1Title   : Kernel Probes (Kprobes)
   2Authors : Jim Keniston <>
   3        : Prasanna S Panchamukhi <>
   4        : Masami Hiramatsu <>
   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
  211. Concepts: Kprobes, Jprobes, Return Probes
  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.
  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
  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.
  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.
  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.
  551.1 How Does a Kprobe Work?
  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).
  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.
  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.)
  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.
  771.2 How Does a Jprobe Work?
  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().
  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.
  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.
 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.
 1051.3 Return Probes
 1071.3.1 How Does a Return Probe Work?
 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.
 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.
 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.
 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.
 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.
 1421.3.2 Kretprobe entry-handler
 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.
 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.
 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.
 1661.4 How Does Jump Optimization Work?
 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.
 1741.4.1 Init a Kprobe
 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.
 1811.4.2 Safety Check
 183Before optimizing a probe, Kprobes performs the following safety checks:
 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
 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).
 200- For each instruction in the optimized region, Kprobes verifies that
 201the instruction can be executed out of line.
 2031.4.3 Preparing Detour Buffer
 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.
 2131.4.4 Pre-optimization
 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
 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.
 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.
 2321.4.5 Optimization
 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.(**)
 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().
 2461.4.6 Unoptimization
 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().
 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.
 261(**)This optimization-safety checking may be replaced with the
 262stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
 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'
 2762. Architectures Supported
 278Kprobes, jprobes, and return probes are implemented on the following
 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
 2903. Configuring Kprobes
 292When configuring the kernel using make menuconfig/xconfig/oldconfig,
 293ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
 294Support", look for "Kprobes".
 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".
 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.
 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.
 3094. API Reference
 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.
 3184.1 register_kprobe
 320#include <linux/kprobes.h>
 321int register_kprobe(struct kprobe *kp);
 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).
 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:
 337        kp.symbol_name = "symbol_name";
 339(64-bit powerpc intricacies such as function descriptors are handled
 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
 3463. Specify either the kprobe "symbol_name" OR the "addr". If both are
 347specified, kprobe registration will fail with -EINVAL.
 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.
 353register_kprobe() returns 0 on success, or a negative errno otherwise.
 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);
 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.
 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);
 370p and regs are as described for the pre_handler.  flags always seems
 371to be zero.
 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);
 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.
 3834.2 register_jprobe
 385#include <linux/kprobes.h>
 386int register_jprobe(struct jprobe *jp)
 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.
 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.
 399register_jprobe() returns 0 on success, or a negative errno otherwise.
 4014.3 register_kretprobe
 403#include <linux/kprobes.h>
 404int register_kretprobe(struct kretprobe *rp);
 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.
 411register_kretprobe() returns 0 on success, or a negative errno
 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);
 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.
 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.
 432The handler's return value is currently ignored.
 4344.4 unregister_*probe
 436#include <linux/kprobes.h>
 437void unregister_kprobe(struct kprobe *kp);
 438void unregister_jprobe(struct jprobe *jp);
 439void unregister_kretprobe(struct kretprobe *rp);
 441Removes the specified probe.  The unregister function can be called
 442at any time after the probe has been registered.
 445If the functions find an incorrect probe (ex. an unregistered probe),
 446they clear the addr field of the probe.
 4484.5 register_*probes
 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);
 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.
 463You have to allocate(or define) an array of pointers and set all
 464of the array entries before using these functions.
 4664.6 unregister_*probes
 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);
 473Removes each of the num probes in the specified array at once.
 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.
 4814.7 disable_*probe
 483#include <linux/kprobes.h>
 484int disable_kprobe(struct kprobe *kp);
 485int disable_kretprobe(struct kretprobe *rp);
 486int disable_jprobe(struct jprobe *jp);
 488Temporarily disables the specified *probe. You can enable it again by using
 489enable_*probe(). You must specify the probe which has been registered.
 4914.8 enable_*probe
 493#include <linux/kprobes.h>
 494int enable_kprobe(struct kprobe *kp);
 495int enable_kretprobe(struct kretprobe *rp);
 496int enable_jprobe(struct jprobe *jp);
 498Enables *probe which has been disabled by disable_*probe(). You must specify
 499the probe which has been registered.
 5015. Kprobes Features and Limitations
 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.
 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.
 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).
 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.
 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.
 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.
 538As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
 539the same handler) may run concurrently on different CPUs.
 541Kprobes does not use mutexes or allocate memory except during
 542registration and unregistration.
 544Probe handlers are run with preemption disabled.  Depending on the
 545architecture, handlers may also run with interrupts disabled.  In any
 546case, your handler should not yield the CPU (e.g., by attempting to
 547acquire a semaphore).
 549Since a return probe is implemented by replacing the return
 550address with the trampoline's address, stack backtraces and calls
 551to __builtin_return_address() will typically yield the trampoline's
 552address instead of the real return address for kretprobed functions.
 553(As far as we can tell, __builtin_return_address() is used only
 554for instrumentation and error reporting.)
 556If the number of times a function is called does not match the number
 557of times it returns, registering a return probe on that function may
 558produce undesirable results. In such a case, a line:
 559kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
 560gets printed. With this information, one will be able to correlate the
 561exact instance of the kretprobe that caused the problem. We have the
 562do_exit() case covered. do_execve() and do_fork() are not an issue.
 563We're unaware of other specific cases where this could be a problem.
 565If, upon entry to or exit from a function, the CPU is running on
 566a stack other than that of the current task, registering a return
 567probe on that function may produce undesirable results.  For this
 568reason, Kprobes doesn't support return probes (or kprobes or jprobes)
 569on the x86_64 version of __switch_to(); the registration functions
 570return -EINVAL.
 572On x86/x86-64, since the Jump Optimization of Kprobes modifies
 573instructions widely, there are some limitations to optimization. To
 574explain it, we introduce some terminology. Imagine a 3-instruction
 575sequence consisting of a two 2-byte instructions and one 3-byte
 578        IA
 579         |
 581        [ins1][ins2][  ins3 ]
 582        [<-     DCR       ->]
 583           [<- JTPR ->]
 585ins1: 1st Instruction
 586ins2: 2nd Instruction
 587ins3: 3rd Instruction
 588IA:  Insertion Address
 589JTPR: Jump Target Prohibition Region
 590DCR: Detoured Code Region
 592The instructions in DCR are copied to the out-of-line buffer
 593of the kprobe, because the bytes in DCR are replaced by
 594a 5-byte jump instruction. So there are several limitations.
 596a) The instructions in DCR must be relocatable.
 597b) The instructions in DCR must not include a call instruction.
 598c) JTPR must not be targeted by any jump or call instruction.
 599d) DCR must not straddle the border betweeen functions.
 601Anyway, these limitations are checked by the in-kernel instruction
 602decoder, so you don't need to worry about that.
 6046. Probe Overhead
 606On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
 607microseconds to process.  Specifically, a benchmark that hits the same
 608probepoint repeatedly, firing a simple handler each time, reports 1-2
 609million hits per second, depending on the architecture.  A jprobe or
 610return-probe hit typically takes 50-75% longer than a kprobe hit.
 611When you have a return probe set on a function, adding a kprobe at
 612the entry to that function adds essentially no overhead.
 614Here are sample overhead figures (in usec) for different architectures.
 615k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
 616on same function; jr = jprobe + return probe on same function
 618i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
 619k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
 621x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
 622k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
 624ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
 625k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
 6276.1 Optimized Probe Overhead
 629Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
 630process. Here are sample overhead figures (in usec) for x86 architectures.
 631k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
 632r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
 634i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
 635k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
 637x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
 638k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
 6407. TODO
 642a. SystemTap ( Provides a simplified
 643programming interface for probe-based instrumentation.  Try it out.
 644b. Kernel return probes for sparc64.
 645c. Support for other architectures.
 646d. User-space probes.
 647e. Watchpoint probes (which fire on data references).
 6498. Kprobes Example
 651See samples/kprobes/kprobe_example.c
 6539. Jprobes Example
 655See samples/kprobes/jprobe_example.c
 65710. Kretprobes Example
 659See samples/kprobes/kretprobe_example.c
 661For additional information on Kprobes, refer to the following URLs:
 665 (pages 101-115)
 668Appendix A: The kprobes debugfs interface
 670With recent kernels (> 2.6.20) the list of registered kprobes is visible
 671under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
 673/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
 675c015d71a  k  vfs_read+0x0
 676c011a316  j  do_fork+0x0
 677c03dedc5  r  tcp_v4_rcv+0x0
 679The first column provides the kernel address where the probe is inserted.
 680The second column identifies the type of probe (k - kprobe, r - kretprobe
 681and j - jprobe), while the third column specifies the symbol+offset of
 682the probe. If the probed function belongs to a module, the module name
 683is also specified. Following columns show probe status. If the probe is on
 684a virtual address that is no longer valid (module init sections, module
 685virtual addresses that correspond to modules that've been unloaded),
 686such probes are marked with [GONE]. If the probe is temporarily disabled,
 687such probes are marked with [DISABLED]. If the probe is optimized, it is
 688marked with [OPTIMIZED].
 690/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
 692Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
 693By default, all kprobes are enabled. By echoing "0" to this file, all
 694registered probes will be disarmed, till such time a "1" is echoed to this
 695file. Note that this knob just disarms and arms all kprobes and doesn't
 696change each probe's disabling state. This means that disabled kprobes (marked
 697[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
 700Appendix B: The kprobes sysctl interface
 702/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
 704When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
 705a knob to globally and forcibly turn jump optimization (see section
 7061.4) ON or OFF. By default, jump optimization is allowed (ON).
 707If you echo "0" to this file or set "debug.kprobes_optimization" to
 7080 via sysctl, all optimized probes will be unoptimized, and any new
 709probes registered after that will not be optimized.  Note that this
 710knob *changes* the optimized state. This means that optimized probes
 711(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
 712removed). If the knob is turned on, they will be optimized again.
 714 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.