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2010-03-04 20:29:02

1	Title	: Kernel Probes (Kprobes)
2 Authors : Jim Keniston
3 : Prasanna S Panchamukhi
7 1. Concepts: Kprobes, Jprobes, Return Probes
8 2. Architectures Supported
9 3. Configuring Kprobes
10 4. API Reference
11 5. Kprobes Features and Limitations
12 6. Probe Overhead
13 7. TODO
14 8. Kprobes Example
15 9. Jprobes Example
16 10. Kretprobes Example
17 Appendix A: The kprobes debugfs interface
19 1. Concepts: Kprobes, Jprobes, Return Probes
21 Kprobes enables you to dynamically break into any kernel routine and
22 collect debugging and performance information non-disruptively. You
23 can trap at almost any kernel code address, specifying a handler
24 routine to be invoked when the breakpoint is hit.
26 There are currently three types of probes: kprobes, jprobes, and
27 kretprobes (also called return probes). A kprobe can be inserted
28 on virtually any instruction in the kernel. A jprobe is inserted at
29 the entry to a kernel function, and provides convenient access to the
30 function's arguments. A return probe fires when a specified function
31 returns.
33 In the typical case, Kprobes-based instrumentation is packaged as
34 a kernel module. The module's init function installs ("registers")
35 one or more probes, and the exit function unregisters them. A
36 registration function such as register_kprobe() specifies where
37 the probe is to be inserted and what handler is to be called when
38 the probe is hit.
40 There are also register_/unregister_*probes() functions for batch
41 registration/unregistration of a group of *probes. These functions
42 can speed up unregistration process when you have to unregister
43 a lot of probes at once.
45 The next three subsections explain how the different types of
46 probes work. They explain certain things that you'll need to
47 know in order to make the best use of Kprobes -- e.g., the
48 difference between a pre_handler and a post_handler, and how
49 to use the maxactive and nmissed fields of a kretprobe. But
50 if you're in a hurry to start using Kprobes, you can skip ahead
51 to section 2.
53 1.1 How Does a Kprobe Work?
55 When a kprobe is registered, Kprobes makes a copy of the probed
56 instruction and replaces the first byte(s) of the probed instruction
57 with a breakpoint instruction (e.g., int3 on i386 and x86_64).
59 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
60 registers are saved, and control passes to Kprobes via the
61 notifier_call_chain mechanism. Kprobes executes the "pre_handler"
62 associated with the kprobe, passing the handler the addresses of the
63 kprobe struct and the saved registers.
65 Next, Kprobes single-steps its copy of the probed instruction.
66 (It would be simpler to single-step the actual instruction in place,
67 but then Kprobes would have to temporarily remove the breakpoint
68 instruction. This would open a small time window when another CPU
69 could sail right past the probepoint.)
71 After the instruction is single-stepped, Kprobes executes the
72 "post_handler," if any, that is associated with the kprobe.
73 Execution then continues with the instruction following the probepoint.
75 1.2 How Does a Jprobe Work?
77 A jprobe is implemented using a kprobe that is placed on a function's
78 entry point. It employs a simple mirroring principle to allow
79 seamless access to the probed function's arguments. The jprobe
80 handler routine should have the same signature (arg list and return
81 type) as the function being probed, and must always end by calling
82 the Kprobes function jprobe_return().
84 Here's how it works. When the probe is hit, Kprobes makes a copy of
85 the saved registers and a generous portion of the stack (see below).
86 Kprobes then points the saved instruction pointer at the jprobe's
87 handler routine, and returns from the trap. As a result, control
88 passes to the handler, which is presented with the same register and
89 stack contents as the probed function. When it is done, the handler
90 calls jprobe_return(), which traps again to restore the original stack
91 contents and processor state and switch to the probed function.
93 By convention, the callee owns its arguments, so gcc may produce code
94 that unexpectedly modifies that portion of the stack. This is why
95 Kprobes saves a copy of the stack and restores it after the jprobe
96 handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
97 64 bytes on i386.
99 Note that the probed function's args may be passed on the stack
100 or in registers. The jprobe will work in either case, so long as the
101 handler's prototype matches that of the probed function.
103 1.3 Return Probes
105 1.3.1 How Does a Return Probe Work?
107 When you call register_kretprobe(), Kprobes establishes a kprobe at
108 the entry to the function. When the probed function is called and this
109 probe is hit, Kprobes saves a copy of the return address, and replaces
110 the return address with the address of a "trampoline." The trampoline
111 is an arbitrary piece of code -- typically just a nop instruction.
112 At boot time, Kprobes registers a kprobe at the trampoline.
114 When the probed function executes its return instruction, control
115 passes to the trampoline and that probe is hit. Kprobes' trampoline
116 handler calls the user-specified return handler associated with the
117 kretprobe, then sets the saved instruction pointer to the saved return
118 address, and that's where execution resumes upon return from the trap.
120 While the probed function is executing, its return address is
121 stored in an object of type kretprobe_instance. Before calling
122 register_kretprobe(), the user sets the maxactive field of the
123 kretprobe struct to specify how many instances of the specified
124 function can be probed simultaneously. register_kretprobe()
125 pre-allocates the indicated number of kretprobe_instance objects.
127 For example, if the function is non-recursive and is called with a
128 spinlock held, maxactive = 1 should be enough. If the function is
129 non-recursive and can never relinquish the CPU (e.g., via a semaphore
130 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
131 set to a default value. If CONFIG_PREEMPT is enabled, the default
132 is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
134 It's not a disaster if you set maxactive too low; you'll just miss
135 some probes. In the kretprobe struct, the nmissed field is set to
136 zero when the return probe is registered, and is incremented every
137 time the probed function is entered but there is no kretprobe_instance
138 object available for establishing the return probe.
140 1.3.2 Kretprobe entry-handler
142 Kretprobes also provides an optional user-specified handler which runs
143 on function entry. This handler is specified by setting the entry_handler
144 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
145 function entry is hit, the user-defined entry_handler, if any, is invoked.
146 If the entry_handler returns 0 (success) then a corresponding return handler
147 is guaranteed to be called upon function return. If the entry_handler
148 returns a non-zero error then Kprobes leaves the return address as is, and
149 the kretprobe has no further effect for that particular function instance.
151 Multiple entry and return handler invocations are matched using the unique
152 kretprobe_instance object associated with them. Additionally, a user
153 may also specify per return-instance private data to be part of each
154 kretprobe_instance object. This is especially useful when sharing private
155 data between corresponding user entry and return handlers. The size of each
156 private data object can be specified at kretprobe registration time by
157 setting the data_size field of the kretprobe struct. This data can be
158 accessed through the data field of each kretprobe_instance object.
160 In case probed function is entered but there is no kretprobe_instance
161 object available, then in addition to incrementing the nmissed count,
162 the user entry_handler invocation is also skipped.
164 2. Architectures Supported
166 Kprobes, jprobes, and return probes are implemented on the following
167 architectures:
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
177 3. Configuring Kprobes
179 When configuring the kernel using make menuconfig/xconfig/oldconfig,
180 ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation
181 Support", look for "Kprobes".
183 So that you can load and unload Kprobes-based instrumentation modules,
184 make sure "Loadable module support" (CONFIG_MODULES) and "Module
185 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
187 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
188 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
189 kprobe address resolution code.
191 If you need to insert a probe in the middle of a function, you may find
192 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
193 so you can use "objdump -d -l vmlinux" to see the source-to-object
194 code mapping.
196 4. API Reference
198 The Kprobes API includes a "register" function and an "unregister"
199 function for each type of probe. The API also includes "register_*probes"
200 and "unregister_*probes" functions for (un)registering arrays of probes.
201 Here are terse, mini-man-page specifications for these functions and
202 the associated probe handlers that you'll write. See the files in the
203 samples/kprobes/ sub-directory for examples.
205 4.1 register_kprobe
207 #include
208 int register_kprobe(struct kprobe *kp);
210 Sets a breakpoint at the address kp->addr. When the breakpoint is
211 hit, Kprobes calls kp->pre_handler. After the probed instruction
212 is single-stepped, Kprobe calls kp->post_handler. If a fault
213 occurs during execution of kp->pre_handler or kp->post_handler,
214 or during single-stepping of the probed instruction, Kprobes calls
215 kp->fault_handler. Any or all handlers can be NULL. If kp->flags
216 is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
217 so, it's handlers aren't hit until calling enable_kprobe(kp).
219 NOTE:
220 1. With the introduction of the "symbol_name" field to struct kprobe,
221 the probepoint address resolution will now be taken care of by the kernel.
222 The following will now work:
224 kp.symbol_name = "symbol_name";
226 (64-bit powerpc intricacies such as function descriptors are handled
227 transparently)
229 2. Use the "offset" field of struct kprobe if the offset into the symbol
230 to install a probepoint is known. This field is used to calculate the
231 probepoint.
233 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
234 specified, kprobe registration will fail with -EINVAL.
236 4. With CISC architectures (such as i386 and x86_64), the kprobes code
237 does not validate if the kprobe.addr is at an instruction boundary.
238 Use "offset" with caution.
240 register_kprobe() returns 0 on success, or a negative errno otherwise.
242 User's pre-handler (kp->pre_handler):
243 #include
244 #include
245 int pre_handler(struct kprobe *p, struct pt_regs *regs);
247 Called with p pointing to the kprobe associated with the breakpoint,
248 and regs pointing to the struct containing the registers saved when
249 the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
251 User's post-handler (kp->post_handler):
252 #include
253 #include
254 void post_handler(struct kprobe *p, struct pt_regs *regs,
255 unsigned long flags);
257 p and regs are as described for the pre_handler. flags always seems
258 to be zero.
260 User's fault-handler (kp->fault_handler):
261 #include
262 #include
263 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
265 p and regs are as described for the pre_handler. trapnr is the
266 architecture-specific trap number associated with the fault (e.g.,
267 on i386, 13 for a general protection fault or 14 for a page fault).
268 Returns 1 if it successfully handled the exception.
270 4.2 register_jprobe
272 #include
273 int register_jprobe(struct jprobe *jp)
275 Sets a breakpoint at the address jp->kp.addr, which must be the address
276 of the first instruction of a function. When the breakpoint is hit,
277 Kprobes runs the handler whose address is jp->entry.
279 The handler should have the same arg list and return type as the probed
280 function; and just before it returns, it must call jprobe_return().
281 (The handler never actually returns, since jprobe_return() returns
282 control to Kprobes.) If the probed function is declared asmlinkage
283 or anything else that affects how args are passed, the handler's
284 declaration must match.
286 register_jprobe() returns 0 on success, or a negative errno otherwise.
288 4.3 register_kretprobe
290 #include
291 int register_kretprobe(struct kretprobe *rp);
293 Establishes a return probe for the function whose address is
294 rp->kp.addr. When that function returns, Kprobes calls rp->handler.
295 You must set rp->maxactive appropriately before you call
296 register_kretprobe(); see "How Does a Return Probe Work?" for details.
298 register_kretprobe() returns 0 on success, or a negative errno
299 otherwise.
301 User's return-probe handler (rp->handler):
302 #include
303 #include
304 int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
306 regs is as described for kprobe.pre_handler. ri points to the
307 kretprobe_instance object, of which the following fields may be
308 of 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.
315 The regs_return_value(regs) macro provides a simple abstraction to
316 extract the return value from the appropriate register as defined by
317 the architecture's ABI.
319 The handler's return value is currently ignored.
321 4.4 unregister_*probe
323 #include
324 void unregister_kprobe(struct kprobe *kp);
325 void unregister_jprobe(struct jprobe *jp);
326 void unregister_kretprobe(struct kretprobe *rp);
328 Removes the specified probe. The unregister function can be called
329 at any time after the probe has been registered.
331 NOTE:
332 If the functions find an incorrect probe (ex. an unregistered probe),
333 they clear the addr field of the probe.
335 4.5 register_*probes
337 #include
338 int register_kprobes(struct kprobe **kps, int num);
339 int register_kretprobes(struct kretprobe **rps, int num);
340 int register_jprobes(struct jprobe **jps, int num);
342 Registers each of the num probes in the specified array. If any
343 error occurs during registration, all probes in the array, up to
344 the bad probe, are safely unregistered before the register_*probes
345 function returns.
346 - kps/rps/jps: an array of pointers to *probe data structures
347 - num: the number of the array entries.
349 NOTE:
350 You have to allocate(or define) an array of pointers and set all
351 of the array entries before using these functions.
353 4.6 unregister_*probes
355 #include
356 void unregister_kprobes(struct kprobe **kps, int num);
357 void unregister_kretprobes(struct kretprobe **rps, int num);
358 void unregister_jprobes(struct jprobe **jps, int num);
360 Removes each of the num probes in the specified array at once.
362 NOTE:
363 If the functions find some incorrect probes (ex. unregistered
364 probes) in the specified array, they clear the addr field of those
365 incorrect probes. However, other probes in the array are
366 unregistered correctly.
368 4.7 disable_*probe
370 #include
371 int disable_kprobe(struct kprobe *kp);
372 int disable_kretprobe(struct kretprobe *rp);
373 int disable_jprobe(struct jprobe *jp);
375 Temporarily disables the specified *probe. You can enable it again by using
376 enable_*probe(). You must specify the probe which has been registered.
378 4.8 enable_*probe
380 #include
381 int enable_kprobe(struct kprobe *kp);
382 int enable_kretprobe(struct kretprobe *rp);
383 int enable_jprobe(struct jprobe *jp);
385 Enables *probe which has been disabled by disable_*probe(). You must specify
386 the probe which has been registered.
388 5. Kprobes Features and Limitations
390 Kprobes allows multiple probes at the same address. Currently,
391 however, there cannot be multiple jprobes on the same function at
392 the same time.
394 In general, you can install a probe anywhere in the kernel.
395 In particular, you can probe interrupt handlers. Known exceptions
396 are discussed in this section.
398 The register_*probe functions will return -EINVAL if you attempt
399 to install a probe in the code that implements Kprobes (mostly
400 kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
401 as do_page_fault and notifier_call_chain).
403 If you install a probe in an inline-able function, Kprobes makes
404 no attempt to chase down all inline instances of the function and
405 install probes there. gcc may inline a function without being asked,
406 so keep this in mind if you're not seeing the probe hits you expect.
408 A probe handler can modify the environment of the probed function
409 -- e.g., by modifying kernel data structures, or by modifying the
410 contents of the pt_regs struct (which are restored to the registers
411 upon return from the breakpoint). So Kprobes can be used, for example,
412 to install a bug fix or to inject faults for testing. Kprobes, of
413 course, has no way to distinguish the deliberately injected faults
414 from the accidental ones. Don't drink and probe.
416 Kprobes makes no attempt to prevent probe handlers from stepping on
417 each other -- e.g., probing printk() and then calling printk() from a
418 probe handler. If a probe handler hits a probe, that second probe's
419 handlers won't be run in that instance, and the kprobe.nmissed member
420 of the second probe will be incremented.
422 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
423 the same handler) may run concurrently on different CPUs.
425 Kprobes does not use mutexes or allocate memory except during
426 registration and unregistration.
428 Probe handlers are run with preemption disabled. Depending on the
429 architecture, handlers may also run with interrupts disabled. In any
430 case, your handler should not yield the CPU (e.g., by attempting to
431 acquire a semaphore).
433 Since a return probe is implemented by replacing the return
434 address with the trampoline's address, stack backtraces and calls
435 to __builtin_return_address() will typically yield the trampoline's
436 address instead of the real return address for kretprobed functions.
437 (As far as we can tell, __builtin_return_address() is used only
438 for instrumentation and error reporting.)
440 If the number of times a function is called does not match the number
441 of times it returns, registering a return probe on that function may
442 produce undesirable results. In such a case, a line:
443 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
444 gets printed. With this information, one will be able to correlate the
445 exact instance of the kretprobe that caused the problem. We have the
446 do_exit() case covered. do_execve() and do_fork() are not an issue.
447 We're unaware of other specific cases where this could be a problem.
449 If, upon entry to or exit from a function, the CPU is running on
450 a stack other than that of the current task, registering a return
451 probe on that function may produce undesirable results. For this
452 reason, Kprobes doesn't support return probes (or kprobes or jprobes)
453 on the x86_64 version of __switch_to(); the registration functions
454 return -EINVAL.
456 6. Probe Overhead
458 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
459 microseconds to process. Specifically, a benchmark that hits the same
460 probepoint repeatedly, firing a simple handler each time, reports 1-2
461 million hits per second, depending on the architecture. A jprobe or
462 return-probe hit typically takes 50-75% longer than a kprobe hit.
463 When you have a return probe set on a function, adding a kprobe at
464 the entry to that function adds essentially no overhead.
466 Here are sample overhead figures (in usec) for different architectures.
467 k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
468 on same function; jr = jprobe + return probe on same function
470 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
471 k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
473 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
474 k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
476 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
477 k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
479 7. TODO
481 a. SystemTap ( Provides a simplified
482 programming interface for probe-based instrumentation. Try it out.
483 b. Kernel return probes for sparc64.
484 c. Support for other architectures.
485 d. User-space probes.
486 e. Watchpoint probes (which fire on data references).
488 8. Kprobes Example
490 See samples/kprobes/kprobe_example.c
492 9. Jprobes Example
494 See samples/kprobes/jprobe_example.c
496 10. Kretprobes Example
498 See samples/kprobes/kretprobe_example.c
500 For additional information on Kprobes, refer to the following URLs:
504 (pages 101-115)
507 Appendix A: The kprobes debugfs interface
509 With recent kernels (> 2.6.20) the list of registered kprobes is visible
510 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
512 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system
514 c015d71a k vfs_read+0x0
515 c011a316 j do_fork+0x0
516 c03dedc5 r tcp_v4_rcv+0x0
518 The first column provides the kernel address where the probe is inserted.
519 The second column identifies the type of probe (k - kprobe, r - kretprobe
520 and j - jprobe), while the third column specifies the symbol+offset of
521 the probe. If the probed function belongs to a module, the module name
522 is also specified. Following columns show probe status. If the probe is on
523 a virtual address that is no longer valid (module init sections, module
524 virtual addresses that correspond to modules that've been unloaded),
525 such probes are marked with [GONE]. If the probe is temporarily disabled,
526 such probes are marked with [DISABLED].
528 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
530 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
531 By default, all kprobes are enabled. By echoing "0" to this file, all
532 registered probes will be disarmed, till such time a "1" is echoed to this
533 file. Note that this knob just disarms and arms all kprobes and doesn't
534 change 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.
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