Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 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 The next three subsections explain how the different types of 41 probes work. They explain certain things that you'll need to 42 know in order to make the best use of Kprobes -- e.g., the 43 difference between a pre_handler and a post_handler, and how 44 to use the maxactive and nmissed fields of a kretprobe. But 45 if you're in a hurry to start using Kprobes, you can skip ahead 46 to section 2. 47 48 1.1 How Does a Kprobe Work? 49 50 When a kprobe is registered, Kprobes makes a copy of the probed 51 instruction and replaces the first byte(s) of the probed instruction 52 with a breakpoint instruction (e.g., int3 on i386 and x86_64). 53 54 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's 55 registers are saved, and control passes to Kprobes via the 56 notifier_call_chain mechanism. Kprobes executes the "pre_handler" 57 associated with the kprobe, passing the handler the addresses of the 58 kprobe struct and the saved registers. 59 60 Next, Kprobes single-steps its copy of the probed instruction. 61 (It would be simpler to single-step the actual instruction in place, 62 but then Kprobes would have to temporarily remove the breakpoint 63 instruction. This would open a small time window when another CPU 64 could sail right past the probepoint.) 65 66 After the instruction is single-stepped, Kprobes executes the 67 "post_handler," if any, that is associated with the kprobe. 68 Execution then continues with the instruction following the probepoint. 69 70 1.2 How Does a Jprobe Work? 71 72 A jprobe is implemented using a kprobe that is placed on a function's 73 entry point. It employs a simple mirroring principle to allow 74 seamless access to the probed function's arguments. The jprobe 75 handler routine should have the same signature (arg list and return 76 type) as the function being probed, and must always end by calling 77 the Kprobes function jprobe_return(). 78 79 Here's how it works. When the probe is hit, Kprobes makes a copy of 80 the saved registers and a generous portion of the stack (see below). 81 Kprobes then points the saved instruction pointer at the jprobe's 82 handler routine, and returns from the trap. As a result, control 83 passes to the handler, which is presented with the same register and 84 stack contents as the probed function. When it is done, the handler 85 calls jprobe_return(), which traps again to restore the original stack 86 contents and processor state and switch to the probed function. 87 88 By convention, the callee owns its arguments, so gcc may produce code 89 that unexpectedly modifies that portion of the stack. This is why 90 Kprobes saves a copy of the stack and restores it after the jprobe 91 handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g., 92 64 bytes on i386. 93 94 Note that the probed function's args may be passed on the stack 95 or in registers. The jprobe will work in either case, so long as the 96 handler's prototype matches that of the probed function. 97 98 1.3 Return Probes 99 100 1.3.1 How Does a Return Probe Work? 101 102 When you call register_kretprobe(), Kprobes establishes a kprobe at 103 the entry to the function. When the probed function is called and this 104 probe is hit, Kprobes saves a copy of the return address, and replaces 105 the return address with the address of a "trampoline." The trampoline 106 is an arbitrary piece of code -- typically just a nop instruction. 107 At boot time, Kprobes registers a kprobe at the trampoline. 108 109 When the probed function executes its return instruction, control 110 passes to the trampoline and that probe is hit. Kprobes' trampoline 111 handler calls the user-specified return handler associated with the 112 kretprobe, then sets the saved instruction pointer to the saved return 113 address, and that's where execution resumes upon return from the trap. 114 115 While the probed function is executing, its return address is 116 stored in an object of type kretprobe_instance. Before calling 117 register_kretprobe(), the user sets the maxactive field of the 118 kretprobe struct to specify how many instances of the specified 119 function can be probed simultaneously. register_kretprobe() 120 pre-allocates the indicated number of kretprobe_instance objects. 121 122 For example, if the function is non-recursive and is called with a 123 spinlock held, maxactive = 1 should be enough. If the function is 124 non-recursive and can never relinquish the CPU (e.g., via a semaphore 125 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is 126 set to a default value. If CONFIG_PREEMPT is enabled, the default 127 is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS. 128 129 It's not a disaster if you set maxactive too low; you'll just miss 130 some probes. In the kretprobe struct, the nmissed field is set to 131 zero when the return probe is registered, and is incremented every 132 time the probed function is entered but there is no kretprobe_instance 133 object available for establishing the return probe. 134 135 1.3.2 Kretprobe entry-handler 136 137 Kretprobes also provides an optional user-specified handler which runs 138 on function entry. This handler is specified by setting the entry_handler 139 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the 140 function entry is hit, the user-defined entry_handler, if any, is invoked. 141 If the entry_handler returns 0 (success) then a corresponding return handler 142 is guaranteed to be called upon function return. If the entry_handler 143 returns a non-zero error then Kprobes leaves the return address as is, and 144 the kretprobe has no further effect for that particular function instance. 145 146 Multiple entry and return handler invocations are matched using the unique 147 kretprobe_instance object associated with them. Additionally, a user 148 may also specify per return-instance private data to be part of each 149 kretprobe_instance object. This is especially useful when sharing private 150 data between corresponding user entry and return handlers. The size of each 151 private data object can be specified at kretprobe registration time by 152 setting the data_size field of the kretprobe struct. This data can be 153 accessed through the data field of each kretprobe_instance object. 154 155 In case probed function is entered but there is no kretprobe_instance 156 object available, then in addition to incrementing the nmissed count, 157 the user entry_handler invocation is also skipped. 158 159 2. Architectures Supported 160 161 Kprobes, jprobes, and return probes are implemented on the following 162 architectures: 163 164 - i386 165 - x86_64 (AMD-64, EM64T) 166 - ppc64 167 - ia64 (Does not support probes on instruction slot1.) 168 - sparc64 (Return probes not yet implemented.) 169 - arm 170 171 3. Configuring Kprobes 172 173 When configuring the kernel using make menuconfig/xconfig/oldconfig, 174 ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation 175 Support", look for "Kprobes". 176 177 So that you can load and unload Kprobes-based instrumentation modules, 178 make sure "Loadable module support" (CONFIG_MODULES) and "Module 179 unloading" (CONFIG_MODULE_UNLOAD) are set to "y". 180 181 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL 182 are set to "y", since kallsyms_lookup_name() is used by the in-kernel 183 kprobe address resolution code. 184 185 If you need to insert a probe in the middle of a function, you may find 186 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), 187 so you can use "objdump -d -l vmlinux" to see the source-to-object 188 code mapping. 189 190 4. API Reference 191 192 The Kprobes API includes a "register" function and an "unregister" 193 function for each type of probe. Here are terse, mini-man-page 194 specifications for these functions and the associated probe handlers 195 that you'll write. See the files in the samples/kprobes/ sub-directory 196 for examples. 197 198 4.1 register_kprobe 199 200 #include <linux/kprobes.h> 201 int register_kprobe(struct kprobe *kp); 202 203 Sets a breakpoint at the address kp->addr. When the breakpoint is 204 hit, Kprobes calls kp->pre_handler. After the probed instruction 205 is single-stepped, Kprobe calls kp->post_handler. If a fault 206 occurs during execution of kp->pre_handler or kp->post_handler, 207 or during single-stepping of the probed instruction, Kprobes calls 208 kp->fault_handler. Any or all handlers can be NULL. 209 210 NOTE: 211 1. With the introduction of the "symbol_name" field to struct kprobe, 212 the probepoint address resolution will now be taken care of by the kernel. 213 The following will now work: 214 215 kp.symbol_name = "symbol_name"; 216 217 (64-bit powerpc intricacies such as function descriptors are handled 218 transparently) 219 220 2. Use the "offset" field of struct kprobe if the offset into the symbol 221 to install a probepoint is known. This field is used to calculate the 222 probepoint. 223 224 3. Specify either the kprobe "symbol_name" OR the "addr". If both are 225 specified, kprobe registration will fail with -EINVAL. 226 227 4. With CISC architectures (such as i386 and x86_64), the kprobes code 228 does not validate if the kprobe.addr is at an instruction boundary. 229 Use "offset" with caution. 230 231 register_kprobe() returns 0 on success, or a negative errno otherwise. 232 233 User's pre-handler (kp->pre_handler): 234 #include <linux/kprobes.h> 235 #include <linux/ptrace.h> 236 int pre_handler(struct kprobe *p, struct pt_regs *regs); 237 238 Called with p pointing to the kprobe associated with the breakpoint, 239 and regs pointing to the struct containing the registers saved when 240 the breakpoint was hit. Return 0 here unless you're a Kprobes geek. 241 242 User's post-handler (kp->post_handler): 243 #include <linux/kprobes.h> 244 #include <linux/ptrace.h> 245 void post_handler(struct kprobe *p, struct pt_regs *regs, 246 unsigned long flags); 247 248 p and regs are as described for the pre_handler. flags always seems 249 to be zero. 250 251 User's fault-handler (kp->fault_handler): 252 #include <linux/kprobes.h> 253 #include <linux/ptrace.h> 254 int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); 255 256 p and regs are as described for the pre_handler. trapnr is the 257 architecture-specific trap number associated with the fault (e.g., 258 on i386, 13 for a general protection fault or 14 for a page fault). 259 Returns 1 if it successfully handled the exception. 260 261 4.2 register_jprobe 262 263 #include <linux/kprobes.h> 264 int register_jprobe(struct jprobe *jp) 265 266 Sets a breakpoint at the address jp->kp.addr, which must be the address 267 of the first instruction of a function. When the breakpoint is hit, 268 Kprobes runs the handler whose address is jp->entry. 269 270 The handler should have the same arg list and return type as the probed 271 function; and just before it returns, it must call jprobe_return(). 272 (The handler never actually returns, since jprobe_return() returns 273 control to Kprobes.) If the probed function is declared asmlinkage 274 or anything else that affects how args are passed, the handler's 275 declaration must match. 276 277 register_jprobe() returns 0 on success, or a negative errno otherwise. 278 279 4.3 register_kretprobe 280 281 #include <linux/kprobes.h> 282 int register_kretprobe(struct kretprobe *rp); 283 284 Establishes a return probe for the function whose address is 285 rp->kp.addr. When that function returns, Kprobes calls rp->handler. 286 You must set rp->maxactive appropriately before you call 287 register_kretprobe(); see "How Does a Return Probe Work?" for details. 288 289 register_kretprobe() returns 0 on success, or a negative errno 290 otherwise. 291 292 User's return-probe handler (rp->handler): 293 #include <linux/kprobes.h> 294 #include <linux/ptrace.h> 295 int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); 296 297 regs is as described for kprobe.pre_handler. ri points to the 298 kretprobe_instance object, of which the following fields may be 299 of interest: 300 - ret_addr: the return address 301 - rp: points to the corresponding kretprobe object 302 - task: points to the corresponding task struct 303 - data: points to per return-instance private data; see "Kretprobe 304 entry-handler" for details. 305 306 The regs_return_value(regs) macro provides a simple abstraction to 307 extract the return value from the appropriate register as defined by 308 the architecture's ABI. 309 310 The handler's return value is currently ignored. 311 312 4.4 unregister_*probe 313 314 #include <linux/kprobes.h> 315 void unregister_kprobe(struct kprobe *kp); 316 void unregister_jprobe(struct jprobe *jp); 317 void unregister_kretprobe(struct kretprobe *rp); 318 319 Removes the specified probe. The unregister function can be called 320 at any time after the probe has been registered. 321 322 5. Kprobes Features and Limitations 323 324 Kprobes allows multiple probes at the same address. Currently, 325 however, there cannot be multiple jprobes on the same function at 326 the same time. 327 328 In general, you can install a probe anywhere in the kernel. 329 In particular, you can probe interrupt handlers. Known exceptions 330 are discussed in this section. 331 332 The register_*probe functions will return -EINVAL if you attempt 333 to install a probe in the code that implements Kprobes (mostly 334 kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such 335 as do_page_fault and notifier_call_chain). 336 337 If you install a probe in an inline-able function, Kprobes makes 338 no attempt to chase down all inline instances of the function and 339 install probes there. gcc may inline a function without being asked, 340 so keep this in mind if you're not seeing the probe hits you expect. 341 342 A probe handler can modify the environment of the probed function 343 -- e.g., by modifying kernel data structures, or by modifying the 344 contents of the pt_regs struct (which are restored to the registers 345 upon return from the breakpoint). So Kprobes can be used, for example, 346 to install a bug fix or to inject faults for testing. Kprobes, of 347 course, has no way to distinguish the deliberately injected faults 348 from the accidental ones. Don't drink and probe. 349 350 Kprobes makes no attempt to prevent probe handlers from stepping on 351 each other -- e.g., probing printk() and then calling printk() from a 352 probe handler. If a probe handler hits a probe, that second probe's 353 handlers won't be run in that instance, and the kprobe.nmissed member 354 of the second probe will be incremented. 355 356 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of 357 the same handler) may run concurrently on different CPUs. 358 359 Kprobes does not use mutexes or allocate memory except during 360 registration and unregistration. 361 362 Probe handlers are run with preemption disabled. Depending on the 363 architecture, handlers may also run with interrupts disabled. In any 364 case, your handler should not yield the CPU (e.g., by attempting to 365 acquire a semaphore). 366 367 Since a return probe is implemented by replacing the return 368 address with the trampoline's address, stack backtraces and calls 369 to __builtin_return_address() will typically yield the trampoline's 370 address instead of the real return address for kretprobed functions. 371 (As far as we can tell, __builtin_return_address() is used only 372 for instrumentation and error reporting.) 373 374 If the number of times a function is called does not match the number 375 of times it returns, registering a return probe on that function may 376 produce undesirable results. In such a case, a line: 377 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c 378 gets printed. With this information, one will be able to correlate the 379 exact instance of the kretprobe that caused the problem. We have the 380 do_exit() case covered. do_execve() and do_fork() are not an issue. 381 We're unaware of other specific cases where this could be a problem. 382 383 If, upon entry to or exit from a function, the CPU is running on 384 a stack other than that of the current task, registering a return 385 probe on that function may produce undesirable results. For this 386 reason, Kprobes doesn't support return probes (or kprobes or jprobes) 387 on the x86_64 version of __switch_to(); the registration functions 388 return -EINVAL. 389 390 6. Probe Overhead 391 392 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 393 microseconds to process. Specifically, a benchmark that hits the same 394 probepoint repeatedly, firing a simple handler each time, reports 1-2 395 million hits per second, depending on the architecture. A jprobe or 396 return-probe hit typically takes 50-75% longer than a kprobe hit. 397 When you have a return probe set on a function, adding a kprobe at 398 the entry to that function adds essentially no overhead. 399 400 Here are sample overhead figures (in usec) for different architectures. 401 k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe 402 on same function; jr = jprobe + return probe on same function 403 404 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips 405 k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 406 407 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips 408 k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 409 410 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) 411 k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 412 413 7. TODO 414 415 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified 416 programming interface for probe-based instrumentation. Try it out. 417 b. Kernel return probes for sparc64. 418 c. Support for other architectures. 419 d. User-space probes. 420 e. Watchpoint probes (which fire on data references). 421 422 8. Kprobes Example 423 424 See samples/kprobes/kprobe_example.c 425 426 9. Jprobes Example 427 428 See samples/kprobes/jprobe_example.c 429 430 10. Kretprobes Example 431 432 See samples/kprobes/kretprobe_example.c 433 434 For additional information on Kprobes, refer to the following URLs: 435 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe 436 http://www.redhat.com/magazine/005mar05/features/kprobes/ 437 http://www-users.cs.umn.edu/~boutcher/kprobes/ 438 http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) 439 440 441 Appendix A: The kprobes debugfs interface 442 443 With recent kernels (> 2.6.20) the list of registered kprobes is visible 444 under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug). 445 446 /debug/kprobes/list: Lists all registered probes on the system 447 448 c015d71a k vfs_read+0x0 449 c011a316 j do_fork+0x0 450 c03dedc5 r tcp_v4_rcv+0x0 451 452 The first column provides the kernel address where the probe is inserted. 453 The second column identifies the type of probe (k - kprobe, r - kretprobe 454 and j - jprobe), while the third column specifies the symbol+offset of 455 the probe. If the probed function belongs to a module, the module name 456 is also specified. 457 458 /debug/kprobes/enabled: Turn kprobes ON/OFF 459 460 Provides a knob to globally turn registered kprobes ON or OFF. By default, 461 all kprobes are enabled. By echoing "0" to this file, all registered probes 462 will be disarmed, till such time a "1" is echoed to this file.
