linux/Documentation/kprobes.txt
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   1Title   : Kernel Probes (Kprobes)
   2Authors : Jim Keniston <jkenisto@us.ibm.com>
   3        : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
   4        : Masami Hiramatsu <mhiramat@redhat.com>
   5
   6CONTENTS
   7
   81. Concepts: Kprobes, Jprobes, Return Probes
   92. Architectures Supported
  103. Configuring Kprobes
  114. API Reference
  125. Kprobes Features and Limitations
  136. Probe Overhead
  147. TODO
  158. Kprobes Example
  169. Jprobes Example
  1710. Kretprobes Example
  18Appendix A: The kprobes debugfs interface
  19Appendix B: The kprobes sysctl interface
  20
  211. Concepts: Kprobes, Jprobes, Return Probes
  22
  23Kprobes enables you to dynamically break into any kernel routine and
  24collect debugging and performance information non-disruptively. You
  25can trap at almost any kernel code address, specifying a handler
  26routine to be invoked when the breakpoint is hit.
  27
  28There are currently three types of probes: kprobes, jprobes, and
  29kretprobes (also called return probes).  A kprobe can be inserted
  30on virtually any instruction in the kernel.  A jprobe is inserted at
  31the entry to a kernel function, and provides convenient access to the
  32function's arguments.  A return probe fires when a specified function
  33returns.
  34
  35In the typical case, Kprobes-based instrumentation is packaged as
  36a kernel module.  The module's init function installs ("registers")
  37one or more probes, and the exit function unregisters them.  A
  38registration function such as register_kprobe() specifies where
  39the probe is to be inserted and what handler is to be called when
  40the probe is hit.
  41
  42There are also register_/unregister_*probes() functions for batch
  43registration/unregistration of a group of *probes. These functions
  44can speed up unregistration process when you have to unregister
  45a lot of probes at once.
  46
  47The next four subsections explain how the different types of
  48probes work and how jump optimization works.  They explain certain
  49things that you'll need to know in order to make the best use of
  50Kprobes -- e.g., the difference between a pre_handler and
  51a post_handler, and how to use the maxactive and nmissed fields of
  52a kretprobe.  But if you're in a hurry to start using Kprobes, you
  53can skip ahead to section 2.
  54
  551.1 How Does a Kprobe Work?
  56
  57When a kprobe is registered, Kprobes makes a copy of the probed
  58instruction and replaces the first byte(s) of the probed instruction
  59with a breakpoint instruction (e.g., int3 on i386 and x86_64).
  60
  61When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
  62registers are saved, and control passes to Kprobes via the
  63notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
  64associated with the kprobe, passing the handler the addresses of the
  65kprobe struct and the saved registers.
  66
  67Next, Kprobes single-steps its copy of the probed instruction.
  68(It would be simpler to single-step the actual instruction in place,
  69but then Kprobes would have to temporarily remove the breakpoint
  70instruction.  This would open a small time window when another CPU
  71could sail right past the probepoint.)
  72
  73After the instruction is single-stepped, Kprobes executes the
  74"post_handler," if any, that is associated with the kprobe.
  75Execution then continues with the instruction following the probepoint.
  76
  771.2 How Does a Jprobe Work?
  78
  79A jprobe is implemented using a kprobe that is placed on a function's
  80entry point.  It employs a simple mirroring principle to allow
  81seamless access to the probed function's arguments.  The jprobe
  82handler routine should have the same signature (arg list and return
  83type) as the function being probed, and must always end by calling
  84the Kprobes function jprobe_return().
  85
  86Here's how it works.  When the probe is hit, Kprobes makes a copy of
  87the saved registers and a generous portion of the stack (see below).
  88Kprobes then points the saved instruction pointer at the jprobe's
  89handler routine, and returns from the trap.  As a result, control
  90passes to the handler, which is presented with the same register and
  91stack contents as the probed function.  When it is done, the handler
  92calls jprobe_return(), which traps again to restore the original stack
  93contents and processor state and switch to the probed function.
  94
  95By convention, the callee owns its arguments, so gcc may produce code
  96that unexpectedly modifies that portion of the stack.  This is why
  97Kprobes saves a copy of the stack and restores it after the jprobe
  98handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
  9964 bytes on i386.
 100
 101Note that the probed function's args may be passed on the stack
 102or in registers.  The jprobe will work in either case, so long as the
 103handler's prototype matches that of the probed function.
 104
 1051.3 Return Probes
 106
 1071.3.1 How Does a Return Probe Work?
 108
 109When you call register_kretprobe(), Kprobes establishes a kprobe at
 110the entry to the function.  When the probed function is called and this
 111probe is hit, Kprobes saves a copy of the return address, and replaces
 112the return address with the address of a "trampoline."  The trampoline
 113is an arbitrary piece of code -- typically just a nop instruction.
 114At boot time, Kprobes registers a kprobe at the trampoline.
 115
 116When the probed function executes its return instruction, control
 117passes to the trampoline and that probe is hit.  Kprobes' trampoline
 118handler calls the user-specified return handler associated with the
 119kretprobe, then sets the saved instruction pointer to the saved return
 120address, and that's where execution resumes upon return from the trap.
 121
 122While the probed function is executing, its return address is
 123stored in an object of type kretprobe_instance.  Before calling
 124register_kretprobe(), the user sets the maxactive field of the
 125kretprobe struct to specify how many instances of the specified
 126function can be probed simultaneously.  register_kretprobe()
 127pre-allocates the indicated number of kretprobe_instance objects.
 128
 129For example, if the function is non-recursive and is called with a
 130spinlock held, maxactive = 1 should be enough.  If the function is
 131non-recursive and can never relinquish the CPU (e.g., via a semaphore
 132or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
 133set to a default value.  If CONFIG_PREEMPT is enabled, the default
 134is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
 135
 136It's not a disaster if you set maxactive too low; you'll just miss
 137some probes.  In the kretprobe struct, the nmissed field is set to
 138zero when the return probe is registered, and is incremented every
 139time the probed function is entered but there is no kretprobe_instance
 140object available for establishing the return probe.
 141
 1421.3.2 Kretprobe entry-handler
 143
 144Kretprobes also provides an optional user-specified handler which runs
 145on function entry. This handler is specified by setting the entry_handler
 146field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
 147function entry is hit, the user-defined entry_handler, if any, is invoked.
 148If the entry_handler returns 0 (success) then a corresponding return handler
 149is guaranteed to be called upon function return. If the entry_handler
 150returns a non-zero error then Kprobes leaves the return address as is, and
 151the kretprobe has no further effect for that particular function instance.
 152
 153Multiple entry and return handler invocations are matched using the unique
 154kretprobe_instance object associated with them. Additionally, a user
 155may also specify per return-instance private data to be part of each
 156kretprobe_instance object. This is especially useful when sharing private
 157data between corresponding user entry and return handlers. The size of each
 158private data object can be specified at kretprobe registration time by
 159setting the data_size field of the kretprobe struct. This data can be
 160accessed through the data field of each kretprobe_instance object.
 161
 162In case probed function is entered but there is no kretprobe_instance
 163object available, then in addition to incrementing the nmissed count,
 164the user entry_handler invocation is also skipped.
 165
 1661.4 How Does Jump Optimization Work?
 167
 168If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
 169is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
 170the "debug.kprobes_optimization" kernel parameter is set to 1 (see
 171sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
 172instruction instead of a breakpoint instruction at each probepoint.
 173
 1741.4.1 Init a Kprobe
 175
 176When a probe is registered, before attempting this optimization,
 177Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
 178address. So, even if it's not possible to optimize this particular
 179probepoint, there'll be a probe there.
 180
 1811.4.2 Safety Check
 182
 183Before optimizing a probe, Kprobes performs the following safety checks:
 184
 185- Kprobes verifies that the region that will be replaced by the jump
 186instruction (the "optimized region") lies entirely within one function.
 187(A jump instruction is multiple bytes, and so may overlay multiple
 188instructions.)
 189
 190- Kprobes analyzes the entire function and verifies that there is no
 191jump into the optimized region.  Specifically:
 192  - the function contains no indirect jump;
 193  - the function contains no instruction that causes an exception (since
 194  the fixup code triggered by the exception could jump back into the
 195  optimized region -- Kprobes checks the exception tables to verify this);
 196  and
 197  - there is no near jump to the optimized region (other than to the first
 198  byte).
 199
 200- For each instruction in the optimized region, Kprobes verifies that
 201the instruction can be executed out of line.
 202
 2031.4.3 Preparing Detour Buffer
 204
 205Next, Kprobes prepares a "detour" buffer, which contains the following
 206instruction sequence:
 207- code to push the CPU's registers (emulating a breakpoint trap)
 208- a call to the trampoline code which calls user's probe handlers.
 209- code to restore registers
 210- the instructions from the optimized region
 211- a jump back to the original execution path.
 212
 2131.4.4 Pre-optimization
 214
 215After preparing the detour buffer, Kprobes verifies that none of the
 216following situations exist:
 217- The probe has either a break_handler (i.e., it's a jprobe) or a
 218post_handler.
 219- Other instructions in the optimized region are probed.
 220- The probe is disabled.
 221In any of the above cases, Kprobes won't start optimizing the probe.
 222Since these are temporary situations, Kprobes tries to start
 223optimizing it again if the situation is changed.
 224
 225If the kprobe can be optimized, Kprobes enqueues the kprobe to an
 226optimizing list, and kicks the kprobe-optimizer workqueue to optimize
 227it.  If the to-be-optimized probepoint is hit before being optimized,
 228Kprobes returns control to the original instruction path by setting
 229the CPU's instruction pointer to the copied code in the detour buffer
 230-- thus at least avoiding the single-step.
 231
 2321.4.5 Optimization
 233
 234The Kprobe-optimizer doesn't insert the jump instruction immediately;
 235rather, it calls synchronize_sched() for safety first, because it's
 236possible for a CPU to be interrupted in the middle of executing the
 237optimized region(*).  As you know, synchronize_sched() can ensure
 238that all interruptions that were active when synchronize_sched()
 239was called are done, but only if CONFIG_PREEMPT=n.  So, this version
 240of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
 241
 242After that, the Kprobe-optimizer calls stop_machine() to replace
 243the optimized region with a jump instruction to the detour buffer,
 244using text_poke_smp().
 245
 2461.4.6 Unoptimization
 247
 248When an optimized kprobe is unregistered, disabled, or blocked by
 249another kprobe, it will be unoptimized.  If this happens before
 250the optimization is complete, the kprobe is just dequeued from the
 251optimized list.  If the optimization has been done, the jump is
 252replaced with the original code (except for an int3 breakpoint in
 253the first byte) by using text_poke_smp().
 254
 255(*)Please imagine that the 2nd instruction is interrupted and then
 256the optimizer replaces the 2nd instruction with the jump *address*
 257while the interrupt handler is running. When the interrupt
 258returns to original address, there is no valid instruction,
 259and it causes an unexpected result.
 260
 261(**)This optimization-safety checking may be replaced with the
 262stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
 263kernel.
 264
 265NOTE for geeks:
 266The jump optimization changes the kprobe's pre_handler behavior.
 267Without optimization, the pre_handler can change the kernel's execution
 268path by changing regs->ip and returning 1.  However, when the probe
 269is optimized, that modification is ignored.  Thus, if you want to
 270tweak the kernel's execution path, you need to suppress optimization,
 271using one of the following techniques:
 272- Specify an empty function for the kprobe's post_handler or break_handler.
 273 or
 274- Execute 'sysctl -w debug.kprobes_optimization=n'
 275
 2762. Architectures Supported
 277
 278Kprobes, jprobes, and return probes are implemented on the following
 279architectures:
 280
 281- i386 (Supports jump optimization)
 282- x86_64 (AMD-64, EM64T) (Supports jump optimization)
 283- ppc64
 284- ia64 (Does not support probes on instruction slot1.)
 285- sparc64 (Return probes not yet implemented.)
 286- arm
 287- ppc
 288
 2893. Configuring Kprobes
 290
 291When configuring the kernel using make menuconfig/xconfig/oldconfig,
 292ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
 293Support", look for "Kprobes".
 294
 295So that you can load and unload Kprobes-based instrumentation modules,
 296make sure "Loadable module support" (CONFIG_MODULES) and "Module
 297unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
 298
 299Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
 300are set to "y", since kallsyms_lookup_name() is used by the in-kernel
 301kprobe address resolution code.
 302
 303If you need to insert a probe in the middle of a function, you may find
 304it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
 305so you can use "objdump -d -l vmlinux" to see the source-to-object
 306code mapping.
 307
 3084. API Reference
 309
 310The Kprobes API includes a "register" function and an "unregister"
 311function for each type of probe. The API also includes "register_*probes"
 312and "unregister_*probes" functions for (un)registering arrays of probes.
 313Here are terse, mini-man-page specifications for these functions and
 314the associated probe handlers that you'll write. See the files in the
 315samples/kprobes/ sub-directory for examples.
 316
 3174.1 register_kprobe
 318
 319#include <linux/kprobes.h>
 320int register_kprobe(struct kprobe *kp);
 321
 322Sets a breakpoint at the address kp->addr.  When the breakpoint is
 323hit, Kprobes calls kp->pre_handler.  After the probed instruction
 324is single-stepped, Kprobe calls kp->post_handler.  If a fault
 325occurs during execution of kp->pre_handler or kp->post_handler,
 326or during single-stepping of the probed instruction, Kprobes calls
 327kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
 328is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
 329so, its handlers aren't hit until calling enable_kprobe(kp).
 330
 331NOTE:
 3321. With the introduction of the "symbol_name" field to struct kprobe,
 333the probepoint address resolution will now be taken care of by the kernel.
 334The following will now work:
 335
 336        kp.symbol_name = "symbol_name";
 337
 338(64-bit powerpc intricacies such as function descriptors are handled
 339transparently)
 340
 3412. Use the "offset" field of struct kprobe if the offset into the symbol
 342to install a probepoint is known. This field is used to calculate the
 343probepoint.
 344
 3453. Specify either the kprobe "symbol_name" OR the "addr". If both are
 346specified, kprobe registration will fail with -EINVAL.
 347
 3484. With CISC architectures (such as i386 and x86_64), the kprobes code
 349does not validate if the kprobe.addr is at an instruction boundary.
 350Use "offset" with caution.
 351
 352register_kprobe() returns 0 on success, or a negative errno otherwise.
 353
 354User's pre-handler (kp->pre_handler):
 355#include <linux/kprobes.h>
 356#include <linux/ptrace.h>
 357int pre_handler(struct kprobe *p, struct pt_regs *regs);
 358
 359Called with p pointing to the kprobe associated with the breakpoint,
 360and regs pointing to the struct containing the registers saved when
 361the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
 362
 363User's post-handler (kp->post_handler):
 364#include <linux/kprobes.h>
 365#include <linux/ptrace.h>
 366void post_handler(struct kprobe *p, struct pt_regs *regs,
 367        unsigned long flags);
 368
 369p and regs are as described for the pre_handler.  flags always seems
 370to be zero.
 371
 372User's fault-handler (kp->fault_handler):
 373#include <linux/kprobes.h>
 374#include <linux/ptrace.h>
 375int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
 376
 377p and regs are as described for the pre_handler.  trapnr is the
 378architecture-specific trap number associated with the fault (e.g.,
 379on i386, 13 for a general protection fault or 14 for a page fault).
 380Returns 1 if it successfully handled the exception.
 381
 3824.2 register_jprobe
 383
 384#include <linux/kprobes.h>
 385int register_jprobe(struct jprobe *jp)
 386
 387Sets a breakpoint at the address jp->kp.addr, which must be the address
 388of the first instruction of a function.  When the breakpoint is hit,
 389Kprobes runs the handler whose address is jp->entry.
 390
 391The handler should have the same arg list and return type as the probed
 392function; and just before it returns, it must call jprobe_return().
 393(The handler never actually returns, since jprobe_return() returns
 394control to Kprobes.)  If the probed function is declared asmlinkage
 395or anything else that affects how args are passed, the handler's
 396declaration must match.
 397
 398register_jprobe() returns 0 on success, or a negative errno otherwise.
 399
 4004.3 register_kretprobe
 401
 402#include <linux/kprobes.h>
 403int register_kretprobe(struct kretprobe *rp);
 404
 405Establishes a return probe for the function whose address is
 406rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
 407You must set rp->maxactive appropriately before you call
 408register_kretprobe(); see "How Does a Return Probe Work?" for details.
 409
 410register_kretprobe() returns 0 on success, or a negative errno
 411otherwise.
 412
 413User's return-probe handler (rp->handler):
 414#include <linux/kprobes.h>
 415#include <linux/ptrace.h>
 416int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
 417
 418regs is as described for kprobe.pre_handler.  ri points to the
 419kretprobe_instance object, of which the following fields may be
 420of interest:
 421- ret_addr: the return address
 422- rp: points to the corresponding kretprobe object
 423- task: points to the corresponding task struct
 424- data: points to per return-instance private data; see "Kretprobe
 425        entry-handler" for details.
 426
 427The regs_return_value(regs) macro provides a simple abstraction to
 428extract the return value from the appropriate register as defined by
 429the architecture's ABI.
 430
 431The handler's return value is currently ignored.
 432
 4334.4 unregister_*probe
 434
 435#include <linux/kprobes.h>
 436void unregister_kprobe(struct kprobe *kp);
 437void unregister_jprobe(struct jprobe *jp);
 438void unregister_kretprobe(struct kretprobe *rp);
 439
 440Removes the specified probe.  The unregister function can be called
 441at any time after the probe has been registered.
 442
 443NOTE:
 444If the functions find an incorrect probe (ex. an unregistered probe),
 445they clear the addr field of the probe.
 446
 4474.5 register_*probes
 448
 449#include <linux/kprobes.h>
 450int register_kprobes(struct kprobe **kps, int num);
 451int register_kretprobes(struct kretprobe **rps, int num);
 452int register_jprobes(struct jprobe **jps, int num);
 453
 454Registers each of the num probes in the specified array.  If any
 455error occurs during registration, all probes in the array, up to
 456the bad probe, are safely unregistered before the register_*probes
 457function returns.
 458- kps/rps/jps: an array of pointers to *probe data structures
 459- num: the number of the array entries.
 460
 461NOTE:
 462You have to allocate(or define) an array of pointers and set all
 463of the array entries before using these functions.
 464
 4654.6 unregister_*probes
 466
 467#include <linux/kprobes.h>
 468void unregister_kprobes(struct kprobe **kps, int num);
 469void unregister_kretprobes(struct kretprobe **rps, int num);
 470void unregister_jprobes(struct jprobe **jps, int num);
 471
 472Removes each of the num probes in the specified array at once.
 473
 474NOTE:
 475If the functions find some incorrect probes (ex. unregistered
 476probes) in the specified array, they clear the addr field of those
 477incorrect probes. However, other probes in the array are
 478unregistered correctly.
 479
 4804.7 disable_*probe
 481
 482#include <linux/kprobes.h>
 483int disable_kprobe(struct kprobe *kp);
 484int disable_kretprobe(struct kretprobe *rp);
 485int disable_jprobe(struct jprobe *jp);
 486
 487Temporarily disables the specified *probe. You can enable it again by using
 488enable_*probe(). You must specify the probe which has been registered.
 489
 4904.8 enable_*probe
 491
 492#include <linux/kprobes.h>
 493int enable_kprobe(struct kprobe *kp);
 494int enable_kretprobe(struct kretprobe *rp);
 495int enable_jprobe(struct jprobe *jp);
 496
 497Enables *probe which has been disabled by disable_*probe(). You must specify
 498the probe which has been registered.
 499
 5005. Kprobes Features and Limitations
 501
 502Kprobes allows multiple probes at the same address.  Currently,
 503however, there cannot be multiple jprobes on the same function at
 504the same time.  Also, a probepoint for which there is a jprobe or
 505a post_handler cannot be optimized.  So if you install a jprobe,
 506or a kprobe with a post_handler, at an optimized probepoint, the
 507probepoint will be unoptimized automatically.
 508
 509In general, you can install a probe anywhere in the kernel.
 510In particular, you can probe interrupt handlers.  Known exceptions
 511are discussed in this section.
 512
 513The register_*probe functions will return -EINVAL if you attempt
 514to install a probe in the code that implements Kprobes (mostly
 515kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
 516as do_page_fault and notifier_call_chain).
 517
 518If you install a probe in an inline-able function, Kprobes makes
 519no attempt to chase down all inline instances of the function and
 520install probes there.  gcc may inline a function without being asked,
 521so keep this in mind if you're not seeing the probe hits you expect.
 522
 523A probe handler can modify the environment of the probed function
 524-- e.g., by modifying kernel data structures, or by modifying the
 525contents of the pt_regs struct (which are restored to the registers
 526upon return from the breakpoint).  So Kprobes can be used, for example,
 527to install a bug fix or to inject faults for testing.  Kprobes, of
 528course, has no way to distinguish the deliberately injected faults
 529from the accidental ones.  Don't drink and probe.
 530
 531Kprobes makes no attempt to prevent probe handlers from stepping on
 532each other -- e.g., probing printk() and then calling printk() from a
 533probe handler.  If a probe handler hits a probe, that second probe's
 534handlers won't be run in that instance, and the kprobe.nmissed member
 535of the second probe will be incremented.
 536
 537As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
 538the same handler) may run concurrently on different CPUs.
 539
 540Kprobes does not use mutexes or allocate memory except during
 541registration and unregistration.
 542
 543Probe handlers are run with preemption disabled.  Depending on the
 544architecture, handlers may also run with interrupts disabled.  In any
 545case, your handler should not yield the CPU (e.g., by attempting to
 546acquire a semaphore).
 547
 548Since a return probe is implemented by replacing the return
 549address with the trampoline's address, stack backtraces and calls
 550to __builtin_return_address() will typically yield the trampoline's
 551address instead of the real return address for kretprobed functions.
 552(As far as we can tell, __builtin_return_address() is used only
 553for instrumentation and error reporting.)
 554
 555If the number of times a function is called does not match the number
 556of times it returns, registering a return probe on that function may
 557produce undesirable results. In such a case, a line:
 558kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
 559gets printed. With this information, one will be able to correlate the
 560exact instance of the kretprobe that caused the problem. We have the
 561do_exit() case covered. do_execve() and do_fork() are not an issue.
 562We're unaware of other specific cases where this could be a problem.
 563
 564If, upon entry to or exit from a function, the CPU is running on
 565a stack other than that of the current task, registering a return
 566probe on that function may produce undesirable results.  For this
 567reason, Kprobes doesn't support return probes (or kprobes or jprobes)
 568on the x86_64 version of __switch_to(); the registration functions
 569return -EINVAL.
 570
 571On x86/x86-64, since the Jump Optimization of Kprobes modifies
 572instructions widely, there are some limitations to optimization. To
 573explain it, we introduce some terminology. Imagine a 3-instruction
 574sequence consisting of a two 2-byte instructions and one 3-byte
 575instruction.
 576
 577        IA
 578         |
 579[-2][-1][0][1][2][3][4][5][6][7]
 580        [ins1][ins2][  ins3 ]
 581        [<-     DCR       ->]
 582           [<- JTPR ->]
 583
 584ins1: 1st Instruction
 585ins2: 2nd Instruction
 586ins3: 3rd Instruction
 587IA:  Insertion Address
 588JTPR: Jump Target Prohibition Region
 589DCR: Detoured Code Region
 590
 591The instructions in DCR are copied to the out-of-line buffer
 592of the kprobe, because the bytes in DCR are replaced by
 593a 5-byte jump instruction. So there are several limitations.
 594
 595a) The instructions in DCR must be relocatable.
 596b) The instructions in DCR must not include a call instruction.
 597c) JTPR must not be targeted by any jump or call instruction.
 598d) DCR must not straddle the border betweeen functions.
 599
 600Anyway, these limitations are checked by the in-kernel instruction
 601decoder, so you don't need to worry about that.
 602
 6036. Probe Overhead
 604
 605On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
 606microseconds to process.  Specifically, a benchmark that hits the same
 607probepoint repeatedly, firing a simple handler each time, reports 1-2
 608million hits per second, depending on the architecture.  A jprobe or
 609return-probe hit typically takes 50-75% longer than a kprobe hit.
 610When you have a return probe set on a function, adding a kprobe at
 611the entry to that function adds essentially no overhead.
 612
 613Here are sample overhead figures (in usec) for different architectures.
 614k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
 615on same function; jr = jprobe + return probe on same function
 616
 617i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
 618k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
 619
 620x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
 621k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
 622
 623ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
 624k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
 625
 6266.1 Optimized Probe Overhead
 627
 628Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
 629process. Here are sample overhead figures (in usec) for x86 architectures.
 630k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
 631r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
 632
 633i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
 634k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
 635
 636x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
 637k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
 638
 6397. TODO
 640
 641a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
 642programming interface for probe-based instrumentation.  Try it out.
 643b. Kernel return probes for sparc64.
 644c. Support for other architectures.
 645d. User-space probes.
 646e. Watchpoint probes (which fire on data references).
 647
 6488. Kprobes Example
 649
 650See samples/kprobes/kprobe_example.c
 651
 6529. Jprobes Example
 653
 654See samples/kprobes/jprobe_example.c
 655
 65610. Kretprobes Example
 657
 658See samples/kprobes/kretprobe_example.c
 659
 660For additional information on Kprobes, refer to the following URLs:
 661http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
 662http://www.redhat.com/magazine/005mar05/features/kprobes/
 663http://www-users.cs.umn.edu/~boutcher/kprobes/
 664http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
 665
 666
 667Appendix A: The kprobes debugfs interface
 668
 669With recent kernels (> 2.6.20) the list of registered kprobes is visible
 670under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
 671
 672/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
 673
 674c015d71a  k  vfs_read+0x0
 675c011a316  j  do_fork+0x0
 676c03dedc5  r  tcp_v4_rcv+0x0
 677
 678The first column provides the kernel address where the probe is inserted.
 679The second column identifies the type of probe (k - kprobe, r - kretprobe
 680and j - jprobe), while the third column specifies the symbol+offset of
 681the probe. If the probed function belongs to a module, the module name
 682is also specified. Following columns show probe status. If the probe is on
 683a virtual address that is no longer valid (module init sections, module
 684virtual addresses that correspond to modules that've been unloaded),
 685such probes are marked with [GONE]. If the probe is temporarily disabled,
 686such probes are marked with [DISABLED]. If the probe is optimized, it is
 687marked with [OPTIMIZED].
 688
 689/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
 690
 691Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
 692By default, all kprobes are enabled. By echoing "0" to this file, all
 693registered probes will be disarmed, till such time a "1" is echoed to this
 694file. Note that this knob just disarms and arms all kprobes and doesn't
 695change each probe's disabling state. This means that disabled kprobes (marked
 696[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
 697
 698
 699Appendix B: The kprobes sysctl interface
 700
 701/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
 702
 703When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
 704a knob to globally and forcibly turn jump optimization (see section
 7051.4) ON or OFF. By default, jump optimization is allowed (ON).
 706If you echo "0" to this file or set "debug.kprobes_optimization" to
 7070 via sysctl, all optimized probes will be unoptimized, and any new
 708probes registered after that will not be optimized.  Note that this
 709knob *changes* the optimized state. This means that optimized probes
 710(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
 711removed). If the knob is turned on, they will be optimized again.
 712
 713
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