1Title : Kernel Probes (Kprobes) 2Authors : Jim Keniston <jkenisto@us.ibm.com> 3 : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com> 4 : Masami Hiramatsu <mhiramat@redhat.com> 5 6CONTENTS 7 81. Concepts: Kprobes, Jprobes, Return Probes 92. Architectures Supported 103. Configuring Kprobes 114. API Reference 125. Kprobes Features and Limitations 136. Probe Overhead 147. TODO 158. Kprobes Example 169. Jprobes Example 1710. Kretprobes Example 18Appendix A: The kprobes debugfs interface 19Appendix B: The kprobes sysctl interface 20 211. Concepts: Kprobes, Jprobes, Return Probes 22 23Kprobes enables you to dynamically break into any kernel routine and 24collect debugging and performance information non-disruptively. You 25can trap at almost any kernel code address, specifying a handler 26routine to be invoked when the breakpoint is hit. 27 28There are currently three types of probes: kprobes, jprobes, and 29kretprobes (also called return probes). A kprobe can be inserted 30on virtually any instruction in the kernel. A jprobe is inserted at 31the entry to a kernel function, and provides convenient access to the 32function's arguments. A return probe fires when a specified function 33returns. 34 35In the typical case, Kprobes-based instrumentation is packaged as 36a kernel module. The module's init function installs ("registers") 37one or more probes, and the exit function unregisters them. A 38registration function such as register_kprobe() specifies where 39the probe is to be inserted and what handler is to be called when 40the probe is hit. 41 42There are also register_/unregister_*probes() functions for batch 43registration/unregistration of a group of *probes. These functions 44can speed up unregistration process when you have to unregister 45a lot of probes at once. 46 47The next four subsections explain how the different types of 48probes work and how jump optimization works. They explain certain 49things that you'll need to know in order to make the best use of 50Kprobes -- e.g., the difference between a pre_handler and 51a post_handler, and how to use the maxactive and nmissed fields of 52a kretprobe. But if you're in a hurry to start using Kprobes, you 53can skip ahead to section 2. 54 551.1 How Does a Kprobe Work? 56 57When a kprobe is registered, Kprobes makes a copy of the probed 58instruction and replaces the first byte(s) of the probed instruction 59with a breakpoint instruction (e.g., int3 on i386 and x86_64). 60 61When a CPU hits the breakpoint instruction, a trap occurs, the CPU's 62registers are saved, and control passes to Kprobes via the 63notifier_call_chain mechanism. Kprobes executes the "pre_handler" 64associated with the kprobe, passing the handler the addresses of the 65kprobe struct and the saved registers. 66 67Next, Kprobes single-steps its copy of the probed instruction. 68(It would be simpler to single-step the actual instruction in place, 69but then Kprobes would have to temporarily remove the breakpoint 70instruction. This would open a small time window when another CPU 71could sail right past the probepoint.) 72 73After the instruction is single-stepped, Kprobes executes the 74"post_handler," if any, that is associated with the kprobe. 75Execution then continues with the instruction following the probepoint. 76 771.2 How Does a Jprobe Work? 78 79A jprobe is implemented using a kprobe that is placed on a function's 80entry point. It employs a simple mirroring principle to allow 81seamless access to the probed function's arguments. The jprobe 82handler routine should have the same signature (arg list and return 83type) as the function being probed, and must always end by calling 84the Kprobes function jprobe_return(). 85 86Here's how it works. When the probe is hit, Kprobes makes a copy of 87the saved registers and a generous portion of the stack (see below). 88Kprobes then points the saved instruction pointer at the jprobe's 89handler routine, and returns from the trap. As a result, control 90passes to the handler, which is presented with the same register and 91stack contents as the probed function. When it is done, the handler 92calls jprobe_return(), which traps again to restore the original stack 93contents and processor state and switch to the probed function. 94 95By convention, the callee owns its arguments, so gcc may produce code 96that unexpectedly modifies that portion of the stack. This is why 97Kprobes saves a copy of the stack and restores it after the jprobe 98handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g., 9964 bytes on i386. 100 101Note that the probed function's args may be passed on the stack 102or in registers. The jprobe will work in either case, so long as the 103handler's prototype matches that of the probed function. 104 1051.3 Return Probes 106 1071.3.1 How Does a Return Probe Work? 108 109When you call register_kretprobe(), Kprobes establishes a kprobe at 110the entry to the function. When the probed function is called and this 111probe is hit, Kprobes saves a copy of the return address, and replaces 112the return address with the address of a "trampoline." The trampoline 113is an arbitrary piece of code -- typically just a nop instruction. 114At boot time, Kprobes registers a kprobe at the trampoline. 115 116When the probed function executes its return instruction, control 117passes to the trampoline and that probe is hit. Kprobes' trampoline 118handler calls the user-specified return handler associated with the 119kretprobe, then sets the saved instruction pointer to the saved return 120address, and that's where execution resumes upon return from the trap. 121 122While the probed function is executing, its return address is 123stored in an object of type kretprobe_instance. Before calling 124register_kretprobe(), the user sets the maxactive field of the 125kretprobe struct to specify how many instances of the specified 126function can be probed simultaneously. register_kretprobe() 127pre-allocates the indicated number of kretprobe_instance objects. 128 129For example, if the function is non-recursive and is called with a 130spinlock held, maxactive = 1 should be enough. If the function is 131non-recursive and can never relinquish the CPU (e.g., via a semaphore 132or preemption), NR_CPUS should be enough. If maxactive <= 0, it is 133set to a default value. If CONFIG_PREEMPT is enabled, the default 134is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS. 135 136It's not a disaster if you set maxactive too low; you'll just miss 137some probes. In the kretprobe struct, the nmissed field is set to 138zero when the return probe is registered, and is incremented every 139time the probed function is entered but there is no kretprobe_instance 140object available for establishing the return probe. 141 1421.3.2 Kretprobe entry-handler 143 144Kretprobes also provides an optional user-specified handler which runs 145on function entry. This handler is specified by setting the entry_handler 146field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the 147function entry is hit, the user-defined entry_handler, if any, is invoked. 148If the entry_handler returns 0 (success) then a corresponding return handler 149is guaranteed to be called upon function return. If the entry_handler 150returns a non-zero error then Kprobes leaves the return address as is, and 151the kretprobe has no further effect for that particular function instance. 152 153Multiple entry and return handler invocations are matched using the unique 154kretprobe_instance object associated with them. Additionally, a user 155may also specify per return-instance private data to be part of each 156kretprobe_instance object. This is especially useful when sharing private 157data between corresponding user entry and return handlers. The size of each 158private data object can be specified at kretprobe registration time by 159setting the data_size field of the kretprobe struct. This data can be 160accessed through the data field of each kretprobe_instance object. 161 162In case probed function is entered but there is no kretprobe_instance 163object available, then in addition to incrementing the nmissed count, 164the user entry_handler invocation is also skipped. 165 1661.4 How Does Jump Optimization Work? 167 168If your kernel is built with CONFIG_OPTPROBES=y (currently this flag 169is automatically set 'y' on x86/x86-64, non-preemptive kernel) and 170the "debug.kprobes_optimization" kernel parameter is set to 1 (see 171sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump 172instruction instead of a breakpoint instruction at each probepoint. 173 1741.4.1 Init a Kprobe 175 176When a probe is registered, before attempting this optimization, 177Kprobes inserts an ordinary, breakpoint-based kprobe at the specified 178address. So, even if it's not possible to optimize this particular 179probepoint, there'll be a probe there. 180 1811.4.2 Safety Check 182 183Before optimizing a probe, Kprobes performs the following safety checks: 184 185- Kprobes verifies that the region that will be replaced by the jump 186instruction (the "optimized region") lies entirely within one function. 187(A jump instruction is multiple bytes, and so may overlay multiple 188instructions.) 189 190- Kprobes analyzes the entire function and verifies that there is no 191jump 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 201the instruction can be executed out of line. 202 2031.4.3 Preparing Detour Buffer 204 205Next, Kprobes prepares a "detour" buffer, which contains the following 206instruction 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 2131.4.4 Pre-optimization 214 215After preparing the detour buffer, Kprobes verifies that none of the 216following situations exist: 217- The probe has either a break_handler (i.e., it's a jprobe) or a 218post_handler. 219- Other instructions in the optimized region are probed. 220- The probe is disabled. 221In any of the above cases, Kprobes won't start optimizing the probe. 222Since these are temporary situations, Kprobes tries to start 223optimizing it again if the situation is changed. 224 225If the kprobe can be optimized, Kprobes enqueues the kprobe to an 226optimizing list, and kicks the kprobe-optimizer workqueue to optimize 227it. If the to-be-optimized probepoint is hit before being optimized, 228Kprobes returns control to the original instruction path by setting 229the CPU's instruction pointer to the copied code in the detour buffer 230-- thus at least avoiding the single-step. 231 2321.4.5 Optimization 233 234The Kprobe-optimizer doesn't insert the jump instruction immediately; 235rather, it calls synchronize_sched() for safety first, because it's 236possible for a CPU to be interrupted in the middle of executing the 237optimized region(*). As you know, synchronize_sched() can ensure 238that all interruptions that were active when synchronize_sched() 239was called are done, but only if CONFIG_PREEMPT=n. So, this version 240of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**) 241 242After that, the Kprobe-optimizer calls stop_machine() to replace 243the optimized region with a jump instruction to the detour buffer, 244using text_poke_smp(). 245 2461.4.6 Unoptimization 247 248When an optimized kprobe is unregistered, disabled, or blocked by 249another kprobe, it will be unoptimized. If this happens before 250the optimization is complete, the kprobe is just dequeued from the 251optimized list. If the optimization has been done, the jump is 252replaced with the original code (except for an int3 breakpoint in 253the first byte) by using text_poke_smp(). 254 255(*)Please imagine that the 2nd instruction is interrupted and then 256the optimizer replaces the 2nd instruction with the jump *address* 257while the interrupt handler is running. When the interrupt 258returns to original address, there is no valid instruction, 259and it causes an unexpected result. 260 261(**)This optimization-safety checking may be replaced with the 262stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y 263kernel. 264 265NOTE for geeks: 266The jump optimization changes the kprobe's pre_handler behavior. 267Without optimization, the pre_handler can change the kernel's execution 268path by changing regs->ip and returning 1. However, when the probe 269is optimized, that modification is ignored. Thus, if you want to 270tweak the kernel's execution path, you need to suppress optimization, 271using 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 2762. Architectures Supported 277 278Kprobes, jprobes, and return probes are implemented on the following 279architectures: 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 2903. Configuring Kprobes 291 292When configuring the kernel using make menuconfig/xconfig/oldconfig, 293ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation 294Support", look for "Kprobes". 295 296So that you can load and unload Kprobes-based instrumentation modules, 297make sure "Loadable module support" (CONFIG_MODULES) and "Module 298unloading" (CONFIG_MODULE_UNLOAD) are set to "y". 299 300Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL 301are set to "y", since kallsyms_lookup_name() is used by the in-kernel 302kprobe address resolution code. 303 304If you need to insert a probe in the middle of a function, you may find 305it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), 306so you can use "objdump -d -l vmlinux" to see the source-to-object 307code mapping. 308 3094. API Reference 310 311The Kprobes API includes a "register" function and an "unregister" 312function for each type of probe. The API also includes "register_*probes" 313and "unregister_*probes" functions for (un)registering arrays of probes. 314Here are terse, mini-man-page specifications for these functions and 315the associated probe handlers that you'll write. See the files in the 316samples/kprobes/ sub-directory for examples. 317 3184.1 register_kprobe 319 320#include <linux/kprobes.h> 321int register_kprobe(struct kprobe *kp); 322 323Sets a breakpoint at the address kp->addr. When the breakpoint is 324hit, Kprobes calls kp->pre_handler. After the probed instruction 325is single-stepped, Kprobe calls kp->post_handler. If a fault 326occurs during execution of kp->pre_handler or kp->post_handler, 327or during single-stepping of the probed instruction, Kprobes calls 328kp->fault_handler. Any or all handlers can be NULL. If kp->flags 329is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled, 330so, its handlers aren't hit until calling enable_kprobe(kp). 331 332NOTE: 3331. With the introduction of the "symbol_name" field to struct kprobe, 334the probepoint address resolution will now be taken care of by the kernel. 335The following will now work: 336 337 kp.symbol_name = "symbol_name"; 338 339(64-bit powerpc intricacies such as function descriptors are handled 340transparently) 341 3422. Use the "offset" field of struct kprobe if the offset into the symbol 343to install a probepoint is known. This field is used to calculate the 344probepoint. 345 3463. Specify either the kprobe "symbol_name" OR the "addr". If both are 347specified, kprobe registration will fail with -EINVAL. 348 3494. With CISC architectures (such as i386 and x86_64), the kprobes code 350does not validate if the kprobe.addr is at an instruction boundary. 351Use "offset" with caution. 352 353register_kprobe() returns 0 on success, or a negative errno otherwise. 354 355User's pre-handler (kp->pre_handler): 356#include <linux/kprobes.h> 357#include <linux/ptrace.h> 358int pre_handler(struct kprobe *p, struct pt_regs *regs); 359 360Called with p pointing to the kprobe associated with the breakpoint, 361and regs pointing to the struct containing the registers saved when 362the breakpoint was hit. Return 0 here unless you're a Kprobes geek. 363 364User's post-handler (kp->post_handler): 365#include <linux/kprobes.h> 366#include <linux/ptrace.h> 367void post_handler(struct kprobe *p, struct pt_regs *regs, 368 unsigned long flags); 369 370p and regs are as described for the pre_handler. flags always seems 371to be zero. 372 373User's fault-handler (kp->fault_handler): 374#include <linux/kprobes.h> 375#include <linux/ptrace.h> 376int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); 377 378p and regs are as described for the pre_handler. trapnr is the 379architecture-specific trap number associated with the fault (e.g., 380on i386, 13 for a general protection fault or 14 for a page fault). 381Returns 1 if it successfully handled the exception. 382 3834.2 register_jprobe 384 385#include <linux/kprobes.h> 386int register_jprobe(struct jprobe *jp) 387 388Sets a breakpoint at the address jp->kp.addr, which must be the address 389of the first instruction of a function. When the breakpoint is hit, 390Kprobes runs the handler whose address is jp->entry. 391 392The handler should have the same arg list and return type as the probed 393function; and just before it returns, it must call jprobe_return(). 394(The handler never actually returns, since jprobe_return() returns 395control to Kprobes.) If the probed function is declared asmlinkage 396or anything else that affects how args are passed, the handler's 397declaration must match. 398 399register_jprobe() returns 0 on success, or a negative errno otherwise. 400 4014.3 register_kretprobe 402 403#include <linux/kprobes.h> 404int register_kretprobe(struct kretprobe *rp); 405 406Establishes a return probe for the function whose address is 407rp->kp.addr. When that function returns, Kprobes calls rp->handler. 408You must set rp->maxactive appropriately before you call 409register_kretprobe(); see "How Does a Return Probe Work?" for details. 410 411register_kretprobe() returns 0 on success, or a negative errno 412otherwise. 413 414User's return-probe handler (rp->handler): 415#include <linux/kprobes.h> 416#include <linux/ptrace.h> 417int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); 418 419regs is as described for kprobe.pre_handler. ri points to the 420kretprobe_instance object, of which the following fields may be 421of 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 428The regs_return_value(regs) macro provides a simple abstraction to 429extract the return value from the appropriate register as defined by 430the architecture's ABI. 431 432The handler's return value is currently ignored. 433 4344.4 unregister_*probe 435 436#include <linux/kprobes.h> 437void unregister_kprobe(struct kprobe *kp); 438void unregister_jprobe(struct jprobe *jp); 439void unregister_kretprobe(struct kretprobe *rp); 440 441Removes the specified probe. The unregister function can be called 442at any time after the probe has been registered. 443 444NOTE: 445If the functions find an incorrect probe (ex. an unregistered probe), 446they clear the addr field of the probe. 447 4484.5 register_*probes 449 450#include <linux/kprobes.h> 451int register_kprobes(struct kprobe **kps, int num); 452int register_kretprobes(struct kretprobe **rps, int num); 453int register_jprobes(struct jprobe **jps, int num); 454 455Registers each of the num probes in the specified array. If any 456error occurs during registration, all probes in the array, up to 457the bad probe, are safely unregistered before the register_*probes 458function returns. 459- kps/rps/jps: an array of pointers to *probe data structures 460- num: the number of the array entries. 461 462NOTE: 463You have to allocate(or define) an array of pointers and set all 464of the array entries before using these functions. 465 4664.6 unregister_*probes 467 468#include <linux/kprobes.h> 469void unregister_kprobes(struct kprobe **kps, int num); 470void unregister_kretprobes(struct kretprobe **rps, int num); 471void unregister_jprobes(struct jprobe **jps, int num); 472 473Removes each of the num probes in the specified array at once. 474 475NOTE: 476If the functions find some incorrect probes (ex. unregistered 477probes) in the specified array, they clear the addr field of those 478incorrect probes. However, other probes in the array are 479unregistered correctly. 480 4814.7 disable_*probe 482 483#include <linux/kprobes.h> 484int disable_kprobe(struct kprobe *kp); 485int disable_kretprobe(struct kretprobe *rp); 486int disable_jprobe(struct jprobe *jp); 487 488Temporarily disables the specified *probe. You can enable it again by using 489enable_*probe(). You must specify the probe which has been registered. 490 4914.8 enable_*probe 492 493#include <linux/kprobes.h> 494int enable_kprobe(struct kprobe *kp); 495int enable_kretprobe(struct kretprobe *rp); 496int enable_jprobe(struct jprobe *jp); 497 498Enables *probe which has been disabled by disable_*probe(). You must specify 499the probe which has been registered. 500 5015. Kprobes Features and Limitations 502 503Kprobes allows multiple probes at the same address. Currently, 504however, there cannot be multiple jprobes on the same function at 505the same time. Also, a probepoint for which there is a jprobe or 506a post_handler cannot be optimized. So if you install a jprobe, 507or a kprobe with a post_handler, at an optimized probepoint, the 508probepoint will be unoptimized automatically. 509 510In general, you can install a probe anywhere in the kernel. 511In particular, you can probe interrupt handlers. Known exceptions 512are discussed in this section. 513 514The register_*probe functions will return -EINVAL if you attempt 515to install a probe in the code that implements Kprobes (mostly 516kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such 517as do_page_fault and notifier_call_chain). 518 519If you install a probe in an inline-able function, Kprobes makes 520no attempt to chase down all inline instances of the function and 521install probes there. gcc may inline a function without being asked, 522so keep this in mind if you're not seeing the probe hits you expect. 523 524A probe handler can modify the environment of the probed function 525-- e.g., by modifying kernel data structures, or by modifying the 526contents of the pt_regs struct (which are restored to the registers 527upon return from the breakpoint). So Kprobes can be used, for example, 528to install a bug fix or to inject faults for testing. Kprobes, of 529course, has no way to distinguish the deliberately injected faults 530from the accidental ones. Don't drink and probe. 531 532Kprobes makes no attempt to prevent probe handlers from stepping on 533each other -- e.g., probing printk() and then calling printk() from a 534probe handler. If a probe handler hits a probe, that second probe's 535handlers won't be run in that instance, and the kprobe.nmissed member 536of the second probe will be incremented. 537 538As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of 539the same handler) may run concurrently on different CPUs. 540 541Kprobes does not use mutexes or allocate memory except during 542registration and unregistration. 543 544Probe handlers are run with preemption disabled. Depending on the 545architecture and optimization state, handlers may also run with 546interrupts disabled (e.g., kretprobe handlers and optimized kprobe 547handlers run without interrupt disabled on x86/x86-64). In any case, 548your handler should not yield the CPU (e.g., by attempting to acquire 549a semaphore). 550 551Since a return probe is implemented by replacing the return 552address with the trampoline's address, stack backtraces and calls 553to __builtin_return_address() will typically yield the trampoline's 554address instead of the real return address for kretprobed functions. 555(As far as we can tell, __builtin_return_address() is used only 556for instrumentation and error reporting.) 557 558If the number of times a function is called does not match the number 559of times it returns, registering a return probe on that function may 560produce undesirable results. In such a case, a line: 561kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c 562gets printed. With this information, one will be able to correlate the 563exact instance of the kretprobe that caused the problem. We have the 564do_exit() case covered. do_execve() and do_fork() are not an issue. 565We're unaware of other specific cases where this could be a problem. 566 567If, upon entry to or exit from a function, the CPU is running on 568a stack other than that of the current task, registering a return 569probe on that function may produce undesirable results. For this 570reason, Kprobes doesn't support return probes (or kprobes or jprobes) 571on the x86_64 version of __switch_to(); the registration functions 572return -EINVAL. 573 574On x86/x86-64, since the Jump Optimization of Kprobes modifies 575instructions widely, there are some limitations to optimization. To 576explain it, we introduce some terminology. Imagine a 3-instruction 577sequence consisting of a two 2-byte instructions and one 3-byte 578instruction. 579 580 IA 581 | 582[-2][-1][0][1][2][3][4][5][6][7] 583 [ins1][ins2][ ins3 ] 584 [<- DCR ->] 585 [<- JTPR ->] 586 587ins1: 1st Instruction 588ins2: 2nd Instruction 589ins3: 3rd Instruction 590IA: Insertion Address 591JTPR: Jump Target Prohibition Region 592DCR: Detoured Code Region 593 594The instructions in DCR are copied to the out-of-line buffer 595of the kprobe, because the bytes in DCR are replaced by 596a 5-byte jump instruction. So there are several limitations. 597 598a) The instructions in DCR must be relocatable. 599b) The instructions in DCR must not include a call instruction. 600c) JTPR must not be targeted by any jump or call instruction. 601d) DCR must not straddle the border between functions. 602 603Anyway, these limitations are checked by the in-kernel instruction 604decoder, so you don't need to worry about that. 605 6066. Probe Overhead 607 608On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 609microseconds to process. Specifically, a benchmark that hits the same 610probepoint repeatedly, firing a simple handler each time, reports 1-2 611million hits per second, depending on the architecture. A jprobe or 612return-probe hit typically takes 50-75% longer than a kprobe hit. 613When you have a return probe set on a function, adding a kprobe at 614the entry to that function adds essentially no overhead. 615 616Here are sample overhead figures (in usec) for different architectures. 617k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe 618on same function; jr = jprobe + return probe on same function 619 620i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips 621k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 622 623x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips 624k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 625 626ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) 627k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 628 6296.1 Optimized Probe Overhead 630 631Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to 632process. Here are sample overhead figures (in usec) for x86 architectures. 633k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe, 634r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe. 635 636i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 637k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33 638 639x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 640k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30 641 6427. TODO 643 644a. SystemTap (http://sourceware.org/systemtap): Provides a simplified 645programming interface for probe-based instrumentation. Try it out. 646b. Kernel return probes for sparc64. 647c. Support for other architectures. 648d. User-space probes. 649e. Watchpoint probes (which fire on data references). 650 6518. Kprobes Example 652 653See samples/kprobes/kprobe_example.c 654 6559. Jprobes Example 656 657See samples/kprobes/jprobe_example.c 658 65910. Kretprobes Example 660 661See samples/kprobes/kretprobe_example.c 662 663For additional information on Kprobes, refer to the following URLs: 664http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe 665http://www.redhat.com/magazine/005mar05/features/kprobes/ 666http://www-users.cs.umn.edu/~boutcher/kprobes/ 667http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) 668 669 670Appendix A: The kprobes debugfs interface 671 672With recent kernels (> 2.6.20) the list of registered kprobes is visible 673under 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 677c015d71a k vfs_read+0x0 678c011a316 j do_fork+0x0 679c03dedc5 r tcp_v4_rcv+0x0 680 681The first column provides the kernel address where the probe is inserted. 682The second column identifies the type of probe (k - kprobe, r - kretprobe 683and j - jprobe), while the third column specifies the symbol+offset of 684the probe. If the probed function belongs to a module, the module name 685is also specified. Following columns show probe status. If the probe is on 686a virtual address that is no longer valid (module init sections, module 687virtual addresses that correspond to modules that've been unloaded), 688such probes are marked with [GONE]. If the probe is temporarily disabled, 689such probes are marked with [DISABLED]. If the probe is optimized, it is 690marked with [OPTIMIZED]. 691 692/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. 693 694Provides a knob to globally and forcibly turn registered kprobes ON or OFF. 695By default, all kprobes are enabled. By echoing "0" to this file, all 696registered probes will be disarmed, till such time a "1" is echoed to this 697file. Note that this knob just disarms and arms all kprobes and doesn't 698change 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 702Appendix B: The kprobes sysctl interface 703 704/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF. 705 706When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides 707a knob to globally and forcibly turn jump optimization (see section 7081.4) ON or OFF. By default, jump optimization is allowed (ON). 709If you echo "0" to this file or set "debug.kprobes_optimization" to 7100 via sysctl, all optimized probes will be unoptimized, and any new 711probes registered after that will not be optimized. Note that this 712knob *changes* the optimized state. This means that optimized probes 713(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be 714removed). If the knob is turned on, they will be optimized again. 715 716