1======================= 2Kernel Probes (Kprobes) 3======================= 4 5:Author: Jim Keniston <jkenisto@us.ibm.com> 6:Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com> 7:Author: Masami Hiramatsu <mhiramat@redhat.com> 8 9.. CONTENTS 10 11 1. Concepts: Kprobes, and Return Probes 12 2. Architectures Supported 13 3. Configuring Kprobes 14 4. API Reference 15 5. Kprobes Features and Limitations 16 6. Probe Overhead 17 7. TODO 18 8. Kprobes Example 19 9. Kretprobes Example 20 10. Deprecated Features 21 Appendix A: The kprobes debugfs interface 22 Appendix B: The kprobes sysctl interface 23 Appendix C: References 24 25Concepts: Kprobes and Return Probes 26========================================= 27 28Kprobes enables you to dynamically break into any kernel routine and 29collect debugging and performance information non-disruptively. You 30can trap at almost any kernel code address [1]_, specifying a handler 31routine to be invoked when the breakpoint is hit. 32 33.. [1] some parts of the kernel code can not be trapped, see 34 :ref:`kprobes_blacklist`) 35 36There are currently two types of probes: kprobes, and kretprobes 37(also called return probes). A kprobe can be inserted on virtually 38any instruction in the kernel. A return probe fires when a specified 39function returns. 40 41In the typical case, Kprobes-based instrumentation is packaged as 42a kernel module. The module's init function installs ("registers") 43one or more probes, and the exit function unregisters them. A 44registration function such as register_kprobe() specifies where 45the probe is to be inserted and what handler is to be called when 46the probe is hit. 47 48There are also ``register_/unregister_*probes()`` functions for batch 49registration/unregistration of a group of ``*probes``. These functions 50can speed up unregistration process when you have to unregister 51a lot of probes at once. 52 53The next four subsections explain how the different types of 54probes work and how jump optimization works. They explain certain 55things that you'll need to know in order to make the best use of 56Kprobes -- e.g., the difference between a pre_handler and 57a post_handler, and how to use the maxactive and nmissed fields of 58a kretprobe. But if you're in a hurry to start using Kprobes, you 59can skip ahead to :ref:`kprobes_archs_supported`. 60 61How Does a Kprobe Work? 62----------------------- 63 64When a kprobe is registered, Kprobes makes a copy of the probed 65instruction and replaces the first byte(s) of the probed instruction 66with a breakpoint instruction (e.g., int3 on i386 and x86_64). 67 68When a CPU hits the breakpoint instruction, a trap occurs, the CPU's 69registers are saved, and control passes to Kprobes via the 70notifier_call_chain mechanism. Kprobes executes the "pre_handler" 71associated with the kprobe, passing the handler the addresses of the 72kprobe struct and the saved registers. 73 74Next, Kprobes single-steps its copy of the probed instruction. 75(It would be simpler to single-step the actual instruction in place, 76but then Kprobes would have to temporarily remove the breakpoint 77instruction. This would open a small time window when another CPU 78could sail right past the probepoint.) 79 80After the instruction is single-stepped, Kprobes executes the 81"post_handler," if any, that is associated with the kprobe. 82Execution then continues with the instruction following the probepoint. 83 84Changing Execution Path 85----------------------- 86 87Since kprobes can probe into a running kernel code, it can change the 88register set, including instruction pointer. This operation requires 89maximum care, such as keeping the stack frame, recovering the execution 90path etc. Since it operates on a running kernel and needs deep knowledge 91of computer architecture and concurrent computing, you can easily shoot 92your foot. 93 94If you change the instruction pointer (and set up other related 95registers) in pre_handler, you must return !0 so that kprobes stops 96single stepping and just returns to the given address. 97This also means post_handler should not be called anymore. 98 99Note that this operation may be harder on some architectures which use 100TOC (Table of Contents) for function call, since you have to setup a new 101TOC for your function in your module, and recover the old one after 102returning from it. 103 104Return Probes 105------------- 106 107How Does a Return Probe Work? 108^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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: max(10, 2*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 142Kretprobe entry-handler 143^^^^^^^^^^^^^^^^^^^^^^^ 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. 166 167.. _kprobes_jump_optimization: 168 169How Does Jump Optimization Work? 170-------------------------------- 171 172If your kernel is built with CONFIG_OPTPROBES=y (currently this flag 173is automatically set 'y' on x86/x86-64, non-preemptive kernel) and 174the "debug.kprobes_optimization" kernel parameter is set to 1 (see 175sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump 176instruction instead of a breakpoint instruction at each probepoint. 177 178Init a Kprobe 179^^^^^^^^^^^^^ 180 181When a probe is registered, before attempting this optimization, 182Kprobes inserts an ordinary, breakpoint-based kprobe at the specified 183address. So, even if it's not possible to optimize this particular 184probepoint, there'll be a probe there. 185 186Safety Check 187^^^^^^^^^^^^ 188 189Before optimizing a probe, Kprobes performs the following safety checks: 190 191- Kprobes verifies that the region that will be replaced by the jump 192 instruction (the "optimized region") lies entirely within one function. 193 (A jump instruction is multiple bytes, and so may overlay multiple 194 instructions.) 195 196- Kprobes analyzes the entire function and verifies that there is no 197 jump into the optimized region. Specifically: 198 199 - the function contains no indirect jump; 200 - the function contains no instruction that causes an exception (since 201 the fixup code triggered by the exception could jump back into the 202 optimized region -- Kprobes checks the exception tables to verify this); 203 - there is no near jump to the optimized region (other than to the first 204 byte). 205 206- For each instruction in the optimized region, Kprobes verifies that 207 the instruction can be executed out of line. 208 209Preparing Detour Buffer 210^^^^^^^^^^^^^^^^^^^^^^^ 211 212Next, Kprobes prepares a "detour" buffer, which contains the following 213instruction sequence: 214 215- code to push the CPU's registers (emulating a breakpoint trap) 216- a call to the trampoline code which calls user's probe handlers. 217- code to restore registers 218- the instructions from the optimized region 219- a jump back to the original execution path. 220 221Pre-optimization 222^^^^^^^^^^^^^^^^ 223 224After preparing the detour buffer, Kprobes verifies that none of the 225following situations exist: 226 227- The probe has a post_handler. 228- Other instructions in the optimized region are probed. 229- The probe is disabled. 230 231In any of the above cases, Kprobes won't start optimizing the probe. 232Since these are temporary situations, Kprobes tries to start 233optimizing it again if the situation is changed. 234 235If the kprobe can be optimized, Kprobes enqueues the kprobe to an 236optimizing list, and kicks the kprobe-optimizer workqueue to optimize 237it. If the to-be-optimized probepoint is hit before being optimized, 238Kprobes returns control to the original instruction path by setting 239the CPU's instruction pointer to the copied code in the detour buffer 240-- thus at least avoiding the single-step. 241 242Optimization 243^^^^^^^^^^^^ 244 245The Kprobe-optimizer doesn't insert the jump instruction immediately; 246rather, it calls synchronize_rcu() for safety first, because it's 247possible for a CPU to be interrupted in the middle of executing the 248optimized region [3]_. As you know, synchronize_rcu() can ensure 249that all interruptions that were active when synchronize_rcu() 250was called are done, but only if CONFIG_PREEMPT=n. So, this version 251of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_. 252 253After that, the Kprobe-optimizer calls stop_machine() to replace 254the optimized region with a jump instruction to the detour buffer, 255using text_poke_smp(). 256 257Unoptimization 258^^^^^^^^^^^^^^ 259 260When an optimized kprobe is unregistered, disabled, or blocked by 261another kprobe, it will be unoptimized. If this happens before 262the optimization is complete, the kprobe is just dequeued from the 263optimized list. If the optimization has been done, the jump is 264replaced with the original code (except for an int3 breakpoint in 265the first byte) by using text_poke_smp(). 266 267.. [3] Please imagine that the 2nd instruction is interrupted and then 268 the optimizer replaces the 2nd instruction with the jump *address* 269 while the interrupt handler is running. When the interrupt 270 returns to original address, there is no valid instruction, 271 and it causes an unexpected result. 272 273.. [4] This optimization-safety checking may be replaced with the 274 stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y 275 kernel. 276 277NOTE for geeks: 278The jump optimization changes the kprobe's pre_handler behavior. 279Without optimization, the pre_handler can change the kernel's execution 280path by changing regs->ip and returning 1. However, when the probe 281is optimized, that modification is ignored. Thus, if you want to 282tweak the kernel's execution path, you need to suppress optimization, 283using one of the following techniques: 284 285- Specify an empty function for the kprobe's post_handler. 286 287or 288 289- Execute 'sysctl -w debug.kprobes_optimization=n' 290 291.. _kprobes_blacklist: 292 293Blacklist 294--------- 295 296Kprobes can probe most of the kernel except itself. This means 297that there are some functions where kprobes cannot probe. Probing 298(trapping) such functions can cause a recursive trap (e.g. double 299fault) or the nested probe handler may never be called. 300Kprobes manages such functions as a blacklist. 301If you want to add a function into the blacklist, you just need 302to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro 303to specify a blacklisted function. 304Kprobes checks the given probe address against the blacklist and 305rejects registering it, if the given address is in the blacklist. 306 307.. _kprobes_archs_supported: 308 309Architectures Supported 310======================= 311 312Kprobes and return probes are implemented on the following 313architectures: 314 315- i386 (Supports jump optimization) 316- x86_64 (AMD-64, EM64T) (Supports jump optimization) 317- ppc64 318- ia64 (Does not support probes on instruction slot1.) 319- sparc64 (Return probes not yet implemented.) 320- arm 321- ppc 322- mips 323- s390 324- parisc 325 326Configuring Kprobes 327=================== 328 329When configuring the kernel using make menuconfig/xconfig/oldconfig, 330ensure that CONFIG_KPROBES is set to "y", look for "Kprobes" under 331"General architecture-dependent options". 332 333So that you can load and unload Kprobes-based instrumentation modules, 334make sure "Loadable module support" (CONFIG_MODULES) and "Module 335unloading" (CONFIG_MODULE_UNLOAD) are set to "y". 336 337Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL 338are set to "y", since kallsyms_lookup_name() is used by the in-kernel 339kprobe address resolution code. 340 341If you need to insert a probe in the middle of a function, you may find 342it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), 343so you can use "objdump -d -l vmlinux" to see the source-to-object 344code mapping. 345 346API Reference 347============= 348 349The Kprobes API includes a "register" function and an "unregister" 350function for each type of probe. The API also includes "register_*probes" 351and "unregister_*probes" functions for (un)registering arrays of probes. 352Here are terse, mini-man-page specifications for these functions and 353the associated probe handlers that you'll write. See the files in the 354samples/kprobes/ sub-directory for examples. 355 356register_kprobe 357--------------- 358 359:: 360 361 #include <linux/kprobes.h> 362 int register_kprobe(struct kprobe *kp); 363 364Sets a breakpoint at the address kp->addr. When the breakpoint is hit, Kprobes 365calls kp->pre_handler. After the probed instruction is single-stepped, Kprobe 366calls kp->post_handler. Any or all handlers can be NULL. If kp->flags is set 367KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers 368aren't hit until calling enable_kprobe(kp). 369 370.. note:: 371 372 1. With the introduction of the "symbol_name" field to struct kprobe, 373 the probepoint address resolution will now be taken care of by the kernel. 374 The following will now work:: 375 376 kp.symbol_name = "symbol_name"; 377 378 (64-bit powerpc intricacies such as function descriptors are handled 379 transparently) 380 381 2. Use the "offset" field of struct kprobe if the offset into the symbol 382 to install a probepoint is known. This field is used to calculate the 383 probepoint. 384 385 3. Specify either the kprobe "symbol_name" OR the "addr". If both are 386 specified, kprobe registration will fail with -EINVAL. 387 388 4. With CISC architectures (such as i386 and x86_64), the kprobes code 389 does not validate if the kprobe.addr is at an instruction boundary. 390 Use "offset" with caution. 391 392register_kprobe() returns 0 on success, or a negative errno otherwise. 393 394User's pre-handler (kp->pre_handler):: 395 396 #include <linux/kprobes.h> 397 #include <linux/ptrace.h> 398 int pre_handler(struct kprobe *p, struct pt_regs *regs); 399 400Called with p pointing to the kprobe associated with the breakpoint, 401and regs pointing to the struct containing the registers saved when 402the breakpoint was hit. Return 0 here unless you're a Kprobes geek. 403 404User's post-handler (kp->post_handler):: 405 406 #include <linux/kprobes.h> 407 #include <linux/ptrace.h> 408 void post_handler(struct kprobe *p, struct pt_regs *regs, 409 unsigned long flags); 410 411p and regs are as described for the pre_handler. flags always seems 412to be zero. 413 414register_kretprobe 415------------------ 416 417:: 418 419 #include <linux/kprobes.h> 420 int register_kretprobe(struct kretprobe *rp); 421 422Establishes a return probe for the function whose address is 423rp->kp.addr. When that function returns, Kprobes calls rp->handler. 424You must set rp->maxactive appropriately before you call 425register_kretprobe(); see "How Does a Return Probe Work?" for details. 426 427register_kretprobe() returns 0 on success, or a negative errno 428otherwise. 429 430User's return-probe handler (rp->handler):: 431 432 #include <linux/kprobes.h> 433 #include <linux/ptrace.h> 434 int kretprobe_handler(struct kretprobe_instance *ri, 435 struct pt_regs *regs); 436 437regs is as described for kprobe.pre_handler. ri points to the 438kretprobe_instance object, of which the following fields may be 439of interest: 440 441- ret_addr: the return address 442- rp: points to the corresponding kretprobe object 443- task: points to the corresponding task struct 444- data: points to per return-instance private data; see "Kretprobe 445 entry-handler" for details. 446 447The regs_return_value(regs) macro provides a simple abstraction to 448extract the return value from the appropriate register as defined by 449the architecture's ABI. 450 451The handler's return value is currently ignored. 452 453unregister_*probe 454------------------ 455 456:: 457 458 #include <linux/kprobes.h> 459 void unregister_kprobe(struct kprobe *kp); 460 void unregister_kretprobe(struct kretprobe *rp); 461 462Removes the specified probe. The unregister function can be called 463at any time after the probe has been registered. 464 465.. note:: 466 467 If the functions find an incorrect probe (ex. an unregistered probe), 468 they clear the addr field of the probe. 469 470register_*probes 471---------------- 472 473:: 474 475 #include <linux/kprobes.h> 476 int register_kprobes(struct kprobe **kps, int num); 477 int register_kretprobes(struct kretprobe **rps, int num); 478 479Registers each of the num probes in the specified array. If any 480error occurs during registration, all probes in the array, up to 481the bad probe, are safely unregistered before the register_*probes 482function returns. 483 484- kps/rps: an array of pointers to ``*probe`` data structures 485- num: the number of the array entries. 486 487.. note:: 488 489 You have to allocate(or define) an array of pointers and set all 490 of the array entries before using these functions. 491 492unregister_*probes 493------------------ 494 495:: 496 497 #include <linux/kprobes.h> 498 void unregister_kprobes(struct kprobe **kps, int num); 499 void unregister_kretprobes(struct kretprobe **rps, int num); 500 501Removes each of the num probes in the specified array at once. 502 503.. note:: 504 505 If the functions find some incorrect probes (ex. unregistered 506 probes) in the specified array, they clear the addr field of those 507 incorrect probes. However, other probes in the array are 508 unregistered correctly. 509 510disable_*probe 511-------------- 512 513:: 514 515 #include <linux/kprobes.h> 516 int disable_kprobe(struct kprobe *kp); 517 int disable_kretprobe(struct kretprobe *rp); 518 519Temporarily disables the specified ``*probe``. You can enable it again by using 520enable_*probe(). You must specify the probe which has been registered. 521 522enable_*probe 523------------- 524 525:: 526 527 #include <linux/kprobes.h> 528 int enable_kprobe(struct kprobe *kp); 529 int enable_kretprobe(struct kretprobe *rp); 530 531Enables ``*probe`` which has been disabled by disable_*probe(). You must specify 532the probe which has been registered. 533 534Kprobes Features and Limitations 535================================ 536 537Kprobes allows multiple probes at the same address. Also, 538a probepoint for which there is a post_handler cannot be optimized. 539So if you install a kprobe with a post_handler, at an optimized 540probepoint, the probepoint will be unoptimized automatically. 541 542In general, you can install a probe anywhere in the kernel. 543In particular, you can probe interrupt handlers. Known exceptions 544are discussed in this section. 545 546The register_*probe functions will return -EINVAL if you attempt 547to install a probe in the code that implements Kprobes (mostly 548kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such 549as do_page_fault and notifier_call_chain). 550 551If you install a probe in an inline-able function, Kprobes makes 552no attempt to chase down all inline instances of the function and 553install probes there. gcc may inline a function without being asked, 554so keep this in mind if you're not seeing the probe hits you expect. 555 556A probe handler can modify the environment of the probed function 557-- e.g., by modifying kernel data structures, or by modifying the 558contents of the pt_regs struct (which are restored to the registers 559upon return from the breakpoint). So Kprobes can be used, for example, 560to install a bug fix or to inject faults for testing. Kprobes, of 561course, has no way to distinguish the deliberately injected faults 562from the accidental ones. Don't drink and probe. 563 564Kprobes makes no attempt to prevent probe handlers from stepping on 565each other -- e.g., probing printk() and then calling printk() from a 566probe handler. If a probe handler hits a probe, that second probe's 567handlers won't be run in that instance, and the kprobe.nmissed member 568of the second probe will be incremented. 569 570As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of 571the same handler) may run concurrently on different CPUs. 572 573Kprobes does not use mutexes or allocate memory except during 574registration and unregistration. 575 576Probe handlers are run with preemption disabled or interrupt disabled, 577which depends on the architecture and optimization state. (e.g., 578kretprobe handlers and optimized kprobe handlers run without interrupt 579disabled on x86/x86-64). In any case, your handler should not yield 580the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O). 581 582Since a return probe is implemented by replacing the return 583address with the trampoline's address, stack backtraces and calls 584to __builtin_return_address() will typically yield the trampoline's 585address instead of the real return address for kretprobed functions. 586(As far as we can tell, __builtin_return_address() is used only 587for instrumentation and error reporting.) 588 589If the number of times a function is called does not match the number 590of times it returns, registering a return probe on that function may 591produce undesirable results. In such a case, a line: 592kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c 593gets printed. With this information, one will be able to correlate the 594exact instance of the kretprobe that caused the problem. We have the 595do_exit() case covered. do_execve() and do_fork() are not an issue. 596We're unaware of other specific cases where this could be a problem. 597 598If, upon entry to or exit from a function, the CPU is running on 599a stack other than that of the current task, registering a return 600probe on that function may produce undesirable results. For this 601reason, Kprobes doesn't support return probes (or kprobes) 602on the x86_64 version of __switch_to(); the registration functions 603return -EINVAL. 604 605On x86/x86-64, since the Jump Optimization of Kprobes modifies 606instructions widely, there are some limitations to optimization. To 607explain it, we introduce some terminology. Imagine a 3-instruction 608sequence consisting of a two 2-byte instructions and one 3-byte 609instruction. 610 611:: 612 613 IA 614 | 615 [-2][-1][0][1][2][3][4][5][6][7] 616 [ins1][ins2][ ins3 ] 617 [<- DCR ->] 618 [<- JTPR ->] 619 620 ins1: 1st Instruction 621 ins2: 2nd Instruction 622 ins3: 3rd Instruction 623 IA: Insertion Address 624 JTPR: Jump Target Prohibition Region 625 DCR: Detoured Code Region 626 627The instructions in DCR are copied to the out-of-line buffer 628of the kprobe, because the bytes in DCR are replaced by 629a 5-byte jump instruction. So there are several limitations. 630 631a) The instructions in DCR must be relocatable. 632b) The instructions in DCR must not include a call instruction. 633c) JTPR must not be targeted by any jump or call instruction. 634d) DCR must not straddle the border between functions. 635 636Anyway, these limitations are checked by the in-kernel instruction 637decoder, so you don't need to worry about that. 638 639Probe Overhead 640============== 641 642On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 643microseconds to process. Specifically, a benchmark that hits the same 644probepoint repeatedly, firing a simple handler each time, reports 1-2 645million hits per second, depending on the architecture. A return-probe 646hit typically takes 50-75% longer than a kprobe hit. 647When you have a return probe set on a function, adding a kprobe at 648the entry to that function adds essentially no overhead. 649 650Here are sample overhead figures (in usec) for different architectures:: 651 652 k = kprobe; r = return probe; kr = kprobe + return probe 653 on same function 654 655 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips 656 k = 0.57 usec; r = 0.92; kr = 0.99 657 658 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips 659 k = 0.49 usec; r = 0.80; kr = 0.82 660 661 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) 662 k = 0.77 usec; r = 1.26; kr = 1.45 663 664Optimized Probe Overhead 665------------------------ 666 667Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to 668process. Here are sample overhead figures (in usec) for x86 architectures:: 669 670 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe, 671 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe. 672 673 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 674 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33 675 676 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips 677 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30 678 679TODO 680==== 681 682a. SystemTap (http://sourceware.org/systemtap): Provides a simplified 683 programming interface for probe-based instrumentation. Try it out. 684b. Kernel return probes for sparc64. 685c. Support for other architectures. 686d. User-space probes. 687e. Watchpoint probes (which fire on data references). 688 689Kprobes Example 690=============== 691 692See samples/kprobes/kprobe_example.c 693 694Kretprobes Example 695================== 696 697See samples/kprobes/kretprobe_example.c 698 699Deprecated Features 700=================== 701 702Jprobes is now a deprecated feature. People who are depending on it should 703migrate to other tracing features or use older kernels. Please consider to 704migrate your tool to one of the following options: 705 706- Use trace-event to trace target function with arguments. 707 708 trace-event is a low-overhead (and almost no visible overhead if it 709 is off) statically defined event interface. You can define new events 710 and trace it via ftrace or any other tracing tools. 711 712 See the following urls: 713 714 - https://lwn.net/Articles/379903/ 715 - https://lwn.net/Articles/381064/ 716 - https://lwn.net/Articles/383362/ 717 718- Use ftrace dynamic events (kprobe event) with perf-probe. 719 720 If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can 721 find which register/stack is assigned to which local variable or arguments 722 by using perf-probe and set up new event to trace it. 723 724 See following documents: 725 726 - Documentation/trace/kprobetrace.rst 727 - Documentation/trace/events.rst 728 - tools/perf/Documentation/perf-probe.txt 729 730 731The kprobes debugfs interface 732============================= 733 734 735With recent kernels (> 2.6.20) the list of registered kprobes is visible 736under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug). 737 738/sys/kernel/debug/kprobes/list: Lists all registered probes on the system:: 739 740 c015d71a k vfs_read+0x0 741 c03dedc5 r tcp_v4_rcv+0x0 742 743The first column provides the kernel address where the probe is inserted. 744The second column identifies the type of probe (k - kprobe and r - kretprobe) 745while the third column specifies the symbol+offset of the probe. 746If the probed function belongs to a module, the module name is also 747specified. Following columns show probe status. If the probe is on 748a virtual address that is no longer valid (module init sections, module 749virtual addresses that correspond to modules that've been unloaded), 750such probes are marked with [GONE]. If the probe is temporarily disabled, 751such probes are marked with [DISABLED]. If the probe is optimized, it is 752marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with 753[FTRACE]. 754 755/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. 756 757Provides a knob to globally and forcibly turn registered kprobes ON or OFF. 758By default, all kprobes are enabled. By echoing "0" to this file, all 759registered probes will be disarmed, till such time a "1" is echoed to this 760file. Note that this knob just disarms and arms all kprobes and doesn't 761change each probe's disabling state. This means that disabled kprobes (marked 762[DISABLED]) will be not enabled if you turn ON all kprobes by this knob. 763 764 765The kprobes sysctl interface 766============================ 767 768/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF. 769 770When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides 771a knob to globally and forcibly turn jump optimization (see section 772:ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization 773is allowed (ON). If you echo "0" to this file or set 774"debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be 775unoptimized, and any new probes registered after that will not be optimized. 776 777Note that this knob *changes* the optimized state. This means that optimized 778probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be 779removed). If the knob is turned on, they will be optimized again. 780 781References 782========== 783 784For additional information on Kprobes, refer to the following URLs: 785 786- https://lwn.net/Articles/132196/ 787- https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf 788 789