Based on kernel version 2.6.30. Page generated on 2009-06-11 10:12 EST.
1 Title : Kernel Probes (Kprobes) 2 Authors : Jim Keniston <jkenisto[AT]us.ibm[DOT]com> 3 : Prasanna S Panchamukhi <prasanna[AT]in.ibm[DOT]com> 4 5 CONTENTS 6 7 1. Concepts: Kprobes, Jprobes, Return Probes 8 2. Architectures Supported 9 3. Configuring Kprobes 10 4. API Reference 11 5. Kprobes Features and Limitations 12 6. Probe Overhead 13 7. TODO 14 8. Kprobes Example 15 9. Jprobes Example 16 10. Kretprobes Example 17 Appendix A: The kprobes debugfs interface 18 19 1. Concepts: Kprobes, Jprobes, Return Probes 20 21 Kprobes enables you to dynamically break into any kernel routine and 22 collect debugging and performance information non-disruptively. You 23 can trap at almost any kernel code address, specifying a handler 24 routine to be invoked when the breakpoint is hit. 25 26 There are currently three types of probes: kprobes, jprobes, and 27 kretprobes (also called return probes). A kprobe can be inserted 28 on virtually any instruction in the kernel. A jprobe is inserted at 29 the entry to a kernel function, and provides convenient access to the 30 function's arguments. A return probe fires when a specified function 31 returns. 32 33 In the typical case, Kprobes-based instrumentation is packaged as 34 a kernel module. The module's init function installs ("registers") 35 one or more probes, and the exit function unregisters them. A 36 registration function such as register_kprobe() specifies where 37 the probe is to be inserted and what handler is to be called when 38 the probe is hit. 39 40 There are also register_/unregister_*probes() functions for batch 41 registration/unregistration of a group of *probes. These functions 42 can speed up unregistration process when you have to unregister 43 a lot of probes at once. 44 45 The next three subsections explain how the different types of 46 probes work. They explain certain things that you'll need to 47 know in order to make the best use of Kprobes -- e.g., the 48 difference between a pre_handler and a post_handler, and how 49 to use the maxactive and nmissed fields of a kretprobe. But 50 if you're in a hurry to start using Kprobes, you can skip ahead 51 to section 2. 52 53 1.1 How Does a Kprobe Work? 54 55 When a kprobe is registered, Kprobes makes a copy of the probed 56 instruction and replaces the first byte(s) of the probed instruction 57 with a breakpoint instruction (e.g., int3 on i386 and x86_64). 58 59 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's 60 registers are saved, and control passes to Kprobes via the 61 notifier_call_chain mechanism. Kprobes executes the "pre_handler" 62 associated with the kprobe, passing the handler the addresses of the 63 kprobe struct and the saved registers. 64 65 Next, Kprobes single-steps its copy of the probed instruction. 66 (It would be simpler to single-step the actual instruction in place, 67 but then Kprobes would have to temporarily remove the breakpoint 68 instruction. This would open a small time window when another CPU 69 could sail right past the probepoint.) 70 71 After the instruction is single-stepped, Kprobes executes the 72 "post_handler," if any, that is associated with the kprobe. 73 Execution then continues with the instruction following the probepoint. 74 75 1.2 How Does a Jprobe Work? 76 77 A jprobe is implemented using a kprobe that is placed on a function's 78 entry point. It employs a simple mirroring principle to allow 79 seamless access to the probed function's arguments. The jprobe 80 handler routine should have the same signature (arg list and return 81 type) as the function being probed, and must always end by calling 82 the Kprobes function jprobe_return(). 83 84 Here's how it works. When the probe is hit, Kprobes makes a copy of 85 the saved registers and a generous portion of the stack (see below). 86 Kprobes then points the saved instruction pointer at the jprobe's 87 handler routine, and returns from the trap. As a result, control 88 passes to the handler, which is presented with the same register and 89 stack contents as the probed function. When it is done, the handler 90 calls jprobe_return(), which traps again to restore the original stack 91 contents and processor state and switch to the probed function. 92 93 By convention, the callee owns its arguments, so gcc may produce code 94 that unexpectedly modifies that portion of the stack. This is why 95 Kprobes saves a copy of the stack and restores it after the jprobe 96 handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g., 97 64 bytes on i386. 98 99 Note that the probed function's args may be passed on the stack 100 or in registers. The jprobe will work in either case, so long as the 101 handler's prototype matches that of the probed function. 102 103 1.3 Return Probes 104 105 1.3.1 How Does a Return Probe Work? 106 107 When you call register_kretprobe(), Kprobes establishes a kprobe at 108 the entry to the function. When the probed function is called and this 109 probe is hit, Kprobes saves a copy of the return address, and replaces 110 the return address with the address of a "trampoline." The trampoline 111 is an arbitrary piece of code -- typically just a nop instruction. 112 At boot time, Kprobes registers a kprobe at the trampoline. 113 114 When the probed function executes its return instruction, control 115 passes to the trampoline and that probe is hit. Kprobes' trampoline 116 handler calls the user-specified return handler associated with the 117 kretprobe, then sets the saved instruction pointer to the saved return 118 address, and that's where execution resumes upon return from the trap. 119 120 While the probed function is executing, its return address is 121 stored in an object of type kretprobe_instance. Before calling 122 register_kretprobe(), the user sets the maxactive field of the 123 kretprobe struct to specify how many instances of the specified 124 function can be probed simultaneously. register_kretprobe() 125 pre-allocates the indicated number of kretprobe_instance objects. 126 127 For example, if the function is non-recursive and is called with a 128 spinlock held, maxactive = 1 should be enough. If the function is 129 non-recursive and can never relinquish the CPU (e.g., via a semaphore 130 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is 131 set to a default value. If CONFIG_PREEMPT is enabled, the default 132 is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS. 133 134 It's not a disaster if you set maxactive too low; you'll just miss 135 some probes. In the kretprobe struct, the nmissed field is set to 136 zero when the return probe is registered, and is incremented every 137 time the probed function is entered but there is no kretprobe_instance 138 object available for establishing the return probe. 139 140 1.3.2 Kretprobe entry-handler 141 142 Kretprobes also provides an optional user-specified handler which runs 143 on function entry. This handler is specified by setting the entry_handler 144 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the 145 function entry is hit, the user-defined entry_handler, if any, is invoked. 146 If the entry_handler returns 0 (success) then a corresponding return handler 147 is guaranteed to be called upon function return. If the entry_handler 148 returns a non-zero error then Kprobes leaves the return address as is, and 149 the kretprobe has no further effect for that particular function instance. 150 151 Multiple entry and return handler invocations are matched using the unique 152 kretprobe_instance object associated with them. Additionally, a user 153 may also specify per return-instance private data to be part of each 154 kretprobe_instance object. This is especially useful when sharing private 155 data between corresponding user entry and return handlers. The size of each 156 private data object can be specified at kretprobe registration time by 157 setting the data_size field of the kretprobe struct. This data can be 158 accessed through the data field of each kretprobe_instance object. 159 160 In case probed function is entered but there is no kretprobe_instance 161 object available, then in addition to incrementing the nmissed count, 162 the user entry_handler invocation is also skipped. 163 164 2. Architectures Supported 165 166 Kprobes, jprobes, and return probes are implemented on the following 167 architectures: 168 169 - i386 170 - x86_64 (AMD-64, EM64T) 171 - ppc64 172 - ia64 (Does not support probes on instruction slot1.) 173 - sparc64 (Return probes not yet implemented.) 174 - arm 175 - ppc 176 177 3. Configuring Kprobes 178 179 When configuring the kernel using make menuconfig/xconfig/oldconfig, 180 ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation 181 Support", look for "Kprobes". 182 183 So that you can load and unload Kprobes-based instrumentation modules, 184 make sure "Loadable module support" (CONFIG_MODULES) and "Module 185 unloading" (CONFIG_MODULE_UNLOAD) are set to "y". 186 187 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL 188 are set to "y", since kallsyms_lookup_name() is used by the in-kernel 189 kprobe address resolution code. 190 191 If you need to insert a probe in the middle of a function, you may find 192 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), 193 so you can use "objdump -d -l vmlinux" to see the source-to-object 194 code mapping. 195 196 4. API Reference 197 198 The Kprobes API includes a "register" function and an "unregister" 199 function for each type of probe. The API also includes "register_*probes" 200 and "unregister_*probes" functions for (un)registering arrays of probes. 201 Here are terse, mini-man-page specifications for these functions and 202 the associated probe handlers that you'll write. See the files in the 203 samples/kprobes/ sub-directory for examples. 204 205 4.1 register_kprobe 206 207 #include <linux/kprobes.h> 208 int register_kprobe(struct kprobe *kp); 209 210 Sets a breakpoint at the address kp->addr. When the breakpoint is 211 hit, Kprobes calls kp->pre_handler. After the probed instruction 212 is single-stepped, Kprobe calls kp->post_handler. If a fault 213 occurs during execution of kp->pre_handler or kp->post_handler, 214 or during single-stepping of the probed instruction, Kprobes calls 215 kp->fault_handler. Any or all handlers can be NULL. If kp->flags 216 is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled, 217 so, it's handlers aren't hit until calling enable_kprobe(kp). 218 219 NOTE: 220 1. With the introduction of the "symbol_name" field to struct kprobe, 221 the probepoint address resolution will now be taken care of by the kernel. 222 The following will now work: 223 224 kp.symbol_name = "symbol_name"; 225 226 (64-bit powerpc intricacies such as function descriptors are handled 227 transparently) 228 229 2. Use the "offset" field of struct kprobe if the offset into the symbol 230 to install a probepoint is known. This field is used to calculate the 231 probepoint. 232 233 3. Specify either the kprobe "symbol_name" OR the "addr". If both are 234 specified, kprobe registration will fail with -EINVAL. 235 236 4. With CISC architectures (such as i386 and x86_64), the kprobes code 237 does not validate if the kprobe.addr is at an instruction boundary. 238 Use "offset" with caution. 239 240 register_kprobe() returns 0 on success, or a negative errno otherwise. 241 242 User's pre-handler (kp->pre_handler): 243 #include <linux/kprobes.h> 244 #include <linux/ptrace.h> 245 int pre_handler(struct kprobe *p, struct pt_regs *regs); 246 247 Called with p pointing to the kprobe associated with the breakpoint, 248 and regs pointing to the struct containing the registers saved when 249 the breakpoint was hit. Return 0 here unless you're a Kprobes geek. 250 251 User's post-handler (kp->post_handler): 252 #include <linux/kprobes.h> 253 #include <linux/ptrace.h> 254 void post_handler(struct kprobe *p, struct pt_regs *regs, 255 unsigned long flags); 256 257 p and regs are as described for the pre_handler. flags always seems 258 to be zero. 259 260 User's fault-handler (kp->fault_handler): 261 #include <linux/kprobes.h> 262 #include <linux/ptrace.h> 263 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); 264 265 p and regs are as described for the pre_handler. trapnr is the 266 architecture-specific trap number associated with the fault (e.g., 267 on i386, 13 for a general protection fault or 14 for a page fault). 268 Returns 1 if it successfully handled the exception. 269 270 4.2 register_jprobe 271 272 #include <linux/kprobes.h> 273 int register_jprobe(struct jprobe *jp) 274 275 Sets a breakpoint at the address jp->kp.addr, which must be the address 276 of the first instruction of a function. When the breakpoint is hit, 277 Kprobes runs the handler whose address is jp->entry. 278 279 The handler should have the same arg list and return type as the probed 280 function; and just before it returns, it must call jprobe_return(). 281 (The handler never actually returns, since jprobe_return() returns 282 control to Kprobes.) If the probed function is declared asmlinkage 283 or anything else that affects how args are passed, the handler's 284 declaration must match. 285 286 register_jprobe() returns 0 on success, or a negative errno otherwise. 287 288 4.3 register_kretprobe 289 290 #include <linux/kprobes.h> 291 int register_kretprobe(struct kretprobe *rp); 292 293 Establishes a return probe for the function whose address is 294 rp->kp.addr. When that function returns, Kprobes calls rp->handler. 295 You must set rp->maxactive appropriately before you call 296 register_kretprobe(); see "How Does a Return Probe Work?" for details. 297 298 register_kretprobe() returns 0 on success, or a negative errno 299 otherwise. 300 301 User's return-probe handler (rp->handler): 302 #include <linux/kprobes.h> 303 #include <linux/ptrace.h> 304 int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); 305 306 regs is as described for kprobe.pre_handler. ri points to the 307 kretprobe_instance object, of which the following fields may be 308 of interest: 309 - ret_addr: the return address 310 - rp: points to the corresponding kretprobe object 311 - task: points to the corresponding task struct 312 - data: points to per return-instance private data; see "Kretprobe 313 entry-handler" for details. 314 315 The regs_return_value(regs) macro provides a simple abstraction to 316 extract the return value from the appropriate register as defined by 317 the architecture's ABI. 318 319 The handler's return value is currently ignored. 320 321 4.4 unregister_*probe 322 323 #include <linux/kprobes.h> 324 void unregister_kprobe(struct kprobe *kp); 325 void unregister_jprobe(struct jprobe *jp); 326 void unregister_kretprobe(struct kretprobe *rp); 327 328 Removes the specified probe. The unregister function can be called 329 at any time after the probe has been registered. 330 331 NOTE: 332 If the functions find an incorrect probe (ex. an unregistered probe), 333 they clear the addr field of the probe. 334 335 4.5 register_*probes 336 337 #include <linux/kprobes.h> 338 int register_kprobes(struct kprobe **kps, int num); 339 int register_kretprobes(struct kretprobe **rps, int num); 340 int register_jprobes(struct jprobe **jps, int num); 341 342 Registers each of the num probes in the specified array. If any 343 error occurs during registration, all probes in the array, up to 344 the bad probe, are safely unregistered before the register_*probes 345 function returns. 346 - kps/rps/jps: an array of pointers to *probe data structures 347 - num: the number of the array entries. 348 349 NOTE: 350 You have to allocate(or define) an array of pointers and set all 351 of the array entries before using these functions. 352 353 4.6 unregister_*probes 354 355 #include <linux/kprobes.h> 356 void unregister_kprobes(struct kprobe **kps, int num); 357 void unregister_kretprobes(struct kretprobe **rps, int num); 358 void unregister_jprobes(struct jprobe **jps, int num); 359 360 Removes each of the num probes in the specified array at once. 361 362 NOTE: 363 If the functions find some incorrect probes (ex. unregistered 364 probes) in the specified array, they clear the addr field of those 365 incorrect probes. However, other probes in the array are 366 unregistered correctly. 367 368 4.7 disable_*probe 369 370 #include <linux/kprobes.h> 371 int disable_kprobe(struct kprobe *kp); 372 int disable_kretprobe(struct kretprobe *rp); 373 int disable_jprobe(struct jprobe *jp); 374 375 Temporarily disables the specified *probe. You can enable it again by using 376 enable_*probe(). You must specify the probe which has been registered. 377 378 4.8 enable_*probe 379 380 #include <linux/kprobes.h> 381 int enable_kprobe(struct kprobe *kp); 382 int enable_kretprobe(struct kretprobe *rp); 383 int enable_jprobe(struct jprobe *jp); 384 385 Enables *probe which has been disabled by disable_*probe(). You must specify 386 the probe which has been registered. 387 388 5. Kprobes Features and Limitations 389 390 Kprobes allows multiple probes at the same address. Currently, 391 however, there cannot be multiple jprobes on the same function at 392 the same time. 393 394 In general, you can install a probe anywhere in the kernel. 395 In particular, you can probe interrupt handlers. Known exceptions 396 are discussed in this section. 397 398 The register_*probe functions will return -EINVAL if you attempt 399 to install a probe in the code that implements Kprobes (mostly 400 kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such 401 as do_page_fault and notifier_call_chain). 402 403 If you install a probe in an inline-able function, Kprobes makes 404 no attempt to chase down all inline instances of the function and 405 install probes there. gcc may inline a function without being asked, 406 so keep this in mind if you're not seeing the probe hits you expect. 407 408 A probe handler can modify the environment of the probed function 409 -- e.g., by modifying kernel data structures, or by modifying the 410 contents of the pt_regs struct (which are restored to the registers 411 upon return from the breakpoint). So Kprobes can be used, for example, 412 to install a bug fix or to inject faults for testing. Kprobes, of 413 course, has no way to distinguish the deliberately injected faults 414 from the accidental ones. Don't drink and probe. 415 416 Kprobes makes no attempt to prevent probe handlers from stepping on 417 each other -- e.g., probing printk() and then calling printk() from a 418 probe handler. If a probe handler hits a probe, that second probe's 419 handlers won't be run in that instance, and the kprobe.nmissed member 420 of the second probe will be incremented. 421 422 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of 423 the same handler) may run concurrently on different CPUs. 424 425 Kprobes does not use mutexes or allocate memory except during 426 registration and unregistration. 427 428 Probe handlers are run with preemption disabled. Depending on the 429 architecture, handlers may also run with interrupts disabled. In any 430 case, your handler should not yield the CPU (e.g., by attempting to 431 acquire a semaphore). 432 433 Since a return probe is implemented by replacing the return 434 address with the trampoline's address, stack backtraces and calls 435 to __builtin_return_address() will typically yield the trampoline's 436 address instead of the real return address for kretprobed functions. 437 (As far as we can tell, __builtin_return_address() is used only 438 for instrumentation and error reporting.) 439 440 If the number of times a function is called does not match the number 441 of times it returns, registering a return probe on that function may 442 produce undesirable results. In such a case, a line: 443 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c 444 gets printed. With this information, one will be able to correlate the 445 exact instance of the kretprobe that caused the problem. We have the 446 do_exit() case covered. do_execve() and do_fork() are not an issue. 447 We're unaware of other specific cases where this could be a problem. 448 449 If, upon entry to or exit from a function, the CPU is running on 450 a stack other than that of the current task, registering a return 451 probe on that function may produce undesirable results. For this 452 reason, Kprobes doesn't support return probes (or kprobes or jprobes) 453 on the x86_64 version of __switch_to(); the registration functions 454 return -EINVAL. 455 456 6. Probe Overhead 457 458 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 459 microseconds to process. Specifically, a benchmark that hits the same 460 probepoint repeatedly, firing a simple handler each time, reports 1-2 461 million hits per second, depending on the architecture. A jprobe or 462 return-probe hit typically takes 50-75% longer than a kprobe hit. 463 When you have a return probe set on a function, adding a kprobe at 464 the entry to that function adds essentially no overhead. 465 466 Here are sample overhead figures (in usec) for different architectures. 467 k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe 468 on same function; jr = jprobe + return probe on same function 469 470 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips 471 k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 472 473 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips 474 k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 475 476 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) 477 k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 478 479 7. TODO 480 481 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified 482 programming interface for probe-based instrumentation. Try it out. 483 b. Kernel return probes for sparc64. 484 c. Support for other architectures. 485 d. User-space probes. 486 e. Watchpoint probes (which fire on data references). 487 488 8. Kprobes Example 489 490 See samples/kprobes/kprobe_example.c 491 492 9. Jprobes Example 493 494 See samples/kprobes/jprobe_example.c 495 496 10. Kretprobes Example 497 498 See samples/kprobes/kretprobe_example.c 499 500 For additional information on Kprobes, refer to the following URLs: 501 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe 502 http://www.redhat.com/magazine/005mar05/features/kprobes/ 503 http://www-users.cs.umn.edu/~boutcher/kprobes/ 504 http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) 505 506 507 Appendix A: The kprobes debugfs interface 508 509 With recent kernels (> 2.6.20) the list of registered kprobes is visible 510 under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug). 511 512 /debug/kprobes/list: Lists all registered probes on the system 513 514 c015d71a k vfs_read+0x0 515 c011a316 j do_fork+0x0 516 c03dedc5 r tcp_v4_rcv+0x0 517 518 The first column provides the kernel address where the probe is inserted. 519 The second column identifies the type of probe (k - kprobe, r - kretprobe 520 and j - jprobe), while the third column specifies the symbol+offset of 521 the probe. If the probed function belongs to a module, the module name 522 is also specified. Following columns show probe status. If the probe is on 523 a virtual address that is no longer valid (module init sections, module 524 virtual addresses that correspond to modules that've been unloaded), 525 such probes are marked with [GONE]. If the probe is temporarily disabled, 526 such probes are marked with [DISABLED]. 527 528 /debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. 529 530 Provides a knob to globally and forcibly turn registered kprobes ON or OFF. 531 By default, all kprobes are enabled. By echoing "0" to this file, all 532 registered probes will be disarmed, till such time a "1" is echoed to this 533 file. Note that this knob just disarms and arms all kprobes and doesn't 534 change each probe's disabling state. This means that disabled kprobes (marked 535 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
