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(*: some parts of the kernel code can not be trapped, see 1.5 Blacklist) 28 29There are currently three types of probes: kprobes, jprobes, and 30kretprobes (also called return probes). A kprobe can be inserted 31on virtually any instruction in the kernel. A jprobe is inserted at 32the entry to a kernel function, and provides convenient access to the 33function's arguments. A return probe fires when a specified function 34returns. 35 36In the typical case, Kprobes-based instrumentation is packaged as 37a kernel module. The module's init function installs ("registers") 38one or more probes, and the exit function unregisters them. A 39registration function such as register_kprobe() specifies where 40the probe is to be inserted and what handler is to be called when 41the probe is hit. 42 43There are also register_/unregister_*probes() functions for batch 44registration/unregistration of a group of *probes. These functions 45can speed up unregistration process when you have to unregister 46a lot of probes at once. 47 48The next four subsections explain how the different types of 49probes work and how jump optimization works. They explain certain 50things that you'll need to know in order to make the best use of 51Kprobes -- e.g., the difference between a pre_handler and 52a post_handler, and how to use the maxactive and nmissed fields of 53a kretprobe. But if you're in a hurry to start using Kprobes, you 54can skip ahead to section 2. 55 561.1 How Does a Kprobe Work? 57 58When a kprobe is registered, Kprobes makes a copy of the probed 59instruction and replaces the first byte(s) of the probed instruction 60with a breakpoint instruction (e.g., int3 on i386 and x86_64). 61 62When a CPU hits the breakpoint instruction, a trap occurs, the CPU's 63registers are saved, and control passes to Kprobes via the 64notifier_call_chain mechanism. Kprobes executes the "pre_handler" 65associated with the kprobe, passing the handler the addresses of the 66kprobe struct and the saved registers. 67 68Next, Kprobes single-steps its copy of the probed instruction. 69(It would be simpler to single-step the actual instruction in place, 70but then Kprobes would have to temporarily remove the breakpoint 71instruction. This would open a small time window when another CPU 72could sail right past the probepoint.) 73 74After the instruction is single-stepped, Kprobes executes the 75"post_handler," if any, that is associated with the kprobe. 76Execution then continues with the instruction following the probepoint. 77 781.2 How Does a Jprobe Work? 79 80A jprobe is implemented using a kprobe that is placed on a function's 81entry point. It employs a simple mirroring principle to allow 82seamless access to the probed function's arguments. The jprobe 83handler routine should have the same signature (arg list and return 84type) as the function being probed, and must always end by calling 85the Kprobes function jprobe_return(). 86 87Here's how it works. When the probe is hit, Kprobes makes a copy of 88the saved registers and a generous portion of the stack (see below). 89Kprobes then points the saved instruction pointer at the jprobe's 90handler routine, and returns from the trap. As a result, control 91passes to the handler, which is presented with the same register and 92stack contents as the probed function. When it is done, the handler 93calls jprobe_return(), which traps again to restore the original stack 94contents and processor state and switch to the probed function. 95 96By convention, the callee owns its arguments, so gcc may produce code 97that unexpectedly modifies that portion of the stack. This is why 98Kprobes saves a copy of the stack and restores it after the jprobe 99handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g., 10064 bytes on i386. 101 102Note that the probed function's args may be passed on the stack 103or in registers. The jprobe will work in either case, so long as the 104handler's prototype matches that of the probed function. 105 1061.3 Return Probes 107 1081.3.1 How Does a Return Probe Work? 109 110When you call register_kretprobe(), Kprobes establishes a kprobe at 111the entry to the function. When the probed function is called and this 112probe is hit, Kprobes saves a copy of the return address, and replaces 113the return address with the address of a "trampoline." The trampoline 114is an arbitrary piece of code -- typically just a nop instruction. 115At boot time, Kprobes registers a kprobe at the trampoline. 116 117When the probed function executes its return instruction, control 118passes to the trampoline and that probe is hit. Kprobes' trampoline 119handler calls the user-specified return handler associated with the 120kretprobe, then sets the saved instruction pointer to the saved return 121address, and that's where execution resumes upon return from the trap. 122 123While the probed function is executing, its return address is 124stored in an object of type kretprobe_instance. Before calling 125register_kretprobe(), the user sets the maxactive field of the 126kretprobe struct to specify how many instances of the specified 127function can be probed simultaneously. register_kretprobe() 128pre-allocates the indicated number of kretprobe_instance objects. 129 130For example, if the function is non-recursive and is called with a 131spinlock held, maxactive = 1 should be enough. If the function is 132non-recursive and can never relinquish the CPU (e.g., via a semaphore 133or preemption), NR_CPUS should be enough. If maxactive <= 0, it is 134set to a default value. If CONFIG_PREEMPT is enabled, the default 135is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS. 136 137It's not a disaster if you set maxactive too low; you'll just miss 138some probes. In the kretprobe struct, the nmissed field is set to 139zero when the return probe is registered, and is incremented every 140time the probed function is entered but there is no kretprobe_instance 141object available for establishing the return probe. 142 1431.3.2 Kretprobe entry-handler 144 145Kretprobes also provides an optional user-specified handler which runs 146on function entry. This handler is specified by setting the entry_handler 147field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the 148function entry is hit, the user-defined entry_handler, if any, is invoked. 149If the entry_handler returns 0 (success) then a corresponding return handler 150is guaranteed to be called upon function return. If the entry_handler 151returns a non-zero error then Kprobes leaves the return address as is, and 152the kretprobe has no further effect for that particular function instance. 153 154Multiple entry and return handler invocations are matched using the unique 155kretprobe_instance object associated with them. Additionally, a user 156may also specify per return-instance private data to be part of each 157kretprobe_instance object. This is especially useful when sharing private 158data between corresponding user entry and return handlers. The size of each 159private data object can be specified at kretprobe registration time by 160setting the data_size field of the kretprobe struct. This data can be 161accessed through the data field of each kretprobe_instance object. 162 163In case probed function is entered but there is no kretprobe_instance 164object available, then in addition to incrementing the nmissed count, 165the user entry_handler invocation is also skipped. 166 1671.4 How Does Jump Optimization Work? 168 169If your kernel is built with CONFIG_OPTPROBES=y (currently this flag 170is automatically set 'y' on x86/x86-64, non-preemptive kernel) and 171the "debug.kprobes_optimization" kernel parameter is set to 1 (see 172sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump 173instruction instead of a breakpoint instruction at each probepoint. 174 1751.4.1 Init a Kprobe 176 177When a probe is registered, before attempting this optimization, 178Kprobes inserts an ordinary, breakpoint-based kprobe at the specified 179address. So, even if it's not possible to optimize this particular 180probepoint, there'll be a probe there. 181 1821.4.2 Safety Check 183 184Before optimizing a probe, Kprobes performs the following safety checks: 185 186- Kprobes verifies that the region that will be replaced by the jump 187instruction (the "optimized region") lies entirely within one function. 188(A jump instruction is multiple bytes, and so may overlay multiple 189instructions.) 190 191- Kprobes analyzes the entire function and verifies that there is no 192jump into the optimized region. Specifically: 193 - the function contains no indirect jump; 194 - the function contains no instruction that causes an exception (since 195 the fixup code triggered by the exception could jump back into the 196 optimized region -- Kprobes checks the exception tables to verify this); 197 and 198 - there is no near jump to the optimized region (other than to the first 199 byte). 200 201- For each instruction in the optimized region, Kprobes verifies that 202the instruction can be executed out of line. 203 2041.4.3 Preparing Detour Buffer 205 206Next, Kprobes prepares a "detour" buffer, which contains the following 207instruction sequence: 208- code to push the CPU's registers (emulating a breakpoint trap) 209- a call to the trampoline code which calls user's probe handlers. 210- code to restore registers 211- the instructions from the optimized region 212- a jump back to the original execution path. 213 2141.4.4 Pre-optimization 215 216After preparing the detour buffer, Kprobes verifies that none of the 217following situations exist: 218- The probe has either a break_handler (i.e., it's a jprobe) or a 219post_handler. 220- Other instructions in the optimized region are probed. 221- The probe is disabled. 222In any of the above cases, Kprobes won't start optimizing the probe. 223Since these are temporary situations, Kprobes tries to start 224optimizing it again if the situation is changed. 225 226If the kprobe can be optimized, Kprobes enqueues the kprobe to an 227optimizing list, and kicks the kprobe-optimizer workqueue to optimize 228it. If the to-be-optimized probepoint is hit before being optimized, 229Kprobes returns control to the original instruction path by setting 230the CPU's instruction pointer to the copied code in the detour buffer 231-- thus at least avoiding the single-step. 232 2331.4.5 Optimization 234 235The Kprobe-optimizer doesn't insert the jump instruction immediately; 236rather, it calls synchronize_sched() for safety first, because it's 237possible for a CPU to be interrupted in the middle of executing the 238optimized region(*). As you know, synchronize_sched() can ensure 239that all interruptions that were active when synchronize_sched() 240was called are done, but only if CONFIG_PREEMPT=n. So, this version 241of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**) 242 243After that, the Kprobe-optimizer calls stop_machine() to replace 244the optimized region with a jump instruction to the detour buffer, 245using text_poke_smp(). 246 2471.4.6 Unoptimization 248 249When an optimized kprobe is unregistered, disabled, or blocked by 250another kprobe, it will be unoptimized. If this happens before 251the optimization is complete, the kprobe is just dequeued from the 252optimized list. If the optimization has been done, the jump is 253replaced with the original code (except for an int3 breakpoint in 254the first byte) by using text_poke_smp(). 255 256(*)Please imagine that the 2nd instruction is interrupted and then 257the optimizer replaces the 2nd instruction with the jump *address* 258while the interrupt handler is running. When the interrupt 259returns to original address, there is no valid instruction, 260and it causes an unexpected result. 261 262(**)This optimization-safety checking may be replaced with the 263stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y 264kernel. 265 266NOTE for geeks: 267The jump optimization changes the kprobe's pre_handler behavior. 268Without optimization, the pre_handler can change the kernel's execution 269path by changing regs->ip and returning 1. However, when the probe 270is optimized, that modification is ignored. Thus, if you want to 271tweak the kernel's execution path, you need to suppress optimization, 272using one of the following techniques: 273- Specify an empty function for the kprobe's post_handler or break_handler. 274 or 275- Execute 'sysctl -w debug.kprobes_optimization=n' 276 2771.5 Blacklist 278 279Kprobes can probe most of the kernel except itself. This means 280that there are some functions where kprobes cannot probe. Probing 281(trapping) such functions can cause a recursive trap (e.g. double 282fault) or the nested probe handler may never be called. 283Kprobes manages such functions as a blacklist. 284If you want to add a function into the blacklist, you just need 285to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro 286to specify a blacklisted function. 287Kprobes checks the given probe address against the blacklist and 288rejects registering it, if the given address is in the blacklist. 289 2902. Architectures Supported 291 292Kprobes, jprobes, and return probes are implemented on the following 293architectures: 294 295- i386 (Supports jump optimization) 296- x86_64 (AMD-64, EM64T) (Supports jump optimization) 297- ppc64 298- ia64 (Does not support probes on instruction slot1.) 299- sparc64 (Return probes not yet implemented.) 300- arm 301- ppc 302- mips 303- s390 304 3053. Configuring Kprobes 306 307When configuring the kernel using make menuconfig/xconfig/oldconfig, 308ensure that CONFIG_KPROBES is set to "y". Under "General setup", look 309for "Kprobes". 310 311So that you can load and unload Kprobes-based instrumentation modules, 312make sure "Loadable module support" (CONFIG_MODULES) and "Module 313unloading" (CONFIG_MODULE_UNLOAD) are set to "y". 314 315Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL 316are set to "y", since kallsyms_lookup_name() is used by the in-kernel 317kprobe address resolution code. 318 319If you need to insert a probe in the middle of a function, you may find 320it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), 321so you can use "objdump -d -l vmlinux" to see the source-to-object 322code mapping. 323 3244. API Reference 325 326The Kprobes API includes a "register" function and an "unregister" 327function for each type of probe. The API also includes "register_*probes" 328and "unregister_*probes" functions for (un)registering arrays of probes. 329Here are terse, mini-man-page specifications for these functions and 330the associated probe handlers that you'll write. See the files in the 331samples/kprobes/ sub-directory for examples. 332 3334.1 register_kprobe 334 335#include <linux/kprobes.h> 336int register_kprobe(struct kprobe *kp); 337 338Sets a breakpoint at the address kp->addr. When the breakpoint is 339hit, Kprobes calls kp->pre_handler. After the probed instruction 340is single-stepped, Kprobe calls kp->post_handler. If a fault 341occurs during execution of kp->pre_handler or kp->post_handler, 342or during single-stepping of the probed instruction, Kprobes calls 343kp->fault_handler. Any or all handlers can be NULL. If kp->flags 344is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled, 345so, its handlers aren't hit until calling enable_kprobe(kp). 346 347NOTE: 3481. With the introduction of the "symbol_name" field to struct kprobe, 349the probepoint address resolution will now be taken care of by the kernel. 350The following will now work: 351 352 kp.symbol_name = "symbol_name"; 353 354(64-bit powerpc intricacies such as function descriptors are handled 355transparently) 356 3572. Use the "offset" field of struct kprobe if the offset into the symbol 358to install a probepoint is known. This field is used to calculate the 359probepoint. 360 3613. Specify either the kprobe "symbol_name" OR the "addr". If both are 362specified, kprobe registration will fail with -EINVAL. 363 3644. With CISC architectures (such as i386 and x86_64), the kprobes code 365does not validate if the kprobe.addr is at an instruction boundary. 366Use "offset" with caution. 367 368register_kprobe() returns 0 on success, or a negative errno otherwise. 369 370User's pre-handler (kp->pre_handler): 371#include <linux/kprobes.h> 372#include <linux/ptrace.h> 373int pre_handler(struct kprobe *p, struct pt_regs *regs); 374 375Called with p pointing to the kprobe associated with the breakpoint, 376and regs pointing to the struct containing the registers saved when 377the breakpoint was hit. Return 0 here unless you're a Kprobes geek. 378 379User's post-handler (kp->post_handler): 380#include <linux/kprobes.h> 381#include <linux/ptrace.h> 382void post_handler(struct kprobe *p, struct pt_regs *regs, 383 unsigned long flags); 384 385p and regs are as described for the pre_handler. flags always seems 386to be zero. 387 388User's fault-handler (kp->fault_handler): 389#include <linux/kprobes.h> 390#include <linux/ptrace.h> 391int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); 392 393p and regs are as described for the pre_handler. trapnr is the 394architecture-specific trap number associated with the fault (e.g., 395on i386, 13 for a general protection fault or 14 for a page fault). 396Returns 1 if it successfully handled the exception. 397 3984.2 register_jprobe 399 400#include <linux/kprobes.h> 401int register_jprobe(struct jprobe *jp) 402 403Sets a breakpoint at the address jp->kp.addr, which must be the address 404of the first instruction of a function. When the breakpoint is hit, 405Kprobes runs the handler whose address is jp->entry. 406 407The handler should have the same arg list and return type as the probed 408function; and just before it returns, it must call jprobe_return(). 409(The handler never actually returns, since jprobe_return() returns 410control to Kprobes.) If the probed function is declared asmlinkage 411or anything else that affects how args are passed, the handler's 412declaration must match. 413 414register_jprobe() returns 0 on success, or a negative errno otherwise. 415 4164.3 register_kretprobe 417 418#include <linux/kprobes.h> 419int register_kretprobe(struct kretprobe *rp); 420 421Establishes a return probe for the function whose address is 422rp->kp.addr. When that function returns, Kprobes calls rp->handler. 423You must set rp->maxactive appropriately before you call 424register_kretprobe(); see "How Does a Return Probe Work?" for details. 425 426register_kretprobe() returns 0 on success, or a negative errno 427otherwise. 428 429User's return-probe handler (rp->handler): 430#include <linux/kprobes.h> 431#include <linux/ptrace.h> 432int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); 433 434regs is as described for kprobe.pre_handler. ri points to the 435kretprobe_instance object, of which the following fields may be 436of interest: 437- ret_addr: the return address 438- rp: points to the corresponding kretprobe object 439- task: points to the corresponding task struct 440- data: points to per return-instance private data; see "Kretprobe 441 entry-handler" for details. 442 443The regs_return_value(regs) macro provides a simple abstraction to 444extract the return value from the appropriate register as defined by 445the architecture's ABI. 446 447The handler's return value is currently ignored. 448 4494.4 unregister_*probe 450 451#include <linux/kprobes.h> 452void unregister_kprobe(struct kprobe *kp); 453void unregister_jprobe(struct jprobe *jp); 454void unregister_kretprobe(struct kretprobe *rp); 455 456Removes the specified probe. The unregister function can be called 457at any time after the probe has been registered. 458 459NOTE: 460If the functions find an incorrect probe (ex. an unregistered probe), 461they clear the addr field of the probe. 462 4634.5 register_*probes 464 465#include <linux/kprobes.h> 466int register_kprobes(struct kprobe **kps, int num); 467int register_kretprobes(struct kretprobe **rps, int num); 468int register_jprobes(struct jprobe **jps, int num); 469 470Registers each of the num probes in the specified array. If any 471error occurs during registration, all probes in the array, up to 472the bad probe, are safely unregistered before the register_*probes 473function returns. 474- kps/rps/jps: an array of pointers to *probe data structures 475- num: the number of the array entries. 476 477NOTE: 478You have to allocate(or define) an array of pointers and set all 479of the array entries before using these functions. 480 4814.6 unregister_*probes 482 483#include <linux/kprobes.h> 484void unregister_kprobes(struct kprobe **kps, int num); 485void unregister_kretprobes(struct kretprobe **rps, int num); 486void unregister_jprobes(struct jprobe **jps, int num); 487 488Removes each of the num probes in the specified array at once. 489 490NOTE: 491If the functions find some incorrect probes (ex. unregistered 492probes) in the specified array, they clear the addr field of those 493incorrect probes. However, other probes in the array are 494unregistered correctly. 495 4964.7 disable_*probe 497 498#include <linux/kprobes.h> 499int disable_kprobe(struct kprobe *kp); 500int disable_kretprobe(struct kretprobe *rp); 501int disable_jprobe(struct jprobe *jp); 502 503Temporarily disables the specified *probe. You can enable it again by using 504enable_*probe(). You must specify the probe which has been registered. 505 5064.8 enable_*probe 507 508#include <linux/kprobes.h> 509int enable_kprobe(struct kprobe *kp); 510int enable_kretprobe(struct kretprobe *rp); 511int enable_jprobe(struct jprobe *jp); 512 513Enables *probe which has been disabled by disable_*probe(). You must specify 514the probe which has been registered. 515 5165. Kprobes Features and Limitations 517 518Kprobes allows multiple probes at the same address. Currently, 519however, there cannot be multiple jprobes on the same function at 520the same time. Also, a probepoint for which there is a jprobe or 521a post_handler cannot be optimized. So if you install a jprobe, 522or a kprobe with a post_handler, at an optimized probepoint, the 523probepoint will be unoptimized automatically. 524 525In general, you can install a probe anywhere in the kernel. 526In particular, you can probe interrupt handlers. Known exceptions 527are discussed in this section. 528 529The register_*probe functions will return -EINVAL if you attempt 530to install a probe in the code that implements Kprobes (mostly 531kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such 532as do_page_fault and notifier_call_chain). 533 534If you install a probe in an inline-able function, Kprobes makes 535no attempt to chase down all inline instances of the function and 536install probes there. gcc may inline a function without being asked, 537so keep this in mind if you're not seeing the probe hits you expect. 538 539A probe handler can modify the environment of the probed function 540-- e.g., by modifying kernel data structures, or by modifying the 541contents of the pt_regs struct (which are restored to the registers 542upon return from the breakpoint). So Kprobes can be used, for example, 543to install a bug fix or to inject faults for testing. Kprobes, of 544course, has no way to distinguish the deliberately injected faults 545from the accidental ones. Don't drink and probe. 546 547Kprobes makes no attempt to prevent probe handlers from stepping on 548each other -- e.g., probing printk() and then calling printk() from a 549probe handler. If a probe handler hits a probe, that second probe's 550handlers won't be run in that instance, and the kprobe.nmissed member 551of the second probe will be incremented. 552 553As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of 554the same handler) may run concurrently on different CPUs. 555 556Kprobes does not use mutexes or allocate memory except during 557registration and unregistration. 558 559Probe handlers are run with preemption disabled. Depending on the 560architecture and optimization state, handlers may also run with 561interrupts disabled (e.g., kretprobe handlers and optimized kprobe 562handlers run without interrupt disabled on x86/x86-64). In any case, 563your handler should not yield the CPU (e.g., by attempting to acquire 564a semaphore). 565 566Since a return probe is implemented by replacing the return 567address with the trampoline's address, stack backtraces and calls 568to __builtin_return_address() will typically yield the trampoline's 569address instead of the real return address for kretprobed functions. 570(As far as we can tell, __builtin_return_address() is used only 571for instrumentation and error reporting.) 572 573If the number of times a function is called does not match the number 574of times it returns, registering a return probe on that function may 575produce undesirable results. In such a case, a line: 576kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c 577gets printed. With this information, one will be able to correlate the 578exact instance of the kretprobe that caused the problem. We have the 579do_exit() case covered. do_execve() and do_fork() are not an issue. 580We're unaware of other specific cases where this could be a problem. 581 582If, upon entry to or exit from a function, the CPU is running on 583a stack other than that of the current task, registering a return 584probe on that function may produce undesirable results. For this 585reason, Kprobes doesn't support return probes (or kprobes or jprobes) 586on the x86_64 version of __switch_to(); the registration functions 587return -EINVAL. 588 589On x86/x86-64, since the Jump Optimization of Kprobes modifies 590instructions widely, there are some limitations to optimization. To 591explain it, we introduce some terminology. Imagine a 3-instruction 592sequence consisting of a two 2-byte instructions and one 3-byte 593instruction. 594 595 IA 596 | 597[-2][-1][0][1][2][3][4][5][6][7] 598 [ins1][ins2][ ins3 ] 599 [<- DCR ->] 600 [<- JTPR ->] 601 602ins1: 1st Instruction 603ins2: 2nd Instruction 604ins3: 3rd Instruction 605IA: Insertion Address 606JTPR: Jump Target Prohibition Region 607DCR: Detoured Code Region 608 609The instructions in DCR are copied to the out-of-line buffer 610of the kprobe, because the bytes in DCR are replaced by 611a 5-byte jump instruction. So there are several limitations. 612 613a) The instructions in DCR must be relocatable. 614b) The instructions in DCR must not include a call instruction. 615c) JTPR must not be targeted by any jump or call instruction. 616d) DCR must not straddle the border between functions. 617 618Anyway, these limitations are checked by the in-kernel instruction 619decoder, so you don't need to worry about that. 620 6216. Probe Overhead 622 623On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 624microseconds to process. Specifically, a benchmark that hits the same 625probepoint repeatedly, firing a simple handler each time, reports 1-2 626million hits per second, depending on the architecture. A jprobe or 627return-probe hit typically takes 50-75% longer than a kprobe hit. 628When you have a return probe set on a function, adding a kprobe at 629the entry to that function adds essentially no overhead. 630 631Here are sample overhead figures (in usec) for different architectures. 632k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe 633on same function; jr = jprobe + return probe on same function 634 635i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips 636k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 637 638x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips 639k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 640 641ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) 642k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 643 6446.1 Optimized Probe Overhead 645 646Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to 647process. Here are sample overhead figures (in usec) for x86 architectures. 648k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe, 649r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe. 650 651i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 652k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33 653 654x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 655k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30 656 6577. TODO 658 659a. SystemTap (http://sourceware.org/systemtap): Provides a simplified 660programming interface for probe-based instrumentation. Try it out. 661b. Kernel return probes for sparc64. 662c. Support for other architectures. 663d. User-space probes. 664e. Watchpoint probes (which fire on data references). 665 6668. Kprobes Example 667 668See samples/kprobes/kprobe_example.c 669 6709. Jprobes Example 671 672See samples/kprobes/jprobe_example.c 673 67410. Kretprobes Example 675 676See samples/kprobes/kretprobe_example.c 677 678For additional information on Kprobes, refer to the following URLs: 679http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe 680http://www.redhat.com/magazine/005mar05/features/kprobes/ 681http://www-users.cs.umn.edu/~boutcher/kprobes/ 682http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) 683 684 685Appendix A: The kprobes debugfs interface 686 687With recent kernels (> 2.6.20) the list of registered kprobes is visible 688under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug). 689 690/sys/kernel/debug/kprobes/list: Lists all registered probes on the system 691 692c015d71a k vfs_read+0x0 693c011a316 j do_fork+0x0 694c03dedc5 r tcp_v4_rcv+0x0 695 696The first column provides the kernel address where the probe is inserted. 697The second column identifies the type of probe (k - kprobe, r - kretprobe 698and j - jprobe), while the third column specifies the symbol+offset of 699the probe. If the probed function belongs to a module, the module name 700is also specified. Following columns show probe status. If the probe is on 701a virtual address that is no longer valid (module init sections, module 702virtual addresses that correspond to modules that've been unloaded), 703such probes are marked with [GONE]. If the probe is temporarily disabled, 704such probes are marked with [DISABLED]. If the probe is optimized, it is 705marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with 706[FTRACE]. 707 708/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. 709 710Provides a knob to globally and forcibly turn registered kprobes ON or OFF. 711By default, all kprobes are enabled. By echoing "0" to this file, all 712registered probes will be disarmed, till such time a "1" is echoed to this 713file. Note that this knob just disarms and arms all kprobes and doesn't 714change each probe's disabling state. This means that disabled kprobes (marked 715[DISABLED]) will be not enabled if you turn ON all kprobes by this knob. 716 717 718Appendix B: The kprobes sysctl interface 719 720/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF. 721 722When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides 723a knob to globally and forcibly turn jump optimization (see section 7241.4) ON or OFF. By default, jump optimization is allowed (ON). 725If you echo "0" to this file or set "debug.kprobes_optimization" to 7260 via sysctl, all optimized probes will be unoptimized, and any new 727probes registered after that will not be optimized. Note that this 728knob *changes* the optimized state. This means that optimized probes 729(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be 730removed). If the knob is turned on, they will be optimized again. 731 732