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

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