Root/
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 | - s390 |
304 | |
305 | 3. Configuring Kprobes |
306 | |
307 | When configuring the kernel using make menuconfig/xconfig/oldconfig, |
308 | ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation |
309 | Support", look for "Kprobes". |
310 | |
311 | So that you can load and unload Kprobes-based instrumentation modules, |
312 | make sure "Loadable module support" (CONFIG_MODULES) and "Module |
313 | unloading" (CONFIG_MODULE_UNLOAD) are set to "y". |
314 | |
315 | Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL |
316 | are set to "y", since kallsyms_lookup_name() is used by the in-kernel |
317 | kprobe address resolution code. |
318 | |
319 | If you need to insert a probe in the middle of a function, you may find |
320 | it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), |
321 | so you can use "objdump -d -l vmlinux" to see the source-to-object |
322 | code mapping. |
323 | |
324 | 4. API Reference |
325 | |
326 | The Kprobes API includes a "register" function and an "unregister" |
327 | function for each type of probe. The API also includes "register_*probes" |
328 | and "unregister_*probes" functions for (un)registering arrays of probes. |
329 | Here are terse, mini-man-page specifications for these functions and |
330 | the associated probe handlers that you'll write. See the files in the |
331 | samples/kprobes/ sub-directory for examples. |
332 | |
333 | 4.1 register_kprobe |
334 | |
335 | #include <linux/kprobes.h> |
336 | int register_kprobe(struct kprobe *kp); |
337 | |
338 | Sets a breakpoint at the address kp->addr. When the breakpoint is |
339 | hit, Kprobes calls kp->pre_handler. After the probed instruction |
340 | is single-stepped, Kprobe calls kp->post_handler. If a fault |
341 | occurs during execution of kp->pre_handler or kp->post_handler, |
342 | or during single-stepping of the probed instruction, Kprobes calls |
343 | kp->fault_handler. Any or all handlers can be NULL. If kp->flags |
344 | is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled, |
345 | so, its handlers aren't hit until calling enable_kprobe(kp). |
346 | |
347 | NOTE: |
348 | 1. With the introduction of the "symbol_name" field to struct kprobe, |
349 | the probepoint address resolution will now be taken care of by the kernel. |
350 | The following will now work: |
351 | |
352 | kp.symbol_name = "symbol_name"; |
353 | |
354 | (64-bit powerpc intricacies such as function descriptors are handled |
355 | transparently) |
356 | |
357 | 2. Use the "offset" field of struct kprobe if the offset into the symbol |
358 | to install a probepoint is known. This field is used to calculate the |
359 | probepoint. |
360 | |
361 | 3. Specify either the kprobe "symbol_name" OR the "addr". If both are |
362 | specified, kprobe registration will fail with -EINVAL. |
363 | |
364 | 4. With CISC architectures (such as i386 and x86_64), the kprobes code |
365 | does not validate if the kprobe.addr is at an instruction boundary. |
366 | Use "offset" with caution. |
367 | |
368 | register_kprobe() returns 0 on success, or a negative errno otherwise. |
369 | |
370 | User's pre-handler (kp->pre_handler): |
371 | #include <linux/kprobes.h> |
372 | #include <linux/ptrace.h> |
373 | int pre_handler(struct kprobe *p, struct pt_regs *regs); |
374 | |
375 | Called with p pointing to the kprobe associated with the breakpoint, |
376 | and regs pointing to the struct containing the registers saved when |
377 | the breakpoint was hit. Return 0 here unless you're a Kprobes geek. |
378 | |
379 | User's post-handler (kp->post_handler): |
380 | #include <linux/kprobes.h> |
381 | #include <linux/ptrace.h> |
382 | void post_handler(struct kprobe *p, struct pt_regs *regs, |
383 | unsigned long flags); |
384 | |
385 | p and regs are as described for the pre_handler. flags always seems |
386 | to be zero. |
387 | |
388 | User's fault-handler (kp->fault_handler): |
389 | #include <linux/kprobes.h> |
390 | #include <linux/ptrace.h> |
391 | int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); |
392 | |
393 | p and regs are as described for the pre_handler. trapnr is the |
394 | architecture-specific trap number associated with the fault (e.g., |
395 | on i386, 13 for a general protection fault or 14 for a page fault). |
396 | Returns 1 if it successfully handled the exception. |
397 | |
398 | 4.2 register_jprobe |
399 | |
400 | #include <linux/kprobes.h> |
401 | int register_jprobe(struct jprobe *jp) |
402 | |
403 | Sets a breakpoint at the address jp->kp.addr, which must be the address |
404 | of the first instruction of a function. When the breakpoint is hit, |
405 | Kprobes runs the handler whose address is jp->entry. |
406 | |
407 | The handler should have the same arg list and return type as the probed |
408 | function; and just before it returns, it must call jprobe_return(). |
409 | (The handler never actually returns, since jprobe_return() returns |
410 | control to Kprobes.) If the probed function is declared asmlinkage |
411 | or anything else that affects how args are passed, the handler's |
412 | declaration must match. |
413 | |
414 | register_jprobe() returns 0 on success, or a negative errno otherwise. |
415 | |
416 | 4.3 register_kretprobe |
417 | |
418 | #include <linux/kprobes.h> |
419 | int register_kretprobe(struct kretprobe *rp); |
420 | |
421 | Establishes a return probe for the function whose address is |
422 | rp->kp.addr. When that function returns, Kprobes calls rp->handler. |
423 | You must set rp->maxactive appropriately before you call |
424 | register_kretprobe(); see "How Does a Return Probe Work?" for details. |
425 | |
426 | register_kretprobe() returns 0 on success, or a negative errno |
427 | otherwise. |
428 | |
429 | User's return-probe handler (rp->handler): |
430 | #include <linux/kprobes.h> |
431 | #include <linux/ptrace.h> |
432 | int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); |
433 | |
434 | regs is as described for kprobe.pre_handler. ri points to the |
435 | kretprobe_instance object, of which the following fields may be |
436 | of interest: |
437 | - ret_addr: the return address |
438 | - rp: points to the corresponding kretprobe object |
439 | - task: points to the corresponding task struct |
440 | - data: points to per return-instance private data; see "Kretprobe |
441 | entry-handler" for details. |
442 | |
443 | The regs_return_value(regs) macro provides a simple abstraction to |
444 | extract the return value from the appropriate register as defined by |
445 | the architecture's ABI. |
446 | |
447 | The handler's return value is currently ignored. |
448 | |
449 | 4.4 unregister_*probe |
450 | |
451 | #include <linux/kprobes.h> |
452 | void unregister_kprobe(struct kprobe *kp); |
453 | void unregister_jprobe(struct jprobe *jp); |
454 | void unregister_kretprobe(struct kretprobe *rp); |
455 | |
456 | Removes the specified probe. The unregister function can be called |
457 | at any time after the probe has been registered. |
458 | |
459 | NOTE: |
460 | If the functions find an incorrect probe (ex. an unregistered probe), |
461 | they clear the addr field of the probe. |
462 | |
463 | 4.5 register_*probes |
464 | |
465 | #include <linux/kprobes.h> |
466 | int register_kprobes(struct kprobe **kps, int num); |
467 | int register_kretprobes(struct kretprobe **rps, int num); |
468 | int register_jprobes(struct jprobe **jps, int num); |
469 | |
470 | Registers each of the num probes in the specified array. If any |
471 | error occurs during registration, all probes in the array, up to |
472 | the bad probe, are safely unregistered before the register_*probes |
473 | function returns. |
474 | - kps/rps/jps: an array of pointers to *probe data structures |
475 | - num: the number of the array entries. |
476 | |
477 | NOTE: |
478 | You have to allocate(or define) an array of pointers and set all |
479 | of the array entries before using these functions. |
480 | |
481 | 4.6 unregister_*probes |
482 | |
483 | #include <linux/kprobes.h> |
484 | void unregister_kprobes(struct kprobe **kps, int num); |
485 | void unregister_kretprobes(struct kretprobe **rps, int num); |
486 | void unregister_jprobes(struct jprobe **jps, int num); |
487 | |
488 | Removes each of the num probes in the specified array at once. |
489 | |
490 | NOTE: |
491 | If the functions find some incorrect probes (ex. unregistered |
492 | probes) in the specified array, they clear the addr field of those |
493 | incorrect probes. However, other probes in the array are |
494 | unregistered correctly. |
495 | |
496 | 4.7 disable_*probe |
497 | |
498 | #include <linux/kprobes.h> |
499 | int disable_kprobe(struct kprobe *kp); |
500 | int disable_kretprobe(struct kretprobe *rp); |
501 | int disable_jprobe(struct jprobe *jp); |
502 | |
503 | Temporarily disables the specified *probe. You can enable it again by using |
504 | enable_*probe(). You must specify the probe which has been registered. |
505 | |
506 | 4.8 enable_*probe |
507 | |
508 | #include <linux/kprobes.h> |
509 | int enable_kprobe(struct kprobe *kp); |
510 | int enable_kretprobe(struct kretprobe *rp); |
511 | int enable_jprobe(struct jprobe *jp); |
512 | |
513 | Enables *probe which has been disabled by disable_*probe(). You must specify |
514 | the probe which has been registered. |
515 | |
516 | 5. Kprobes Features and Limitations |
517 | |
518 | Kprobes allows multiple probes at the same address. Currently, |
519 | however, there cannot be multiple jprobes on the same function at |
520 | the same time. Also, a probepoint for which there is a jprobe or |
521 | a post_handler cannot be optimized. So if you install a jprobe, |
522 | or a kprobe with a post_handler, at an optimized probepoint, the |
523 | probepoint will be unoptimized automatically. |
524 | |
525 | In general, you can install a probe anywhere in the kernel. |
526 | In particular, you can probe interrupt handlers. Known exceptions |
527 | are discussed in this section. |
528 | |
529 | The register_*probe functions will return -EINVAL if you attempt |
530 | to install a probe in the code that implements Kprobes (mostly |
531 | kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such |
532 | as do_page_fault and notifier_call_chain). |
533 | |
534 | If you install a probe in an inline-able function, Kprobes makes |
535 | no attempt to chase down all inline instances of the function and |
536 | install probes there. gcc may inline a function without being asked, |
537 | so keep this in mind if you're not seeing the probe hits you expect. |
538 | |
539 | A probe handler can modify the environment of the probed function |
540 | -- e.g., by modifying kernel data structures, or by modifying the |
541 | contents of the pt_regs struct (which are restored to the registers |
542 | upon return from the breakpoint). So Kprobes can be used, for example, |
543 | to install a bug fix or to inject faults for testing. Kprobes, of |
544 | course, has no way to distinguish the deliberately injected faults |
545 | from the accidental ones. Don't drink and probe. |
546 | |
547 | Kprobes makes no attempt to prevent probe handlers from stepping on |
548 | each other -- e.g., probing printk() and then calling printk() from a |
549 | probe handler. If a probe handler hits a probe, that second probe's |
550 | handlers won't be run in that instance, and the kprobe.nmissed member |
551 | of the second probe will be incremented. |
552 | |
553 | As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of |
554 | the same handler) may run concurrently on different CPUs. |
555 | |
556 | Kprobes does not use mutexes or allocate memory except during |
557 | registration and unregistration. |
558 | |
559 | Probe handlers are run with preemption disabled. Depending on the |
560 | architecture and optimization state, handlers may also run with |
561 | interrupts disabled (e.g., kretprobe handlers and optimized kprobe |
562 | handlers run without interrupt disabled on x86/x86-64). In any case, |
563 | your handler should not yield the CPU (e.g., by attempting to acquire |
564 | a semaphore). |
565 | |
566 | Since a return probe is implemented by replacing the return |
567 | address with the trampoline's address, stack backtraces and calls |
568 | to __builtin_return_address() will typically yield the trampoline's |
569 | address instead of the real return address for kretprobed functions. |
570 | (As far as we can tell, __builtin_return_address() is used only |
571 | for instrumentation and error reporting.) |
572 | |
573 | If the number of times a function is called does not match the number |
574 | of times it returns, registering a return probe on that function may |
575 | produce undesirable results. In such a case, a line: |
576 | kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c |
577 | gets printed. With this information, one will be able to correlate the |
578 | exact instance of the kretprobe that caused the problem. We have the |
579 | do_exit() case covered. do_execve() and do_fork() are not an issue. |
580 | We're unaware of other specific cases where this could be a problem. |
581 | |
582 | If, upon entry to or exit from a function, the CPU is running on |
583 | a stack other than that of the current task, registering a return |
584 | probe on that function may produce undesirable results. For this |
585 | reason, Kprobes doesn't support return probes (or kprobes or jprobes) |
586 | on the x86_64 version of __switch_to(); the registration functions |
587 | return -EINVAL. |
588 | |
589 | On x86/x86-64, since the Jump Optimization of Kprobes modifies |
590 | instructions widely, there are some limitations to optimization. To |
591 | explain it, we introduce some terminology. Imagine a 3-instruction |
592 | sequence consisting of a two 2-byte instructions and one 3-byte |
593 | instruction. |
594 | |
595 | IA |
596 | | |
597 | [-2][-1][0][1][2][3][4][5][6][7] |
598 | [ins1][ins2][ ins3 ] |
599 | [<- DCR ->] |
600 | [<- JTPR ->] |
601 | |
602 | ins1: 1st Instruction |
603 | ins2: 2nd Instruction |
604 | ins3: 3rd Instruction |
605 | IA: Insertion Address |
606 | JTPR: Jump Target Prohibition Region |
607 | DCR: Detoured Code Region |
608 | |
609 | The instructions in DCR are copied to the out-of-line buffer |
610 | of the kprobe, because the bytes in DCR are replaced by |
611 | a 5-byte jump instruction. So there are several limitations. |
612 | |
613 | a) The instructions in DCR must be relocatable. |
614 | b) The instructions in DCR must not include a call instruction. |
615 | c) JTPR must not be targeted by any jump or call instruction. |
616 | d) DCR must not straddle the border between functions. |
617 | |
618 | Anyway, these limitations are checked by the in-kernel instruction |
619 | decoder, so you don't need to worry about that. |
620 | |
621 | 6. Probe Overhead |
622 | |
623 | On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 |
624 | microseconds to process. Specifically, a benchmark that hits the same |
625 | probepoint repeatedly, firing a simple handler each time, reports 1-2 |
626 | million hits per second, depending on the architecture. A jprobe or |
627 | return-probe hit typically takes 50-75% longer than a kprobe hit. |
628 | When you have a return probe set on a function, adding a kprobe at |
629 | the entry to that function adds essentially no overhead. |
630 | |
631 | Here are sample overhead figures (in usec) for different architectures. |
632 | k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe |
633 | on same function; jr = jprobe + return probe on same function |
634 | |
635 | i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips |
636 | k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 |
637 | |
638 | x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips |
639 | k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 |
640 | |
641 | ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) |
642 | k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 |
643 | |
644 | 6.1 Optimized Probe Overhead |
645 | |
646 | Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to |
647 | process. Here are sample overhead figures (in usec) for x86 architectures. |
648 | k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe, |
649 | r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe. |
650 | |
651 | i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips |
652 | k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33 |
653 | |
654 | x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips |
655 | k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30 |
656 | |
657 | 7. TODO |
658 | |
659 | a. SystemTap (http://sourceware.org/systemtap): Provides a simplified |
660 | programming interface for probe-based instrumentation. Try it out. |
661 | b. Kernel return probes for sparc64. |
662 | c. Support for other architectures. |
663 | d. User-space probes. |
664 | e. Watchpoint probes (which fire on data references). |
665 | |
666 | 8. Kprobes Example |
667 | |
668 | See samples/kprobes/kprobe_example.c |
669 | |
670 | 9. Jprobes Example |
671 | |
672 | See samples/kprobes/jprobe_example.c |
673 | |
674 | 10. Kretprobes Example |
675 | |
676 | See samples/kprobes/kretprobe_example.c |
677 | |
678 | For additional information on Kprobes, refer to the following URLs: |
679 | http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe |
680 | http://www.redhat.com/magazine/005mar05/features/kprobes/ |
681 | http://www-users.cs.umn.edu/~boutcher/kprobes/ |
682 | http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) |
683 | |
684 | |
685 | Appendix A: The kprobes debugfs interface |
686 | |
687 | With recent kernels (> 2.6.20) the list of registered kprobes is visible |
688 | under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug). |
689 | |
690 | /sys/kernel/debug/kprobes/list: Lists all registered probes on the system |
691 | |
692 | c015d71a k vfs_read+0x0 |
693 | c011a316 j do_fork+0x0 |
694 | c03dedc5 r tcp_v4_rcv+0x0 |
695 | |
696 | The first column provides the kernel address where the probe is inserted. |
697 | The second column identifies the type of probe (k - kprobe, r - kretprobe |
698 | and j - jprobe), while the third column specifies the symbol+offset of |
699 | the probe. If the probed function belongs to a module, the module name |
700 | is also specified. Following columns show probe status. If the probe is on |
701 | a virtual address that is no longer valid (module init sections, module |
702 | virtual addresses that correspond to modules that've been unloaded), |
703 | such probes are marked with [GONE]. If the probe is temporarily disabled, |
704 | such probes are marked with [DISABLED]. If the probe is optimized, it is |
705 | marked with [OPTIMIZED]. |
706 | |
707 | /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. |
708 | |
709 | Provides a knob to globally and forcibly turn registered kprobes ON or OFF. |
710 | By default, all kprobes are enabled. By echoing "0" to this file, all |
711 | registered probes will be disarmed, till such time a "1" is echoed to this |
712 | file. Note that this knob just disarms and arms all kprobes and doesn't |
713 | change each probe's disabling state. This means that disabled kprobes (marked |
714 | [DISABLED]) will be not enabled if you turn ON all kprobes by this knob. |
715 | |
716 | |
717 | Appendix B: The kprobes sysctl interface |
718 | |
719 | /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF. |
720 | |
721 | When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides |
722 | a knob to globally and forcibly turn jump optimization (see section |
723 | 1.4) ON or OFF. By default, jump optimization is allowed (ON). |
724 | If you echo "0" to this file or set "debug.kprobes_optimization" to |
725 | 0 via sysctl, all optimized probes will be unoptimized, and any new |
726 | probes registered after that will not be optimized. Note that this |
727 | knob *changes* the optimized state. This means that optimized probes |
728 | (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be |
729 | removed). If the knob is turned on, they will be optimized again. |
730 | |
731 |
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