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 
 28 There are currently three types of probes: kprobes, jprobes, and
 29 kretprobes (also called return probes).  A kprobe can be inserted
 30 on virtually any instruction in the kernel.  A jprobe is inserted at
 31 the entry to a kernel function, and provides convenient access to the
 32 function's arguments.  A return probe fires when a specified function
 33 returns.
 34 
 35 In the typical case, Kprobes-based instrumentation is packaged as
 36 a kernel module.  The module's init function installs ("registers")
 37 one or more probes, and the exit function unregisters them.  A
 38 registration function such as register_kprobe() specifies where
 39 the probe is to be inserted and what handler is to be called when
 40 the probe is hit.
 41 
 42 There are also register_/unregister_*probes() functions for batch
 43 registration/unregistration of a group of *probes. These functions
 44 can speed up unregistration process when you have to unregister
 45 a lot of probes at once.
 46 
 47 The next four subsections explain how the different types of
 48 probes work and how jump optimization works.  They explain certain
 49 things that you'll need to know in order to make the best use of
 50 Kprobes -- e.g., the difference between a pre_handler and
 51 a post_handler, and how to use the maxactive and nmissed fields of
 52 a kretprobe.  But if you're in a hurry to start using Kprobes, you
 53 can skip ahead to section 2.
 54 
 55 1.1 How Does a Kprobe Work?
 56 
 57 When a kprobe is registered, Kprobes makes a copy of the probed
 58 instruction and replaces the first byte(s) of the probed instruction
 59 with a breakpoint instruction (e.g., int3 on i386 and x86_64).
 60 
 61 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
 62 registers are saved, and control passes to Kprobes via the
 63 notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
 64 associated with the kprobe, passing the handler the addresses of the
 65 kprobe struct and the saved registers.
 66 
 67 Next, Kprobes single-steps its copy of the probed instruction.
 68 (It would be simpler to single-step the actual instruction in place,
 69 but then Kprobes would have to temporarily remove the breakpoint
 70 instruction.  This would open a small time window when another CPU
 71 could sail right past the probepoint.)
 72 
 73 After the instruction is single-stepped, Kprobes executes the
 74 "post_handler," if any, that is associated with the kprobe.
 75 Execution then continues with the instruction following the probepoint.
 76 
 77 1.2 How Does a Jprobe Work?
 78 
 79 A jprobe is implemented using a kprobe that is placed on a function's
 80 entry point.  It employs a simple mirroring principle to allow
 81 seamless access to the probed function's arguments.  The jprobe
 82 handler routine should have the same signature (arg list and return
 83 type) as the function being probed, and must always end by calling
 84 the Kprobes function jprobe_return().
 85 
 86 Here's how it works.  When the probe is hit, Kprobes makes a copy of
 87 the saved registers and a generous portion of the stack (see below).
 88 Kprobes then points the saved instruction pointer at the jprobe's
 89 handler routine, and returns from the trap.  As a result, control
 90 passes to the handler, which is presented with the same register and
 91 stack contents as the probed function.  When it is done, the handler
 92 calls jprobe_return(), which traps again to restore the original stack
 93 contents and processor state and switch to the probed function.
 94 
 95 By convention, the callee owns its arguments, so gcc may produce code
 96 that unexpectedly modifies that portion of the stack.  This is why
 97 Kprobes saves a copy of the stack and restores it after the jprobe
 98 handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
 99 64 bytes on i386.
100 
101 Note that the probed function's args may be passed on the stack
102 or in registers.  The jprobe will work in either case, so long as the
103 handler's prototype matches that of the probed function.
104 
105 1.3 Return Probes
106 
107 1.3.1 How Does a Return Probe Work?
108 
109 When you call register_kretprobe(), Kprobes establishes a kprobe at
110 the entry to the function.  When the probed function is called and this
111 probe is hit, Kprobes saves a copy of the return address, and replaces
112 the return address with the address of a "trampoline."  The trampoline
113 is an arbitrary piece of code -- typically just a nop instruction.
114 At boot time, Kprobes registers a kprobe at the trampoline.
115 
116 When the probed function executes its return instruction, control
117 passes to the trampoline and that probe is hit.  Kprobes' trampoline
118 handler calls the user-specified return handler associated with the
119 kretprobe, then sets the saved instruction pointer to the saved return
120 address, and that's where execution resumes upon return from the trap.
121 
122 While the probed function is executing, its return address is
123 stored in an object of type kretprobe_instance.  Before calling
124 register_kretprobe(), the user sets the maxactive field of the
125 kretprobe struct to specify how many instances of the specified
126 function can be probed simultaneously.  register_kretprobe()
127 pre-allocates the indicated number of kretprobe_instance objects.
128 
129 For example, if the function is non-recursive and is called with a
130 spinlock held, maxactive = 1 should be enough.  If the function is
131 non-recursive and can never relinquish the CPU (e.g., via a semaphore
132 or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
133 set to a default value.  If CONFIG_PREEMPT is enabled, the default
134 is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
135 
136 It's not a disaster if you set maxactive too low; you'll just miss
137 some probes.  In the kretprobe struct, the nmissed field is set to
138 zero when the return probe is registered, and is incremented every
139 time the probed function is entered but there is no kretprobe_instance
140 object available for establishing the return probe.
141 
142 1.3.2 Kretprobe entry-handler
143 
144 Kretprobes also provides an optional user-specified handler which runs
145 on function entry. This handler is specified by setting the entry_handler
146 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
147 function entry is hit, the user-defined entry_handler, if any, is invoked.
148 If the entry_handler returns 0 (success) then a corresponding return handler
149 is guaranteed to be called upon function return. If the entry_handler
150 returns a non-zero error then Kprobes leaves the return address as is, and
151 the kretprobe has no further effect for that particular function instance.
152 
153 Multiple entry and return handler invocations are matched using the unique
154 kretprobe_instance object associated with them. Additionally, a user
155 may also specify per return-instance private data to be part of each
156 kretprobe_instance object. This is especially useful when sharing private
157 data between corresponding user entry and return handlers. The size of each
158 private data object can be specified at kretprobe registration time by
159 setting the data_size field of the kretprobe struct. This data can be
160 accessed through the data field of each kretprobe_instance object.
161 
162 In case probed function is entered but there is no kretprobe_instance
163 object available, then in addition to incrementing the nmissed count,
164 the user entry_handler invocation is also skipped.
165 
166 1.4 How Does Jump Optimization Work?
167 
168 If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
169 is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
170 the "debug.kprobes_optimization" kernel parameter is set to 1 (see
171 sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
172 instruction instead of a breakpoint instruction at each probepoint.
173 
174 1.4.1 Init a Kprobe
175 
176 When a probe is registered, before attempting this optimization,
177 Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
178 address. So, even if it's not possible to optimize this particular
179 probepoint, there'll be a probe there.
180 
181 1.4.2 Safety Check
182 
183 Before optimizing a probe, Kprobes performs the following safety checks:
184 
185 - Kprobes verifies that the region that will be replaced by the jump
186 instruction (the "optimized region") lies entirely within one function.
187 (A jump instruction is multiple bytes, and so may overlay multiple
188 instructions.)
189 
190 - Kprobes analyzes the entire function and verifies that there is no
191 jump into the optimized region.  Specifically:
192   - the function contains no indirect jump;
193   - the function contains no instruction that causes an exception (since
194   the fixup code triggered by the exception could jump back into the
195   optimized region -- Kprobes checks the exception tables to verify this);
196   and
197   - there is no near jump to the optimized region (other than to the first
198   byte).
199 
200 - For each instruction in the optimized region, Kprobes verifies that
201 the instruction can be executed out of line.
202 
203 1.4.3 Preparing Detour Buffer
204 
205 Next, Kprobes prepares a "detour" buffer, which contains the following
206 instruction sequence:
207 - code to push the CPU's registers (emulating a breakpoint trap)
208 - a call to the trampoline code which calls user's probe handlers.
209 - code to restore registers
210 - the instructions from the optimized region
211 - a jump back to the original execution path.
212 
213 1.4.4 Pre-optimization
214 
215 After preparing the detour buffer, Kprobes verifies that none of the
216 following situations exist:
217 - The probe has either a break_handler (i.e., it's a jprobe) or a
218 post_handler.
219 - Other instructions in the optimized region are probed.
220 - The probe is disabled.
221 In any of the above cases, Kprobes won't start optimizing the probe.
222 Since these are temporary situations, Kprobes tries to start
223 optimizing it again if the situation is changed.
224 
225 If the kprobe can be optimized, Kprobes enqueues the kprobe to an
226 optimizing list, and kicks the kprobe-optimizer workqueue to optimize
227 it.  If the to-be-optimized probepoint is hit before being optimized,
228 Kprobes returns control to the original instruction path by setting
229 the CPU's instruction pointer to the copied code in the detour buffer
230 -- thus at least avoiding the single-step.
231 
232 1.4.5 Optimization
233 
234 The Kprobe-optimizer doesn't insert the jump instruction immediately;
235 rather, it calls synchronize_sched() for safety first, because it's
236 possible for a CPU to be interrupted in the middle of executing the
237 optimized region(*).  As you know, synchronize_sched() can ensure
238 that all interruptions that were active when synchronize_sched()
239 was called are done, but only if CONFIG_PREEMPT=n.  So, this version
240 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
241 
242 After that, the Kprobe-optimizer calls stop_machine() to replace
243 the optimized region with a jump instruction to the detour buffer,
244 using text_poke_smp().
245 
246 1.4.6 Unoptimization
247 
248 When an optimized kprobe is unregistered, disabled, or blocked by
249 another kprobe, it will be unoptimized.  If this happens before
250 the optimization is complete, the kprobe is just dequeued from the
251 optimized list.  If the optimization has been done, the jump is
252 replaced with the original code (except for an int3 breakpoint in
253 the first byte) by using text_poke_smp().
254 
255 (*)Please imagine that the 2nd instruction is interrupted and then
256 the optimizer replaces the 2nd instruction with the jump *address*
257 while the interrupt handler is running. When the interrupt
258 returns to original address, there is no valid instruction,
259 and it causes an unexpected result.
260 
261 (**)This optimization-safety checking may be replaced with the
262 stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
263 kernel.
264 
265 NOTE for geeks:
266 The jump optimization changes the kprobe's pre_handler behavior.
267 Without optimization, the pre_handler can change the kernel's execution
268 path by changing regs->ip and returning 1.  However, when the probe
269 is optimized, that modification is ignored.  Thus, if you want to
270 tweak the kernel's execution path, you need to suppress optimization,
271 using one of the following techniques:
272 - Specify an empty function for the kprobe's post_handler or break_handler.
273  or
274 - Execute 'sysctl -w debug.kprobes_optimization=n'
275 
276 2. Architectures Supported
277 
278 Kprobes, jprobes, and return probes are implemented on the following
279 architectures:
280 
281 - i386 (Supports jump optimization)
282 - x86_64 (AMD-64, EM64T) (Supports jump optimization)
283 - ppc64
284 - ia64 (Does not support probes on instruction slot1.)
285 - sparc64 (Return probes not yet implemented.)
286 - arm
287 - ppc
288 - mips
289 
290 3. Configuring Kprobes
291 
292 When configuring the kernel using make menuconfig/xconfig/oldconfig,
293 ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
294 Support", look for "Kprobes".
295 
296 So that you can load and unload Kprobes-based instrumentation modules,
297 make sure "Loadable module support" (CONFIG_MODULES) and "Module
298 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
299 
300 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
301 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
302 kprobe address resolution code.
303 
304 If you need to insert a probe in the middle of a function, you may find
305 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
306 so you can use "objdump -d -l vmlinux" to see the source-to-object
307 code mapping.
308 
309 4. API Reference
310 
311 The Kprobes API includes a "register" function and an "unregister"
312 function for each type of probe. The API also includes "register_*probes"
313 and "unregister_*probes" functions for (un)registering arrays of probes.
314 Here are terse, mini-man-page specifications for these functions and
315 the associated probe handlers that you'll write. See the files in the
316 samples/kprobes/ sub-directory for examples.
317 
318 4.1 register_kprobe
319 
320 #include <linux/kprobes.h>
321 int register_kprobe(struct kprobe *kp);
322 
323 Sets a breakpoint at the address kp->addr.  When the breakpoint is
324 hit, Kprobes calls kp->pre_handler.  After the probed instruction
325 is single-stepped, Kprobe calls kp->post_handler.  If a fault
326 occurs during execution of kp->pre_handler or kp->post_handler,
327 or during single-stepping of the probed instruction, Kprobes calls
328 kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
329 is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
330 so, its handlers aren't hit until calling enable_kprobe(kp).
331 
332 NOTE:
333 1. With the introduction of the "symbol_name" field to struct kprobe,
334 the probepoint address resolution will now be taken care of by the kernel.
335 The following will now work:
336 
337         kp.symbol_name = "symbol_name";
338 
339 (64-bit powerpc intricacies such as function descriptors are handled
340 transparently)
341 
342 2. Use the "offset" field of struct kprobe if the offset into the symbol
343 to install a probepoint is known. This field is used to calculate the
344 probepoint.
345 
346 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
347 specified, kprobe registration will fail with -EINVAL.
348 
349 4. With CISC architectures (such as i386 and x86_64), the kprobes code
350 does not validate if the kprobe.addr is at an instruction boundary.
351 Use "offset" with caution.
352 
353 register_kprobe() returns 0 on success, or a negative errno otherwise.
354 
355 User's pre-handler (kp->pre_handler):
356 #include <linux/kprobes.h>
357 #include <linux/ptrace.h>
358 int pre_handler(struct kprobe *p, struct pt_regs *regs);
359 
360 Called with p pointing to the kprobe associated with the breakpoint,
361 and regs pointing to the struct containing the registers saved when
362 the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
363 
364 User's post-handler (kp->post_handler):
365 #include <linux/kprobes.h>
366 #include <linux/ptrace.h>
367 void post_handler(struct kprobe *p, struct pt_regs *regs,
368         unsigned long flags);
369 
370 p and regs are as described for the pre_handler.  flags always seems
371 to be zero.
372 
373 User's fault-handler (kp->fault_handler):
374 #include <linux/kprobes.h>
375 #include <linux/ptrace.h>
376 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
377 
378 p and regs are as described for the pre_handler.  trapnr is the
379 architecture-specific trap number associated with the fault (e.g.,
380 on i386, 13 for a general protection fault or 14 for a page fault).
381 Returns 1 if it successfully handled the exception.
382 
383 4.2 register_jprobe
384 
385 #include <linux/kprobes.h>
386 int register_jprobe(struct jprobe *jp)
387 
388 Sets a breakpoint at the address jp->kp.addr, which must be the address
389 of the first instruction of a function.  When the breakpoint is hit,
390 Kprobes runs the handler whose address is jp->entry.
391 
392 The handler should have the same arg list and return type as the probed
393 function; and just before it returns, it must call jprobe_return().
394 (The handler never actually returns, since jprobe_return() returns
395 control to Kprobes.)  If the probed function is declared asmlinkage
396 or anything else that affects how args are passed, the handler's
397 declaration must match.
398 
399 register_jprobe() returns 0 on success, or a negative errno otherwise.
400 
401 4.3 register_kretprobe
402 
403 #include <linux/kprobes.h>
404 int register_kretprobe(struct kretprobe *rp);
405 
406 Establishes a return probe for the function whose address is
407 rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
408 You must set rp->maxactive appropriately before you call
409 register_kretprobe(); see "How Does a Return Probe Work?" for details.
410 
411 register_kretprobe() returns 0 on success, or a negative errno
412 otherwise.
413 
414 User's return-probe handler (rp->handler):
415 #include <linux/kprobes.h>
416 #include <linux/ptrace.h>
417 int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
418 
419 regs is as described for kprobe.pre_handler.  ri points to the
420 kretprobe_instance object, of which the following fields may be
421 of interest:
422 - ret_addr: the return address
423 - rp: points to the corresponding kretprobe object
424 - task: points to the corresponding task struct
425 - data: points to per return-instance private data; see "Kretprobe
426         entry-handler" for details.
427 
428 The regs_return_value(regs) macro provides a simple abstraction to
429 extract the return value from the appropriate register as defined by
430 the architecture's ABI.
431 
432 The handler's return value is currently ignored.
433 
434 4.4 unregister_*probe
435 
436 #include <linux/kprobes.h>
437 void unregister_kprobe(struct kprobe *kp);
438 void unregister_jprobe(struct jprobe *jp);
439 void unregister_kretprobe(struct kretprobe *rp);
440 
441 Removes the specified probe.  The unregister function can be called
442 at any time after the probe has been registered.
443 
444 NOTE:
445 If the functions find an incorrect probe (ex. an unregistered probe),
446 they clear the addr field of the probe.
447 
448 4.5 register_*probes
449 
450 #include <linux/kprobes.h>
451 int register_kprobes(struct kprobe **kps, int num);
452 int register_kretprobes(struct kretprobe **rps, int num);
453 int register_jprobes(struct jprobe **jps, int num);
454 
455 Registers each of the num probes in the specified array.  If any
456 error occurs during registration, all probes in the array, up to
457 the bad probe, are safely unregistered before the register_*probes
458 function returns.
459 - kps/rps/jps: an array of pointers to *probe data structures
460 - num: the number of the array entries.
461 
462 NOTE:
463 You have to allocate(or define) an array of pointers and set all
464 of the array entries before using these functions.
465 
466 4.6 unregister_*probes
467 
468 #include <linux/kprobes.h>
469 void unregister_kprobes(struct kprobe **kps, int num);
470 void unregister_kretprobes(struct kretprobe **rps, int num);
471 void unregister_jprobes(struct jprobe **jps, int num);
472 
473 Removes each of the num probes in the specified array at once.
474 
475 NOTE:
476 If the functions find some incorrect probes (ex. unregistered
477 probes) in the specified array, they clear the addr field of those
478 incorrect probes. However, other probes in the array are
479 unregistered correctly.
480 
481 4.7 disable_*probe
482 
483 #include <linux/kprobes.h>
484 int disable_kprobe(struct kprobe *kp);
485 int disable_kretprobe(struct kretprobe *rp);
486 int disable_jprobe(struct jprobe *jp);
487 
488 Temporarily disables the specified *probe. You can enable it again by using
489 enable_*probe(). You must specify the probe which has been registered.
490 
491 4.8 enable_*probe
492 
493 #include <linux/kprobes.h>
494 int enable_kprobe(struct kprobe *kp);
495 int enable_kretprobe(struct kretprobe *rp);
496 int enable_jprobe(struct jprobe *jp);
497 
498 Enables *probe which has been disabled by disable_*probe(). You must specify
499 the probe which has been registered.
500 
501 5. Kprobes Features and Limitations
502 
503 Kprobes allows multiple probes at the same address.  Currently,
504 however, there cannot be multiple jprobes on the same function at
505 the same time.  Also, a probepoint for which there is a jprobe or
506 a post_handler cannot be optimized.  So if you install a jprobe,
507 or a kprobe with a post_handler, at an optimized probepoint, the
508 probepoint will be unoptimized automatically.
509 
510 In general, you can install a probe anywhere in the kernel.
511 In particular, you can probe interrupt handlers.  Known exceptions
512 are discussed in this section.
513 
514 The register_*probe functions will return -EINVAL if you attempt
515 to install a probe in the code that implements Kprobes (mostly
516 kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
517 as do_page_fault and notifier_call_chain).
518 
519 If you install a probe in an inline-able function, Kprobes makes
520 no attempt to chase down all inline instances of the function and
521 install probes there.  gcc may inline a function without being asked,
522 so keep this in mind if you're not seeing the probe hits you expect.
523 
524 A probe handler can modify the environment of the probed function
525 -- e.g., by modifying kernel data structures, or by modifying the
526 contents of the pt_regs struct (which are restored to the registers
527 upon return from the breakpoint).  So Kprobes can be used, for example,
528 to install a bug fix or to inject faults for testing.  Kprobes, of
529 course, has no way to distinguish the deliberately injected faults
530 from the accidental ones.  Don't drink and probe.
531 
532 Kprobes makes no attempt to prevent probe handlers from stepping on
533 each other -- e.g., probing printk() and then calling printk() from a
534 probe handler.  If a probe handler hits a probe, that second probe's
535 handlers won't be run in that instance, and the kprobe.nmissed member
536 of the second probe will be incremented.
537 
538 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
539 the same handler) may run concurrently on different CPUs.
540 
541 Kprobes does not use mutexes or allocate memory except during
542 registration and unregistration.
543 
544 Probe handlers are run with preemption disabled.  Depending on the
545 architecture and optimization state, handlers may also run with
546 interrupts disabled (e.g., kretprobe handlers and optimized kprobe
547 handlers run without interrupt disabled on x86/x86-64).  In any case,
548 your handler should not yield the CPU (e.g., by attempting to acquire
549 a semaphore).
550 
551 Since a return probe is implemented by replacing the return
552 address with the trampoline's address, stack backtraces and calls
553 to __builtin_return_address() will typically yield the trampoline's
554 address instead of the real return address for kretprobed functions.
555 (As far as we can tell, __builtin_return_address() is used only
556 for instrumentation and error reporting.)
557 
558 If the number of times a function is called does not match the number
559 of times it returns, registering a return probe on that function may
560 produce undesirable results. In such a case, a line:
561 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
562 gets printed. With this information, one will be able to correlate the
563 exact instance of the kretprobe that caused the problem. We have the
564 do_exit() case covered. do_execve() and do_fork() are not an issue.
565 We're unaware of other specific cases where this could be a problem.
566 
567 If, upon entry to or exit from a function, the CPU is running on
568 a stack other than that of the current task, registering a return
569 probe on that function may produce undesirable results.  For this
570 reason, Kprobes doesn't support return probes (or kprobes or jprobes)
571 on the x86_64 version of __switch_to(); the registration functions
572 return -EINVAL.
573 
574 On x86/x86-64, since the Jump Optimization of Kprobes modifies
575 instructions widely, there are some limitations to optimization. To
576 explain it, we introduce some terminology. Imagine a 3-instruction
577 sequence consisting of a two 2-byte instructions and one 3-byte
578 instruction.
579 
580         IA
581          |
582 [-2][-1][0][1][2][3][4][5][6][7]
583         [ins1][ins2][  ins3 ]
584         [<-     DCR       ->]
585            [<- JTPR ->]
586 
587 ins1: 1st Instruction
588 ins2: 2nd Instruction
589 ins3: 3rd Instruction
590 IA:  Insertion Address
591 JTPR: Jump Target Prohibition Region
592 DCR: Detoured Code Region
593 
594 The instructions in DCR are copied to the out-of-line buffer
595 of the kprobe, because the bytes in DCR are replaced by
596 a 5-byte jump instruction. So there are several limitations.
597 
598 a) The instructions in DCR must be relocatable.
599 b) The instructions in DCR must not include a call instruction.
600 c) JTPR must not be targeted by any jump or call instruction.
601 d) DCR must not straddle the border between functions.
602 
603 Anyway, these limitations are checked by the in-kernel instruction
604 decoder, so you don't need to worry about that.
605 
606 6. Probe Overhead
607 
608 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
609 microseconds to process.  Specifically, a benchmark that hits the same
610 probepoint repeatedly, firing a simple handler each time, reports 1-2
611 million hits per second, depending on the architecture.  A jprobe or
612 return-probe hit typically takes 50-75% longer than a kprobe hit.
613 When you have a return probe set on a function, adding a kprobe at
614 the entry to that function adds essentially no overhead.
615 
616 Here are sample overhead figures (in usec) for different architectures.
617 k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
618 on same function; jr = jprobe + return probe on same function
619 
620 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
621 k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
622 
623 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
624 k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
625 
626 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
627 k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
628 
629 6.1 Optimized Probe Overhead
630 
631 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
632 process. Here are sample overhead figures (in usec) for x86 architectures.
633 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
634 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
635 
636 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
637 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
638 
639 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
640 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
641 
642 7. TODO
643 
644 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
645 programming interface for probe-based instrumentation.  Try it out.
646 b. Kernel return probes for sparc64.
647 c. Support for other architectures.
648 d. User-space probes.
649 e. Watchpoint probes (which fire on data references).
650 
651 8. Kprobes Example
652 
653 See samples/kprobes/kprobe_example.c
654 
655 9. Jprobes Example
656 
657 See samples/kprobes/jprobe_example.c
658 
659 10. Kretprobes Example
660 
661 See samples/kprobes/kretprobe_example.c
662 
663 For additional information on Kprobes, refer to the following URLs:
664 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
665 http://www.redhat.com/magazine/005mar05/features/kprobes/
666 http://www-users.cs.umn.edu/~boutcher/kprobes/
667 http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
668 
669 
670 Appendix A: The kprobes debugfs interface
671 
672 With recent kernels (> 2.6.20) the list of registered kprobes is visible
673 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
674 
675 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system
676 
677 c015d71a  k  vfs_read+0x0
678 c011a316  j  do_fork+0x0
679 c03dedc5  r  tcp_v4_rcv+0x0
680 
681 The first column provides the kernel address where the probe is inserted.
682 The second column identifies the type of probe (k - kprobe, r - kretprobe
683 and j - jprobe), while the third column specifies the symbol+offset of
684 the probe. If the probed function belongs to a module, the module name
685 is also specified. Following columns show probe status. If the probe is on
686 a virtual address that is no longer valid (module init sections, module
687 virtual addresses that correspond to modules that've been unloaded),
688 such probes are marked with [GONE]. If the probe is temporarily disabled,
689 such probes are marked with [DISABLED]. If the probe is optimized, it is
690 marked with [OPTIMIZED].
691 
692 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
693 
694 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
695 By default, all kprobes are enabled. By echoing "0" to this file, all
696 registered probes will be disarmed, till such time a "1" is echoed to this
697 file. Note that this knob just disarms and arms all kprobes and doesn't
698 change each probe's disabling state. This means that disabled kprobes (marked
699 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
700 
701 
702 Appendix B: The kprobes sysctl interface
703 
704 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
705 
706 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
707 a knob to globally and forcibly turn jump optimization (see section
708 1.4) ON or OFF. By default, jump optimization is allowed (ON).
709 If you echo "0" to this file or set "debug.kprobes_optimization" to
710 0 via sysctl, all optimized probes will be unoptimized, and any new
711 probes registered after that will not be optimized.  Note that this
712 knob *changes* the optimized state. This means that optimized probes
713 (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
714 removed). If the knob is turned on, they will be optimized again.
715 

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