe is registered, before attempting this optimization,
178	Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
179	address. So, even if it's not possible to optimize this particular
180	probepoint, there'll be a probe there.
181	
182	1.4.2 Safety Check
183	
184	Before optimizing a probe, Kprobes performs the following safety checks:
185	
186	- Kprobes verifies that the region that will be replaced by the jump
187	instruction (the "optimized region") lies entirely within one function.
188	(A jump instruction is multiple bytes, and so may overlay multiple
189	instructions.)
190	
191	- Kprobes analyzes the entire function and verifies that there is no
192	jump into the optimized region.  Specifically:
193	  - the function contains no indirect jump;
194	  - the function contains no instruction that causes an exception (since
195	  the fixup code triggered by the exception could jump back into the
196	  optimized region -- Kprobes checks the exception tables to verify this);
197	  and
198	  - there is no near jump to the optimized region (other than to the first
199	  byte).
200	
201	- For each instruction in the optimized region, Kprobes verifies that
202	the instruction can be executed out of line.
203	
204	1.4.3 Preparing Detour Buffer
205	
206	Next, Kprobes prepares a "detour" buffer, which contains the following
207	instruction sequence:
208	- code to push the CPU's registers (emulating a breakpoint trap)
209	- a call to the trampoline code which calls user's probe handlers.
210	- code to restore registers
211	- the instructions from the optimized region
212	- a jump back to the original execution path.
213	
214	1.4.4 Pre-optimization
215	
216	After preparing the detour buffer, Kprobes verifies that none of the
217	following situations exist:
218	- The probe has either a break_handler (i.e., it's a jprobe) or a
219	post_handler.
220	- Other instructions in the optimized region are probed.
221	- The probe is disabled.
222	In any of the above cases, Kprobes won't start optimizing the probe.
223	Since these are temporary situations, Kprobes tries to start
224	optimizing it again if the situation is changed.
225	
226	If the kprobe can be optimized, Kprobes enqueues the kprobe to an
227	optimizing list, and kicks the kprobe-optimizer workqueue to optimize
228	it.  If the to-be-optimized probepoint is hit before being optimized,
229	Kprobes returns control to the original instruction path by setting
230	the CPU's instruction pointer to the copied code in the detour buffer
231	-- thus at least avoiding the single-step.
232	
233	1.4.5 Optimization
234	
235	The Kprobe-optimizer doesn't insert the jump instruction immediately;
236	rather, it calls synchronize_sched() for safety first, because it's
237	possible for a CPU to be interrupted in the middle of executing the
238	optimized region(*).  As you know, synchronize_sched() can ensure
239	that all interruptions that were active when synchronize_sched()
240	was called are done, but only if CONFIG_PREEMPT=n.  So, this version
241	of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
242	
243	After that, the Kprobe-optimizer calls stop_machine() to replace
244	the optimized region with a jump instruction to the detour buffer,
245	using text_poke_smp().
246	
247	1.4.6 Unoptimization
248	
249	When an optimized kprobe is unregistered, disabled, or blocked by
250	another kprobe, it will be unoptimized.  If this happens before
251	the optimization is complete, the kprobe is just dequeued from the
252	optimized list.  If the optimization has been done, the jump is
253	replaced with the original code (except for an int3 breakpoint in
254	the first byte) by using text_poke_smp().
255	
256	(*)Please imagine that the 2nd instruction is interrupted and then
257	the optimizer replaces the 2nd instruction with the jump *address*
258	while the interrupt handler is running. When the interrupt
259	returns to original address, there is no valid instruction,
260	and it causes an unexpected result.
261	
262	(**)This optimization-safety checking may be replaced with the
263	stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
264	kernel.
265	
266	NOTE for geeks:
267	The jump optimization changes the kprobe's pre_handler behavior.
268	Without optimization, the pre_handler can change the kernel's execution
269	path by changing regs->ip and returning 1.  However, when the probe
270	is optimized, that modification is ignored.  Thus, if you want to
271	tweak the kernel's execution path, you need to suppress optimization,
272	using one of the following techniques:
273	- Specify an empty function for the kprobe's post_handler or break_handler.
274	 or
275	- Execute 'sysctl -w debug.kprobes_optimization=n'
276	
277	1.5 Blacklist
278	
279	Kprobes can probe most of the kernel except itself. This means
280	that there are some functions where kprobes cannot probe. Probing
281	(trapping) such functions can cause a recursive trap (e.g. double
282	fault) or the nested probe handler may never be called.
283	Kprobes manages such functions as a blacklist.
284	If you want to add a function into the blacklist, you just need
285	to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
286	to specify a blacklisted function.
287	Kprobes checks the given probe address against the blacklist and
288	rejects registering it, if the given address is in the blacklist.
289	
290	2. Architectures Supported
291	
292	Kprobes, jprobes, and return probes are implemented on the following
293	architectures:
294	
295	- i386 (Supports jump optimization)
296	- x86_64 (AMD-64, EM64T) (Supports jump optimization)
297	- ppc64
298	- ia64 (Does not support probes on instruction slot1.)
299	- sparc64 (Return probes not yet implemented.)
300	- arm
301	- ppc
302	- mips
303	
304	3. Configuring Kprobes
305	
306	When configuring the kernel using make menuconfig/xconfig/oldconfig,
307	ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
308	Support", look for "Kprobes".
309	
310	So that you can load and unload Kprobes-based instrumentation modules,
311	make sure "Loadable module support" (CONFIG_MODULES) and "Module
312	unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
313	
314	Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
315	are set to "y", since kallsyms_lookup_name() is used by the in-kernel
316	kprobe address resolution code.
317	
318	If you need to insert a probe in the middle of a function, you may find
319	it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
320	so you can use "objdump -d -l vmlinux" to see the source-to-object
321	code mapping.
322	
323	4. API Reference
324	
325	The Kprobes API includes a "register" function and an "unregister"
326	function for each type of probe. The API also includes "register_*probes"
327	and "unregister_*probes" functions for (un)registering arrays of probes.
328	Here are terse, mini-man-page specifications for these functions and
329	the associated probe handlers that you'll write. See the files in the
330	samples/kprobes/ sub-directory for examples.
331	
332	4.1 register_kprobe
333	
334	#include <linux/kprobes.h>
335	int register_kprobe(struct kprobe *kp);
336	
337	Sets a breakpoint at the address kp->addr.  When the breakpoint is
338	hit, Kprobes calls kp->pre_handler.  After the probed instruction
339	is single-stepped, Kprobe calls kp->post_handler.  If a fault
340	occurs during execution of kp->pre_handler or kp->post_handler,
341	or during single-stepping of the probed instruction, Kprobes calls
342	kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
343	is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
344	so, its handlers aren't hit until calling enable_kprobe(kp).
345	
346	NOTE:
347	1. With the introduction of the "symbol_name" field to struct kprobe,
348	the probepoint address resolution will now be taken care of by the kernel.
349	The following will now work:
350	
351		kp.symbol_name = "symbol_name";
352	
353	(64-bit powerpc intricacies such as function descriptors are handled
354	transparently)
355	
356	2. Use the "offset" field of struct kprobe if the offset into the symbol
357	to install a probepoint is known. This field is used to calculate the
358	probepoint.
359	
360	3. Specify either the kprobe "symbol_name" OR the "addr". If both are
361	specified, kprobe registration will fail with -EINVAL.
362	
363	4. With CISC architectures (such as i386 and x86_64), the kprobes code
364	does not validate if the kprobe.addr is at an instruction boundary.
365	Use "offset" with caution.
366	
367	register_kprobe() returns 0 on success, or a negative errno otherwise.
368	
369	User's pre-handler (kp->pre_handler):
370	#include <linux/kprobes.h>
371	#include <linux/ptrace.h>
372	int pre_handler(struct kprobe *p, struct pt_regs *regs);
373	
374	Called with p pointing to the kprobe associated with the breakpoint,
375	and regs pointing to the struct containing the registers saved when
376	the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
377	
378	User's post-handler (kp->post_handler):
379	#include <linux/kprobes.h>
380	#include <linux/ptrace.h>
381	void post_handler(struct kprobe *p, struct pt_regs *regs,
382		unsigned long flags);
383	
384	p and regs are as described for the pre_handler.  flags always seems
385	to be zero.
386	
387	User's fault-handler (kp->fault_handler):
388	#include <linux/kprobes.h>
389	#include <linux/ptrace.h>
390	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
391	
392	p and regs are as described for the pre_handler.  trapnr is the
393	architecture-specific trap number associated with the fault (e.g.,
394	on i386, 13 for a general protection fault or 14 for a page fault).
395	Returns 1 if it successfully handled the exception.
396	
397	4.2 register_jprobe
398	
399	#include <linux/kprobes.h>
400	int register_jprobe(struct jprobe *jp)
401	
402	Sets a breakpoint at the address jp->kp.addr, which must be the address
403	of the first instruction of a function.  When the breakpoint is hit,
404	Kprobes runs the handler whose address is jp->entry.
405	
406	The handler should have the same arg list and return type as the probed
407	function; and just before it returns, it must call jprobe_return().
408	(The handler never actually returns, since jprobe_return() returns
409	control to Kprobes.)  If the probed function is declared asmlinkage
410	or anything else that affects how args are passed, the handler's
411	declaration must match.
412	
413	register_jprobe() returns 0 on success, or a negative errno otherwise.
414	
415	4.3 register_kretprobe
416	
417	#include <linux/kprobes.h>
418	int register_kretprobe(struct kretprobe *rp);
419	
420	Establishes a return probe for the function whose address is
421	rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
422	You must set rp->maxactive appropriately before you call
423	register_kretprobe(); see "How Does a Return Probe Work?" for details.
424	
425	register_kretprobe() returns 0 on success, or a negative errno
426	otherwise.
427	
428	User's return-probe handler (rp->handler):
429	#include <linux/kprobes.h>
430	#include <linux/ptrace.h>
431	int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
432	
433	regs is as described for kprobe.pre_handler.  ri points to the
434	kretprobe_instance object, of which the following fields may be
435	of interest:
436	- ret_addr: the return address
437	- rp: points to the corresponding kretprobe object
438	- task: points to the corresponding task struct
439	- data: points to per return-instance private data; see "Kretprobe
440		entry-handler" for details.
441	
442	The regs_return_value(regs) macro provides a simple abstraction to
443	extract the return value from the appropriate register as defined by
444	the architecture's ABI.
445	
446	The handler's return value is currently ignored.
447	
448	4.4 unregister_*probe
449	
450	#include <linux/kprobes.h>
451	void unregister_kprobe(struct kprobe *kp);
452	void unregister_jprobe(struct jprobe *jp);
453	void unregister_kretprobe(struct kretprobe *rp);
454	
455	Removes the specified probe.  The unregister function can be called
456	at any time after the probe has been registered.
457	
458	NOTE:
459	If the functions find an incorrect probe (ex. an unregistered probe),
460	they clear the addr field of the probe.
461	
462	4.5 register_*probes
463	
464	#include <linux/kprobes.h>
465	int register_kprobes(struct kprobe **kps, int num);
466	int register_kretprobes(struct kretprobe **rps, int num);
467	int register_jprobes(struct jprobe **jps, int num);
468	
469	Registers each of the num probes in the specified array.  If any
470	error occurs during registration, all probes in the array, up to
471	the bad probe, are safely unregistered before the register_*probes
472	function returns.
473	- kps/rps/jps: an array of pointers to *probe data structures
474	- num: the number of the array entries.
475	
476	NOTE:
477	You have to allocate(or define) an array of pointers and set all
478	of the array entries before using these functions.
479	
480	4.6 unregister_*probes
481	
482	#include <linux/kprobes.h>
483	void unregister_kprobes(struct kprobe **kps, int num);
484	void unregister_kretprobes(struct kretprobe **rps, int num);
485	void unregister_jprobes(struct jprobe **jps, int num);
486	
487	Removes each of the num probes in the specified array at once.
488	
489	NOTE:
490	If the functions find some incorrect probes (ex. unregistered
491	probes) in the specified array, they clear the addr field of those
492	incorrect probes. However, other probes in the array are
493	unregistered correctly.
494	
495	4.7 disable_*probe
496	
497	#include <linux/kprobes.h>
498	int disable_kprobe(struct kprobe *kp);
499	int disable_kretprobe(struct kretprobe *rp);
500	int disable_jprobe(struct jprobe *jp);
501	
502	Temporarily disables the specified *probe. You can enable it again by using
503	enable_*probe(). You must specify the probe which has been registered.
504	
505	4.8 enable_*probe
506	
507	#include <linux/kprobes.h>
508	int enable_kprobe(struct kprobe *kp);
509	int enable_kretprobe(struct kretprobe *rp);
510	int enable_jprobe(struct jprobe *jp);
511	
512	Enables *probe which has been disabled by disable_*probe(). You must specify
513	the probe which has been registered.
514	
515	5. Kprobes Features and Limitations
516	
517	Kprobes allows multiple probes at the same address.  Currently,
518	however, there cannot be multiple jprobes on the same function at
519	the same time.  Also, a probepoint for which there is a jprobe or
520	a post_handler cannot be optimized.  So if you install a jprobe,
521	or a kprobe with a post_handler, at an optimized probepoint, the
522	probepoint will be unoptimized automatically.
523	
524	In general, you can install a probe anywhere in the kernel.
525	In particular, you can probe interrupt handlers.  Known exceptions
526	are discussed in this section.
527	
528	The register_*probe functions will return -EINVAL if you attempt
529	to install a probe in the code that implements Kprobes (mostly
530	kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
531	as do_page_fault and notifier_call_chain).
532	
533	If you install a probe in an inline-able function, Kprobes makes
534	no attempt to chase down all inline instances of the function and
535	install probes there.  gcc may inline a function without being asked,
536	so keep this in mind if you're not seeing the probe hits you expect.
537	
538	A probe handler can modify the environment of the probed function
539	-- e.g., by modifying kernel data structures, or by modifying the
540	contents of the pt_regs struct (which are restored to the registers
541	upon return from the breakpoint).  So Kprobes can be used, for example,
542	to install a bug fix or to inject faults for testing.  Kprobes, of
543	course, has no way to distinguish the deliberately injected faults
544	from the accidental ones.  Don't drink and probe.
545	
546	Kprobes makes no attempt to prevent probe handlers from stepping on
547	each other -- e.g., probing printk() and then calling printk() from a
548	probe handler.  If a probe handler hits a probe, that second probe's
549	handlers won't be run in that instance, and the kprobe.nmissed member
550	of the second probe will be incremented.
551	
552	As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
553	the same handler) may run concurrently on different CPUs.
554	
555	Kprobes does not use mutexes or allocate memory except during
556	registration and unregistration.
557	
558	Probe handlers are run with preemption disabled.  Depending on the
559	architecture and optimization state, handlers may also run with
560	interrupts disabled (e.g., kretprobe handlers and optimized kprobe
561	handlers run without interrupt disabled on x86/x86-64).  In any case,
562	your handler should not yield the CPU (e.g., by attempting to acquire
563	a semaphore).
564	
565	Since a return probe is implemented by replacing the return
566	address with the trampoline's address, stack backtraces and calls
567	to __builtin_return_address() will typically yield the trampoline's
568	address instead of the real return address for kretprobed functions.
569	(As far as we can tell, __builtin_return_address() is used only
570	for instrumentation and error reporting.)
571	
572	If the number of times a function is called does not match the number
573	of times it returns, registering a return probe on that function may
574	produce undesirable results. In such a case, a line:
575	kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
576	gets printed. With this information, one will be able to correlate the
577	exact instance of the kretprobe that caused the problem. We have the
578	do_exit() case covered. do_execve() and do_fork() are not an issue.
579	We're unaware of other specific cases where this could be a problem.
580	
581	If, upon entry to or exit from a function, the CPU is running on
582	a stack other than that of the current task, registering a return
583	probe on that function may produce undesirable results.  For this
584	reason, Kprobes doesn't support return probes (or kprobes or jprobes)
585	on the x86_64 version of __switch_to(); the registration functions
586	return -EINVAL.
587	
588	On x86/x86-64, since the Jump Optimization of Kprobes modifies
589	instructions widely, there are some limitations to optimization. To
590	explain it, we introduce some terminology. Imagine a 3-instruction
591	sequence consisting of a two 2-byte instructions and one 3-byte
592	instruction.
593	
594	        IA
595	         |
596	[-2][-1][0][1][2][3][4][5][6][7]
597	        [ins1][ins2][  ins3 ]
598		[<-     DCR       ->]
599		   [<- JTPR ->]
600	
601	ins1: 1st Instruction
602	ins2: 2nd Instruction
603	ins3: 3rd Instruction
604	IA:  Insertion Address
605	JTPR: Jump Target Prohibition Region
606	DCR: Detoured Code Region
607	
608	The instructions in DCR are copied to the out-of-line buffer
609	of the kprobe, because the bytes in DCR are replaced by
610	a 5-byte jump instruction. So there are several limitations.
611	
612	a) The instructions in DCR must be relocatable.
613	b) The instructions in DCR must not include a call instruction.
614	c) JTPR must not be targeted by any jump or call instruction.
615	d) DCR must not straddle the border between functions.
616	
617	Anyway, these limitations are checked by the in-kernel instruction
618	decoder, so you don't need to worry about that.
619	
620	6. Probe Overhead
621	
622	On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
623	microseconds to process.  Specifically, a benchmark that hits the same
624	probepoint repeatedly, firing a simple handler each time, reports 1-2
625	million hits per second, depending on the architecture.  A jprobe or
626	return-probe hit typically takes 50-75% longer than a kprobe hit.
627	When you have a return probe set on a function, adding a kprobe at
628	the entry to that function adds essentially no overhead.
629	
630	Here are sample overhead figures (in usec) for different architectures.
631	k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
632	on same function; jr = jprobe + return probe on same function
633	
634	i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
635	k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
636	
637	x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
638	k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
639	
640	ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
641	k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
642	
643	6.1 Optimized Probe Overhead
644	
645	Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
646	process. Here are sample overhead figures (in usec) for x86 architectures.
647	k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
648	r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
649	
650	i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
651	k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
652	
653	x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
654	k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
655	
656	7. TODO
657	
658	a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
659	programming interface for probe-based instrumentation.  Try it out.
660	b. Kernel return probes for sparc64.
661	c. Support for other architectures.
662	d. User-space probes.
663	e. Watchpoint probes (which fire on data references).
664	
665	8. Kprobes Example
666	
667	See samples/kprobes/kprobe_example.c
668	
669	9. Jprobes Example
670	
671	See samples/kprobes/jprobe_example.c
672	
673	10. Kretprobes Example
674	
675	See samples/kprobes/kretprobe_example.c
676	
677	For additional information on Kprobes, refer to the following URLs:
678	http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
679	http://www.redhat.com/magazine/005mar05/features/kprobes/
680	http://www-users.cs.umn.edu/~boutcher/kprobes/
681	http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
682	
683	
684	Appendix A: The kprobes debugfs interface
685	
686	With recent kernels (> 2.6.20) the list of registered kprobes is visible
687	under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
688	
689	/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
690	
691	c015d71a  k  vfs_read+0x0
692	c011a316  j  do_fork+0x0
693	c03dedc5  r  tcp_v4_rcv+0x0
694	
695	The first column provides the kernel address where the probe is inserted.
696	The second column identifies the type of probe (k - kprobe, r - kretprobe
697	and j - jprobe), while the third column specifies the symbol+offset of
698	the probe. If the probed function belongs to a module, the module name
699	is also specified. Following columns show probe status. If the probe is on
700	a virtual address that is no longer valid (module init sections, module
701	virtual addresses that correspond to modules that've been unloaded),
702	such probes are marked with [GONE]. If the probe is temporarily disabled,
703	such probes are marked with [DISABLED]. If the probe is optimized, it is
704	marked with [OPTIMIZED].
705	
706	/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
707	
708	Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
709	By default, all kprobes are enabled. By echoing "0" to this file, all
710	registered probes will be disarmed, till such time a "1" is echoed to this
711	file. Note that this knob just disarms and arms all kprobes and doesn't
712	change each probe's disabling state. This means that disabled kprobes (marked
713	[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
714	
715	
716	Appendix B: The kprobes sysctl interface
717	
718	/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
719	
720	When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
721	a knob to globally and forcibly turn jump optimization (see section
722	1.4) ON or OFF. By default, jump optimization is allowed (ON).
723	If you echo "0" to this file or set "debug.kprobes_optimization" to
724	0 via sysctl, all optimized probes will be unoptimized, and any new
725	probes registered after that will not be optimized.  Note that this
726	knob *changes* the optimized state. This means that optimized probes
727	(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
728	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.