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Linux/Documentation/kprobes.txt

  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 Note that in some architectures (e.g.: arm64 and sparc64) the stack
107 copy is not done, as the actual location of stacked parameters may be
108 outside of a reasonable MAX_STACK_SIZE value and because that location
109 cannot be determined by the jprobes code. In this case the jprobes
110 user must be careful to make certain the calling signature of the
111 function does not cause parameters to be passed on the stack (e.g.:
112 more than eight function arguments, an argument of more than sixteen
113 bytes, or more than 64 bytes of argument data, depending on
114 architecture).
115 
116 1.3 Return Probes
117 
118 1.3.1 How Does a Return Probe Work?
119 
120 When you call register_kretprobe(), Kprobes establishes a kprobe at
121 the entry to the function.  When the probed function is called and this
122 probe is hit, Kprobes saves a copy of the return address, and replaces
123 the return address with the address of a "trampoline."  The trampoline
124 is an arbitrary piece of code -- typically just a nop instruction.
125 At boot time, Kprobes registers a kprobe at the trampoline.
126 
127 When the probed function executes its return instruction, control
128 passes to the trampoline and that probe is hit.  Kprobes' trampoline
129 handler calls the user-specified return handler associated with the
130 kretprobe, then sets the saved instruction pointer to the saved return
131 address, and that's where execution resumes upon return from the trap.
132 
133 While the probed function is executing, its return address is
134 stored in an object of type kretprobe_instance.  Before calling
135 register_kretprobe(), the user sets the maxactive field of the
136 kretprobe struct to specify how many instances of the specified
137 function can be probed simultaneously.  register_kretprobe()
138 pre-allocates the indicated number of kretprobe_instance objects.
139 
140 For example, if the function is non-recursive and is called with a
141 spinlock held, maxactive = 1 should be enough.  If the function is
142 non-recursive and can never relinquish the CPU (e.g., via a semaphore
143 or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
144 set to a default value.  If CONFIG_PREEMPT is enabled, the default
145 is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
146 
147 It's not a disaster if you set maxactive too low; you'll just miss
148 some probes.  In the kretprobe struct, the nmissed field is set to
149 zero when the return probe is registered, and is incremented every
150 time the probed function is entered but there is no kretprobe_instance
151 object available for establishing the return probe.
152 
153 1.3.2 Kretprobe entry-handler
154 
155 Kretprobes also provides an optional user-specified handler which runs
156 on function entry. This handler is specified by setting the entry_handler
157 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
158 function entry is hit, the user-defined entry_handler, if any, is invoked.
159 If the entry_handler returns 0 (success) then a corresponding return handler
160 is guaranteed to be called upon function return. If the entry_handler
161 returns a non-zero error then Kprobes leaves the return address as is, and
162 the kretprobe has no further effect for that particular function instance.
163 
164 Multiple entry and return handler invocations are matched using the unique
165 kretprobe_instance object associated with them. Additionally, a user
166 may also specify per return-instance private data to be part of each
167 kretprobe_instance object. This is especially useful when sharing private
168 data between corresponding user entry and return handlers. The size of each
169 private data object can be specified at kretprobe registration time by
170 setting the data_size field of the kretprobe struct. This data can be
171 accessed through the data field of each kretprobe_instance object.
172 
173 In case probed function is entered but there is no kretprobe_instance
174 object available, then in addition to incrementing the nmissed count,
175 the user entry_handler invocation is also skipped.
176 
177 1.4 How Does Jump Optimization Work?
178 
179 If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
180 is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
181 the "debug.kprobes_optimization" kernel parameter is set to 1 (see
182 sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
183 instruction instead of a breakpoint instruction at each probepoint.
184 
185 1.4.1 Init a Kprobe
186 
187 When a probe is registered, before attempting this optimization,
188 Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
189 address. So, even if it's not possible to optimize this particular
190 probepoint, there'll be a probe there.
191 
192 1.4.2 Safety Check
193 
194 Before optimizing a probe, Kprobes performs the following safety checks:
195 
196 - Kprobes verifies that the region that will be replaced by the jump
197 instruction (the "optimized region") lies entirely within one function.
198 (A jump instruction is multiple bytes, and so may overlay multiple
199 instructions.)
200 
201 - Kprobes analyzes the entire function and verifies that there is no
202 jump into the optimized region.  Specifically:
203   - the function contains no indirect jump;
204   - the function contains no instruction that causes an exception (since
205   the fixup code triggered by the exception could jump back into the
206   optimized region -- Kprobes checks the exception tables to verify this);
207   and
208   - there is no near jump to the optimized region (other than to the first
209   byte).
210 
211 - For each instruction in the optimized region, Kprobes verifies that
212 the instruction can be executed out of line.
213 
214 1.4.3 Preparing Detour Buffer
215 
216 Next, Kprobes prepares a "detour" buffer, which contains the following
217 instruction sequence:
218 - code to push the CPU's registers (emulating a breakpoint trap)
219 - a call to the trampoline code which calls user's probe handlers.
220 - code to restore registers
221 - the instructions from the optimized region
222 - a jump back to the original execution path.
223 
224 1.4.4 Pre-optimization
225 
226 After preparing the detour buffer, Kprobes verifies that none of the
227 following situations exist:
228 - The probe has either a break_handler (i.e., it's a jprobe) or a
229 post_handler.
230 - Other instructions in the optimized region are probed.
231 - The probe is disabled.
232 In any of the above cases, Kprobes won't start optimizing the probe.
233 Since these are temporary situations, Kprobes tries to start
234 optimizing it again if the situation is changed.
235 
236 If the kprobe can be optimized, Kprobes enqueues the kprobe to an
237 optimizing list, and kicks the kprobe-optimizer workqueue to optimize
238 it.  If the to-be-optimized probepoint is hit before being optimized,
239 Kprobes returns control to the original instruction path by setting
240 the CPU's instruction pointer to the copied code in the detour buffer
241 -- thus at least avoiding the single-step.
242 
243 1.4.5 Optimization
244 
245 The Kprobe-optimizer doesn't insert the jump instruction immediately;
246 rather, it calls synchronize_sched() for safety first, because it's
247 possible for a CPU to be interrupted in the middle of executing the
248 optimized region(*).  As you know, synchronize_sched() can ensure
249 that all interruptions that were active when synchronize_sched()
250 was called are done, but only if CONFIG_PREEMPT=n.  So, this version
251 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
252 
253 After that, the Kprobe-optimizer calls stop_machine() to replace
254 the optimized region with a jump instruction to the detour buffer,
255 using text_poke_smp().
256 
257 1.4.6 Unoptimization
258 
259 When an optimized kprobe is unregistered, disabled, or blocked by
260 another kprobe, it will be unoptimized.  If this happens before
261 the optimization is complete, the kprobe is just dequeued from the
262 optimized list.  If the optimization has been done, the jump is
263 replaced with the original code (except for an int3 breakpoint in
264 the first byte) by using text_poke_smp().
265 
266 (*)Please imagine that the 2nd instruction is interrupted and then
267 the optimizer replaces the 2nd instruction with the jump *address*
268 while the interrupt handler is running. When the interrupt
269 returns to original address, there is no valid instruction,
270 and it causes an unexpected result.
271 
272 (**)This optimization-safety checking may be replaced with the
273 stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
274 kernel.
275 
276 NOTE for geeks:
277 The jump optimization changes the kprobe's pre_handler behavior.
278 Without optimization, the pre_handler can change the kernel's execution
279 path by changing regs->ip and returning 1.  However, when the probe
280 is optimized, that modification is ignored.  Thus, if you want to
281 tweak the kernel's execution path, you need to suppress optimization,
282 using one of the following techniques:
283 - Specify an empty function for the kprobe's post_handler or break_handler.
284  or
285 - Execute 'sysctl -w debug.kprobes_optimization=n'
286 
287 1.5 Blacklist
288 
289 Kprobes can probe most of the kernel except itself. This means
290 that there are some functions where kprobes cannot probe. Probing
291 (trapping) such functions can cause a recursive trap (e.g. double
292 fault) or the nested probe handler may never be called.
293 Kprobes manages such functions as a blacklist.
294 If you want to add a function into the blacklist, you just need
295 to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
296 to specify a blacklisted function.
297 Kprobes checks the given probe address against the blacklist and
298 rejects registering it, if the given address is in the blacklist.
299 
300 2. Architectures Supported
301 
302 Kprobes, jprobes, and return probes are implemented on the following
303 architectures:
304 
305 - i386 (Supports jump optimization)
306 - x86_64 (AMD-64, EM64T) (Supports jump optimization)
307 - ppc64
308 - ia64 (Does not support probes on instruction slot1.)
309 - sparc64 (Return probes not yet implemented.)
310 - arm
311 - ppc
312 - mips
313 - s390
314 
315 3. Configuring Kprobes
316 
317 When configuring the kernel using make menuconfig/xconfig/oldconfig,
318 ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
319 for "Kprobes".
320 
321 So that you can load and unload Kprobes-based instrumentation modules,
322 make sure "Loadable module support" (CONFIG_MODULES) and "Module
323 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
324 
325 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
326 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
327 kprobe address resolution code.
328 
329 If you need to insert a probe in the middle of a function, you may find
330 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
331 so you can use "objdump -d -l vmlinux" to see the source-to-object
332 code mapping.
333 
334 4. API Reference
335 
336 The Kprobes API includes a "register" function and an "unregister"
337 function for each type of probe. The API also includes "register_*probes"
338 and "unregister_*probes" functions for (un)registering arrays of probes.
339 Here are terse, mini-man-page specifications for these functions and
340 the associated probe handlers that you'll write. See the files in the
341 samples/kprobes/ sub-directory for examples.
342 
343 4.1 register_kprobe
344 
345 #include <linux/kprobes.h>
346 int register_kprobe(struct kprobe *kp);
347 
348 Sets a breakpoint at the address kp->addr.  When the breakpoint is
349 hit, Kprobes calls kp->pre_handler.  After the probed instruction
350 is single-stepped, Kprobe calls kp->post_handler.  If a fault
351 occurs during execution of kp->pre_handler or kp->post_handler,
352 or during single-stepping of the probed instruction, Kprobes calls
353 kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
354 is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
355 so, its handlers aren't hit until calling enable_kprobe(kp).
356 
357 NOTE:
358 1. With the introduction of the "symbol_name" field to struct kprobe,
359 the probepoint address resolution will now be taken care of by the kernel.
360 The following will now work:
361 
362         kp.symbol_name = "symbol_name";
363 
364 (64-bit powerpc intricacies such as function descriptors are handled
365 transparently)
366 
367 2. Use the "offset" field of struct kprobe if the offset into the symbol
368 to install a probepoint is known. This field is used to calculate the
369 probepoint.
370 
371 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
372 specified, kprobe registration will fail with -EINVAL.
373 
374 4. With CISC architectures (such as i386 and x86_64), the kprobes code
375 does not validate if the kprobe.addr is at an instruction boundary.
376 Use "offset" with caution.
377 
378 register_kprobe() returns 0 on success, or a negative errno otherwise.
379 
380 User's pre-handler (kp->pre_handler):
381 #include <linux/kprobes.h>
382 #include <linux/ptrace.h>
383 int pre_handler(struct kprobe *p, struct pt_regs *regs);
384 
385 Called with p pointing to the kprobe associated with the breakpoint,
386 and regs pointing to the struct containing the registers saved when
387 the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
388 
389 User's post-handler (kp->post_handler):
390 #include <linux/kprobes.h>
391 #include <linux/ptrace.h>
392 void post_handler(struct kprobe *p, struct pt_regs *regs,
393         unsigned long flags);
394 
395 p and regs are as described for the pre_handler.  flags always seems
396 to be zero.
397 
398 User's fault-handler (kp->fault_handler):
399 #include <linux/kprobes.h>
400 #include <linux/ptrace.h>
401 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
402 
403 p and regs are as described for the pre_handler.  trapnr is the
404 architecture-specific trap number associated with the fault (e.g.,
405 on i386, 13 for a general protection fault or 14 for a page fault).
406 Returns 1 if it successfully handled the exception.
407 
408 4.2 register_jprobe
409 
410 #include <linux/kprobes.h>
411 int register_jprobe(struct jprobe *jp)
412 
413 Sets a breakpoint at the address jp->kp.addr, which must be the address
414 of the first instruction of a function.  When the breakpoint is hit,
415 Kprobes runs the handler whose address is jp->entry.
416 
417 The handler should have the same arg list and return type as the probed
418 function; and just before it returns, it must call jprobe_return().
419 (The handler never actually returns, since jprobe_return() returns
420 control to Kprobes.)  If the probed function is declared asmlinkage
421 or anything else that affects how args are passed, the handler's
422 declaration must match.
423 
424 register_jprobe() returns 0 on success, or a negative errno otherwise.
425 
426 4.3 register_kretprobe
427 
428 #include <linux/kprobes.h>
429 int register_kretprobe(struct kretprobe *rp);
430 
431 Establishes a return probe for the function whose address is
432 rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
433 You must set rp->maxactive appropriately before you call
434 register_kretprobe(); see "How Does a Return Probe Work?" for details.
435 
436 register_kretprobe() returns 0 on success, or a negative errno
437 otherwise.
438 
439 User's return-probe handler (rp->handler):
440 #include <linux/kprobes.h>
441 #include <linux/ptrace.h>
442 int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
443 
444 regs is as described for kprobe.pre_handler.  ri points to the
445 kretprobe_instance object, of which the following fields may be
446 of interest:
447 - ret_addr: the return address
448 - rp: points to the corresponding kretprobe object
449 - task: points to the corresponding task struct
450 - data: points to per return-instance private data; see "Kretprobe
451         entry-handler" for details.
452 
453 The regs_return_value(regs) macro provides a simple abstraction to
454 extract the return value from the appropriate register as defined by
455 the architecture's ABI.
456 
457 The handler's return value is currently ignored.
458 
459 4.4 unregister_*probe
460 
461 #include <linux/kprobes.h>
462 void unregister_kprobe(struct kprobe *kp);
463 void unregister_jprobe(struct jprobe *jp);
464 void unregister_kretprobe(struct kretprobe *rp);
465 
466 Removes the specified probe.  The unregister function can be called
467 at any time after the probe has been registered.
468 
469 NOTE:
470 If the functions find an incorrect probe (ex. an unregistered probe),
471 they clear the addr field of the probe.
472 
473 4.5 register_*probes
474 
475 #include <linux/kprobes.h>
476 int register_kprobes(struct kprobe **kps, int num);
477 int register_kretprobes(struct kretprobe **rps, int num);
478 int register_jprobes(struct jprobe **jps, int num);
479 
480 Registers each of the num probes in the specified array.  If any
481 error occurs during registration, all probes in the array, up to
482 the bad probe, are safely unregistered before the register_*probes
483 function returns.
484 - kps/rps/jps: an array of pointers to *probe data structures
485 - num: the number of the array entries.
486 
487 NOTE:
488 You have to allocate(or define) an array of pointers and set all
489 of the array entries before using these functions.
490 
491 4.6 unregister_*probes
492 
493 #include <linux/kprobes.h>
494 void unregister_kprobes(struct kprobe **kps, int num);
495 void unregister_kretprobes(struct kretprobe **rps, int num);
496 void unregister_jprobes(struct jprobe **jps, int num);
497 
498 Removes each of the num probes in the specified array at once.
499 
500 NOTE:
501 If the functions find some incorrect probes (ex. unregistered
502 probes) in the specified array, they clear the addr field of those
503 incorrect probes. However, other probes in the array are
504 unregistered correctly.
505 
506 4.7 disable_*probe
507 
508 #include <linux/kprobes.h>
509 int disable_kprobe(struct kprobe *kp);
510 int disable_kretprobe(struct kretprobe *rp);
511 int disable_jprobe(struct jprobe *jp);
512 
513 Temporarily disables the specified *probe. You can enable it again by using
514 enable_*probe(). You must specify the probe which has been registered.
515 
516 4.8 enable_*probe
517 
518 #include <linux/kprobes.h>
519 int enable_kprobe(struct kprobe *kp);
520 int enable_kretprobe(struct kretprobe *rp);
521 int enable_jprobe(struct jprobe *jp);
522 
523 Enables *probe which has been disabled by disable_*probe(). You must specify
524 the probe which has been registered.
525 
526 5. Kprobes Features and Limitations
527 
528 Kprobes allows multiple probes at the same address.  Currently,
529 however, there cannot be multiple jprobes on the same function at
530 the same time.  Also, a probepoint for which there is a jprobe or
531 a post_handler cannot be optimized.  So if you install a jprobe,
532 or a kprobe with a post_handler, at an optimized probepoint, the
533 probepoint will be unoptimized automatically.
534 
535 In general, you can install a probe anywhere in the kernel.
536 In particular, you can probe interrupt handlers.  Known exceptions
537 are discussed in this section.
538 
539 The register_*probe functions will return -EINVAL if you attempt
540 to install a probe in the code that implements Kprobes (mostly
541 kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
542 as do_page_fault and notifier_call_chain).
543 
544 If you install a probe in an inline-able function, Kprobes makes
545 no attempt to chase down all inline instances of the function and
546 install probes there.  gcc may inline a function without being asked,
547 so keep this in mind if you're not seeing the probe hits you expect.
548 
549 A probe handler can modify the environment of the probed function
550 -- e.g., by modifying kernel data structures, or by modifying the
551 contents of the pt_regs struct (which are restored to the registers
552 upon return from the breakpoint).  So Kprobes can be used, for example,
553 to install a bug fix or to inject faults for testing.  Kprobes, of
554 course, has no way to distinguish the deliberately injected faults
555 from the accidental ones.  Don't drink and probe.
556 
557 Kprobes makes no attempt to prevent probe handlers from stepping on
558 each other -- e.g., probing printk() and then calling printk() from a
559 probe handler.  If a probe handler hits a probe, that second probe's
560 handlers won't be run in that instance, and the kprobe.nmissed member
561 of the second probe will be incremented.
562 
563 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
564 the same handler) may run concurrently on different CPUs.
565 
566 Kprobes does not use mutexes or allocate memory except during
567 registration and unregistration.
568 
569 Probe handlers are run with preemption disabled.  Depending on the
570 architecture and optimization state, handlers may also run with
571 interrupts disabled (e.g., kretprobe handlers and optimized kprobe
572 handlers run without interrupt disabled on x86/x86-64).  In any case,
573 your handler should not yield the CPU (e.g., by attempting to acquire
574 a semaphore).
575 
576 Since a return probe is implemented by replacing the return
577 address with the trampoline's address, stack backtraces and calls
578 to __builtin_return_address() will typically yield the trampoline's
579 address instead of the real return address for kretprobed functions.
580 (As far as we can tell, __builtin_return_address() is used only
581 for instrumentation and error reporting.)
582 
583 If the number of times a function is called does not match the number
584 of times it returns, registering a return probe on that function may
585 produce undesirable results. In such a case, a line:
586 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
587 gets printed. With this information, one will be able to correlate the
588 exact instance of the kretprobe that caused the problem. We have the
589 do_exit() case covered. do_execve() and do_fork() are not an issue.
590 We're unaware of other specific cases where this could be a problem.
591 
592 If, upon entry to or exit from a function, the CPU is running on
593 a stack other than that of the current task, registering a return
594 probe on that function may produce undesirable results.  For this
595 reason, Kprobes doesn't support return probes (or kprobes or jprobes)
596 on the x86_64 version of __switch_to(); the registration functions
597 return -EINVAL.
598 
599 On x86/x86-64, since the Jump Optimization of Kprobes modifies
600 instructions widely, there are some limitations to optimization. To
601 explain it, we introduce some terminology. Imagine a 3-instruction
602 sequence consisting of a two 2-byte instructions and one 3-byte
603 instruction.
604 
605         IA
606          |
607 [-2][-1][0][1][2][3][4][5][6][7]
608         [ins1][ins2][  ins3 ]
609         [<-     DCR       ->]
610            [<- JTPR ->]
611 
612 ins1: 1st Instruction
613 ins2: 2nd Instruction
614 ins3: 3rd Instruction
615 IA:  Insertion Address
616 JTPR: Jump Target Prohibition Region
617 DCR: Detoured Code Region
618 
619 The instructions in DCR are copied to the out-of-line buffer
620 of the kprobe, because the bytes in DCR are replaced by
621 a 5-byte jump instruction. So there are several limitations.
622 
623 a) The instructions in DCR must be relocatable.
624 b) The instructions in DCR must not include a call instruction.
625 c) JTPR must not be targeted by any jump or call instruction.
626 d) DCR must not straddle the border between functions.
627 
628 Anyway, these limitations are checked by the in-kernel instruction
629 decoder, so you don't need to worry about that.
630 
631 6. Probe Overhead
632 
633 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
634 microseconds to process.  Specifically, a benchmark that hits the same
635 probepoint repeatedly, firing a simple handler each time, reports 1-2
636 million hits per second, depending on the architecture.  A jprobe or
637 return-probe hit typically takes 50-75% longer than a kprobe hit.
638 When you have a return probe set on a function, adding a kprobe at
639 the entry to that function adds essentially no overhead.
640 
641 Here are sample overhead figures (in usec) for different architectures.
642 k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
643 on same function; jr = jprobe + return probe on same function
644 
645 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
646 k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
647 
648 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
649 k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
650 
651 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
652 k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
653 
654 6.1 Optimized Probe Overhead
655 
656 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
657 process. Here are sample overhead figures (in usec) for x86 architectures.
658 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
659 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
660 
661 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
662 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
663 
664 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
665 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
666 
667 7. TODO
668 
669 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
670 programming interface for probe-based instrumentation.  Try it out.
671 b. Kernel return probes for sparc64.
672 c. Support for other architectures.
673 d. User-space probes.
674 e. Watchpoint probes (which fire on data references).
675 
676 8. Kprobes Example
677 
678 See samples/kprobes/kprobe_example.c
679 
680 9. Jprobes Example
681 
682 See samples/kprobes/jprobe_example.c
683 
684 10. Kretprobes Example
685 
686 See samples/kprobes/kretprobe_example.c
687 
688 For additional information on Kprobes, refer to the following URLs:
689 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
690 http://www.redhat.com/magazine/005mar05/features/kprobes/
691 http://www-users.cs.umn.edu/~boutcher/kprobes/
692 http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
693 
694 
695 Appendix A: The kprobes debugfs interface
696 
697 With recent kernels (> 2.6.20) the list of registered kprobes is visible
698 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
699 
700 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system
701 
702 c015d71a  k  vfs_read+0x0
703 c011a316  j  do_fork+0x0
704 c03dedc5  r  tcp_v4_rcv+0x0
705 
706 The first column provides the kernel address where the probe is inserted.
707 The second column identifies the type of probe (k - kprobe, r - kretprobe
708 and j - jprobe), while the third column specifies the symbol+offset of
709 the probe. If the probed function belongs to a module, the module name
710 is also specified. Following columns show probe status. If the probe is on
711 a virtual address that is no longer valid (module init sections, module
712 virtual addresses that correspond to modules that've been unloaded),
713 such probes are marked with [GONE]. If the probe is temporarily disabled,
714 such probes are marked with [DISABLED]. If the probe is optimized, it is
715 marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
716 [FTRACE].
717 
718 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
719 
720 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
721 By default, all kprobes are enabled. By echoing "0" to this file, all
722 registered probes will be disarmed, till such time a "1" is echoed to this
723 file. Note that this knob just disarms and arms all kprobes and doesn't
724 change each probe's disabling state. This means that disabled kprobes (marked
725 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
726 
727 
728 Appendix B: The kprobes sysctl interface
729 
730 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
731 
732 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
733 a knob to globally and forcibly turn jump optimization (see section
734 1.4) ON or OFF. By default, jump optimization is allowed (ON).
735 If you echo "0" to this file or set "debug.kprobes_optimization" to
736 0 via sysctl, all optimized probes will be unoptimized, and any new
737 probes registered after that will not be optimized.  Note that this
738 knob *changes* the optimized state. This means that optimized probes
739 (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
740 removed). If the knob is turned on, they will be optimized again.
741 

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