Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.
1 Title : Kernel Probes (Kprobes) 2 Authors : Jim Keniston <email@example.com> 3 : Prasanna S Panchamukhi <firstname.lastname@example.org> 4 : Masami Hiramatsu <email@example.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] 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.