Root/Documentation/kprobes.txt

1Title : Kernel Probes (Kprobes)
2Authors : Jim Keniston <jkenisto@us.ibm.com>
3    : Prasanna S Panchamukhi <prasanna@in.ibm.com>
4
5CONTENTS
6
71. Concepts: Kprobes, Jprobes, Return Probes
82. Architectures Supported
93. Configuring Kprobes
104. API Reference
115. Kprobes Features and Limitations
126. Probe Overhead
137. TODO
148. Kprobes Example
159. Jprobes Example
1610. Kretprobes Example
17Appendix A: The kprobes debugfs interface
18
191. Concepts: Kprobes, Jprobes, Return Probes
20
21Kprobes enables you to dynamically break into any kernel routine and
22collect debugging and performance information non-disruptively. You
23can trap at almost any kernel code address, specifying a handler
24routine to be invoked when the breakpoint is hit.
25
26There are currently three types of probes: kprobes, jprobes, and
27kretprobes (also called return probes). A kprobe can be inserted
28on virtually any instruction in the kernel. A jprobe is inserted at
29the entry to a kernel function, and provides convenient access to the
30function's arguments. A return probe fires when a specified function
31returns.
32
33In the typical case, Kprobes-based instrumentation is packaged as
34a kernel module. The module's init function installs ("registers")
35one or more probes, and the exit function unregisters them. A
36registration function such as register_kprobe() specifies where
37the probe is to be inserted and what handler is to be called when
38the probe is hit.
39
40There are also register_/unregister_*probes() functions for batch
41registration/unregistration of a group of *probes. These functions
42can speed up unregistration process when you have to unregister
43a lot of probes at once.
44
45The next three subsections explain how the different types of
46probes work. They explain certain things that you'll need to
47know in order to make the best use of Kprobes -- e.g., the
48difference between a pre_handler and a post_handler, and how
49to use the maxactive and nmissed fields of a kretprobe. But
50if you're in a hurry to start using Kprobes, you can skip ahead
51to section 2.
52
531.1 How Does a Kprobe Work?
54
55When a kprobe is registered, Kprobes makes a copy of the probed
56instruction and replaces the first byte(s) of the probed instruction
57with a breakpoint instruction (e.g., int3 on i386 and x86_64).
58
59When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
60registers are saved, and control passes to Kprobes via the
61notifier_call_chain mechanism. Kprobes executes the "pre_handler"
62associated with the kprobe, passing the handler the addresses of the
63kprobe struct and the saved registers.
64
65Next, Kprobes single-steps its copy of the probed instruction.
66(It would be simpler to single-step the actual instruction in place,
67but then Kprobes would have to temporarily remove the breakpoint
68instruction. This would open a small time window when another CPU
69could sail right past the probepoint.)
70
71After the instruction is single-stepped, Kprobes executes the
72"post_handler," if any, that is associated with the kprobe.
73Execution then continues with the instruction following the probepoint.
74
751.2 How Does a Jprobe Work?
76
77A jprobe is implemented using a kprobe that is placed on a function's
78entry point. It employs a simple mirroring principle to allow
79seamless access to the probed function's arguments. The jprobe
80handler routine should have the same signature (arg list and return
81type) as the function being probed, and must always end by calling
82the Kprobes function jprobe_return().
83
84Here's how it works. When the probe is hit, Kprobes makes a copy of
85the saved registers and a generous portion of the stack (see below).
86Kprobes then points the saved instruction pointer at the jprobe's
87handler routine, and returns from the trap. As a result, control
88passes to the handler, which is presented with the same register and
89stack contents as the probed function. When it is done, the handler
90calls jprobe_return(), which traps again to restore the original stack
91contents and processor state and switch to the probed function.
92
93By convention, the callee owns its arguments, so gcc may produce code
94that unexpectedly modifies that portion of the stack. This is why
95Kprobes saves a copy of the stack and restores it after the jprobe
96handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
9764 bytes on i386.
98
99Note that the probed function's args may be passed on the stack
100or in registers. The jprobe will work in either case, so long as the
101handler's prototype matches that of the probed function.
102
1031.3 Return Probes
104
1051.3.1 How Does a Return Probe Work?
106
107When you call register_kretprobe(), Kprobes establishes a kprobe at
108the entry to the function. When the probed function is called and this
109probe is hit, Kprobes saves a copy of the return address, and replaces
110the return address with the address of a "trampoline." The trampoline
111is an arbitrary piece of code -- typically just a nop instruction.
112At boot time, Kprobes registers a kprobe at the trampoline.
113
114When the probed function executes its return instruction, control
115passes to the trampoline and that probe is hit. Kprobes' trampoline
116handler calls the user-specified return handler associated with the
117kretprobe, then sets the saved instruction pointer to the saved return
118address, and that's where execution resumes upon return from the trap.
119
120While the probed function is executing, its return address is
121stored in an object of type kretprobe_instance. Before calling
122register_kretprobe(), the user sets the maxactive field of the
123kretprobe struct to specify how many instances of the specified
124function can be probed simultaneously. register_kretprobe()
125pre-allocates the indicated number of kretprobe_instance objects.
126
127For example, if the function is non-recursive and is called with a
128spinlock held, maxactive = 1 should be enough. If the function is
129non-recursive and can never relinquish the CPU (e.g., via a semaphore
130or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
131set to a default value. If CONFIG_PREEMPT is enabled, the default
132is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
133
134It's not a disaster if you set maxactive too low; you'll just miss
135some probes. In the kretprobe struct, the nmissed field is set to
136zero when the return probe is registered, and is incremented every
137time the probed function is entered but there is no kretprobe_instance
138object available for establishing the return probe.
139
1401.3.2 Kretprobe entry-handler
141
142Kretprobes also provides an optional user-specified handler which runs
143on function entry. This handler is specified by setting the entry_handler
144field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
145function entry is hit, the user-defined entry_handler, if any, is invoked.
146If the entry_handler returns 0 (success) then a corresponding return handler
147is guaranteed to be called upon function return. If the entry_handler
148returns a non-zero error then Kprobes leaves the return address as is, and
149the kretprobe has no further effect for that particular function instance.
150
151Multiple entry and return handler invocations are matched using the unique
152kretprobe_instance object associated with them. Additionally, a user
153may also specify per return-instance private data to be part of each
154kretprobe_instance object. This is especially useful when sharing private
155data between corresponding user entry and return handlers. The size of each
156private data object can be specified at kretprobe registration time by
157setting the data_size field of the kretprobe struct. This data can be
158accessed through the data field of each kretprobe_instance object.
159
160In case probed function is entered but there is no kretprobe_instance
161object available, then in addition to incrementing the nmissed count,
162the user entry_handler invocation is also skipped.
163
1642. Architectures Supported
165
166Kprobes, jprobes, and return probes are implemented on the following
167architectures:
168
169- i386
170- x86_64 (AMD-64, EM64T)
171- ppc64
172- ia64 (Does not support probes on instruction slot1.)
173- sparc64 (Return probes not yet implemented.)
174- arm
175- ppc
176
1773. Configuring Kprobes
178
179When configuring the kernel using make menuconfig/xconfig/oldconfig,
180ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation
181Support", look for "Kprobes".
182
183So that you can load and unload Kprobes-based instrumentation modules,
184make sure "Loadable module support" (CONFIG_MODULES) and "Module
185unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
186
187Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
188are set to "y", since kallsyms_lookup_name() is used by the in-kernel
189kprobe address resolution code.
190
191If you need to insert a probe in the middle of a function, you may find
192it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
193so you can use "objdump -d -l vmlinux" to see the source-to-object
194code mapping.
195
1964. API Reference
197
198The Kprobes API includes a "register" function and an "unregister"
199function for each type of probe. The API also includes "register_*probes"
200and "unregister_*probes" functions for (un)registering arrays of probes.
201Here are terse, mini-man-page specifications for these functions and
202the associated probe handlers that you'll write. See the files in the
203samples/kprobes/ sub-directory for examples.
204
2054.1 register_kprobe
206
207#include <linux/kprobes.h>
208int register_kprobe(struct kprobe *kp);
209
210Sets a breakpoint at the address kp->addr. When the breakpoint is
211hit, Kprobes calls kp->pre_handler. After the probed instruction
212is single-stepped, Kprobe calls kp->post_handler. If a fault
213occurs during execution of kp->pre_handler or kp->post_handler,
214or during single-stepping of the probed instruction, Kprobes calls
215kp->fault_handler. Any or all handlers can be NULL. If kp->flags
216is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
217so, it's handlers aren't hit until calling enable_kprobe(kp).
218
219NOTE:
2201. With the introduction of the "symbol_name" field to struct kprobe,
221the probepoint address resolution will now be taken care of by the kernel.
222The following will now work:
223
224    kp.symbol_name = "symbol_name";
225
226(64-bit powerpc intricacies such as function descriptors are handled
227transparently)
228
2292. Use the "offset" field of struct kprobe if the offset into the symbol
230to install a probepoint is known. This field is used to calculate the
231probepoint.
232
2333. Specify either the kprobe "symbol_name" OR the "addr". If both are
234specified, kprobe registration will fail with -EINVAL.
235
2364. With CISC architectures (such as i386 and x86_64), the kprobes code
237does not validate if the kprobe.addr is at an instruction boundary.
238Use "offset" with caution.
239
240register_kprobe() returns 0 on success, or a negative errno otherwise.
241
242User's pre-handler (kp->pre_handler):
243#include <linux/kprobes.h>
244#include <linux/ptrace.h>
245int pre_handler(struct kprobe *p, struct pt_regs *regs);
246
247Called with p pointing to the kprobe associated with the breakpoint,
248and regs pointing to the struct containing the registers saved when
249the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
250
251User's post-handler (kp->post_handler):
252#include <linux/kprobes.h>
253#include <linux/ptrace.h>
254void post_handler(struct kprobe *p, struct pt_regs *regs,
255    unsigned long flags);
256
257p and regs are as described for the pre_handler. flags always seems
258to be zero.
259
260User's fault-handler (kp->fault_handler):
261#include <linux/kprobes.h>
262#include <linux/ptrace.h>
263int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
264
265p and regs are as described for the pre_handler. trapnr is the
266architecture-specific trap number associated with the fault (e.g.,
267on i386, 13 for a general protection fault or 14 for a page fault).
268Returns 1 if it successfully handled the exception.
269
2704.2 register_jprobe
271
272#include <linux/kprobes.h>
273int register_jprobe(struct jprobe *jp)
274
275Sets a breakpoint at the address jp->kp.addr, which must be the address
276of the first instruction of a function. When the breakpoint is hit,
277Kprobes runs the handler whose address is jp->entry.
278
279The handler should have the same arg list and return type as the probed
280function; and just before it returns, it must call jprobe_return().
281(The handler never actually returns, since jprobe_return() returns
282control to Kprobes.) If the probed function is declared asmlinkage
283or anything else that affects how args are passed, the handler's
284declaration must match.
285
286register_jprobe() returns 0 on success, or a negative errno otherwise.
287
2884.3 register_kretprobe
289
290#include <linux/kprobes.h>
291int register_kretprobe(struct kretprobe *rp);
292
293Establishes a return probe for the function whose address is
294rp->kp.addr. When that function returns, Kprobes calls rp->handler.
295You must set rp->maxactive appropriately before you call
296register_kretprobe(); see "How Does a Return Probe Work?" for details.
297
298register_kretprobe() returns 0 on success, or a negative errno
299otherwise.
300
301User's return-probe handler (rp->handler):
302#include <linux/kprobes.h>
303#include <linux/ptrace.h>
304int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
305
306regs is as described for kprobe.pre_handler. ri points to the
307kretprobe_instance object, of which the following fields may be
308of interest:
309- ret_addr: the return address
310- rp: points to the corresponding kretprobe object
311- task: points to the corresponding task struct
312- data: points to per return-instance private data; see "Kretprobe
313    entry-handler" for details.
314
315The regs_return_value(regs) macro provides a simple abstraction to
316extract the return value from the appropriate register as defined by
317the architecture's ABI.
318
319The handler's return value is currently ignored.
320
3214.4 unregister_*probe
322
323#include <linux/kprobes.h>
324void unregister_kprobe(struct kprobe *kp);
325void unregister_jprobe(struct jprobe *jp);
326void unregister_kretprobe(struct kretprobe *rp);
327
328Removes the specified probe. The unregister function can be called
329at any time after the probe has been registered.
330
331NOTE:
332If the functions find an incorrect probe (ex. an unregistered probe),
333they clear the addr field of the probe.
334
3354.5 register_*probes
336
337#include <linux/kprobes.h>
338int register_kprobes(struct kprobe **kps, int num);
339int register_kretprobes(struct kretprobe **rps, int num);
340int register_jprobes(struct jprobe **jps, int num);
341
342Registers each of the num probes in the specified array. If any
343error occurs during registration, all probes in the array, up to
344the bad probe, are safely unregistered before the register_*probes
345function returns.
346- kps/rps/jps: an array of pointers to *probe data structures
347- num: the number of the array entries.
348
349NOTE:
350You have to allocate(or define) an array of pointers and set all
351of the array entries before using these functions.
352
3534.6 unregister_*probes
354
355#include <linux/kprobes.h>
356void unregister_kprobes(struct kprobe **kps, int num);
357void unregister_kretprobes(struct kretprobe **rps, int num);
358void unregister_jprobes(struct jprobe **jps, int num);
359
360Removes each of the num probes in the specified array at once.
361
362NOTE:
363If the functions find some incorrect probes (ex. unregistered
364probes) in the specified array, they clear the addr field of those
365incorrect probes. However, other probes in the array are
366unregistered correctly.
367
3684.7 disable_*probe
369
370#include <linux/kprobes.h>
371int disable_kprobe(struct kprobe *kp);
372int disable_kretprobe(struct kretprobe *rp);
373int disable_jprobe(struct jprobe *jp);
374
375Temporarily disables the specified *probe. You can enable it again by using
376enable_*probe(). You must specify the probe which has been registered.
377
3784.8 enable_*probe
379
380#include <linux/kprobes.h>
381int enable_kprobe(struct kprobe *kp);
382int enable_kretprobe(struct kretprobe *rp);
383int enable_jprobe(struct jprobe *jp);
384
385Enables *probe which has been disabled by disable_*probe(). You must specify
386the probe which has been registered.
387
3885. Kprobes Features and Limitations
389
390Kprobes allows multiple probes at the same address. Currently,
391however, there cannot be multiple jprobes on the same function at
392the same time.
393
394In general, you can install a probe anywhere in the kernel.
395In particular, you can probe interrupt handlers. Known exceptions
396are discussed in this section.
397
398The register_*probe functions will return -EINVAL if you attempt
399to install a probe in the code that implements Kprobes (mostly
400kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
401as do_page_fault and notifier_call_chain).
402
403If you install a probe in an inline-able function, Kprobes makes
404no attempt to chase down all inline instances of the function and
405install probes there. gcc may inline a function without being asked,
406so keep this in mind if you're not seeing the probe hits you expect.
407
408A probe handler can modify the environment of the probed function
409-- e.g., by modifying kernel data structures, or by modifying the
410contents of the pt_regs struct (which are restored to the registers
411upon return from the breakpoint). So Kprobes can be used, for example,
412to install a bug fix or to inject faults for testing. Kprobes, of
413course, has no way to distinguish the deliberately injected faults
414from the accidental ones. Don't drink and probe.
415
416Kprobes makes no attempt to prevent probe handlers from stepping on
417each other -- e.g., probing printk() and then calling printk() from a
418probe handler. If a probe handler hits a probe, that second probe's
419handlers won't be run in that instance, and the kprobe.nmissed member
420of the second probe will be incremented.
421
422As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
423the same handler) may run concurrently on different CPUs.
424
425Kprobes does not use mutexes or allocate memory except during
426registration and unregistration.
427
428Probe handlers are run with preemption disabled. Depending on the
429architecture, handlers may also run with interrupts disabled. In any
430case, your handler should not yield the CPU (e.g., by attempting to
431acquire a semaphore).
432
433Since a return probe is implemented by replacing the return
434address with the trampoline's address, stack backtraces and calls
435to __builtin_return_address() will typically yield the trampoline's
436address instead of the real return address for kretprobed functions.
437(As far as we can tell, __builtin_return_address() is used only
438for instrumentation and error reporting.)
439
440If the number of times a function is called does not match the number
441of times it returns, registering a return probe on that function may
442produce undesirable results. In such a case, a line:
443kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
444gets printed. With this information, one will be able to correlate the
445exact instance of the kretprobe that caused the problem. We have the
446do_exit() case covered. do_execve() and do_fork() are not an issue.
447We're unaware of other specific cases where this could be a problem.
448
449If, upon entry to or exit from a function, the CPU is running on
450a stack other than that of the current task, registering a return
451probe on that function may produce undesirable results. For this
452reason, Kprobes doesn't support return probes (or kprobes or jprobes)
453on the x86_64 version of __switch_to(); the registration functions
454return -EINVAL.
455
4566. Probe Overhead
457
458On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
459microseconds to process. Specifically, a benchmark that hits the same
460probepoint repeatedly, firing a simple handler each time, reports 1-2
461million hits per second, depending on the architecture. A jprobe or
462return-probe hit typically takes 50-75% longer than a kprobe hit.
463When you have a return probe set on a function, adding a kprobe at
464the entry to that function adds essentially no overhead.
465
466Here are sample overhead figures (in usec) for different architectures.
467k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
468on same function; jr = jprobe + return probe on same function
469
470i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
471k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
472
473x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
474k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
475
476ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
477k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
478
4797. TODO
480
481a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
482programming interface for probe-based instrumentation. Try it out.
483b. Kernel return probes for sparc64.
484c. Support for other architectures.
485d. User-space probes.
486e. Watchpoint probes (which fire on data references).
487
4888. Kprobes Example
489
490See samples/kprobes/kprobe_example.c
491
4929. Jprobes Example
493
494See samples/kprobes/jprobe_example.c
495
49610. Kretprobes Example
497
498See samples/kprobes/kretprobe_example.c
499
500For additional information on Kprobes, refer to the following URLs:
501http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
502http://www.redhat.com/magazine/005mar05/features/kprobes/
503http://www-users.cs.umn.edu/~boutcher/kprobes/
504http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
505
506
507Appendix A: The kprobes debugfs interface
508
509With recent kernels (> 2.6.20) the list of registered kprobes is visible
510under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
511
512/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
513
514c015d71a k vfs_read+0x0
515c011a316 j do_fork+0x0
516c03dedc5 r tcp_v4_rcv+0x0
517
518The first column provides the kernel address where the probe is inserted.
519The second column identifies the type of probe (k - kprobe, r - kretprobe
520and j - jprobe), while the third column specifies the symbol+offset of
521the probe. If the probed function belongs to a module, the module name
522is also specified. Following columns show probe status. If the probe is on
523a virtual address that is no longer valid (module init sections, module
524virtual addresses that correspond to modules that've been unloaded),
525such probes are marked with [GONE]. If the probe is temporarily disabled,
526such probes are marked with [DISABLED].
527
528/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
529
530Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
531By default, all kprobes are enabled. By echoing "0" to this file, all
532registered probes will be disarmed, till such time a "1" is echoed to this
533file. Note that this knob just disarms and arms all kprobes and doesn't
534change each probe's disabling state. This means that disabled kprobes (marked
535[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
536

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