linux/Documentation/kprobes.txt
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   1Title   : Kernel Probes (Kprobes)
   2Authors : Jim Keniston <jkenisto@us.ibm.com>
   3        : Prasanna S Panchamukhi <prasanna@in.ibm.com>
   4
   5CONTENTS
   6
   71. Concepts: Kprobes, Jprobes, Return Probes
   82. Architectures Supported
   93. Configuring Kprobes
  104. API Reference
  115. Kprobes Features and Limitations
  126. Probe Overhead
  137. TODO
  148. Kprobes Example
  159. Jprobes Example
  1610. Kretprobes Example
  17Appendix A: The kprobes debugfs interface
  18
  191. Concepts: Kprobes, Jprobes, Return Probes
  20
  21Kprobes enables you to dynamically break into any kernel routine and
  22collect debugging and performance information non-disruptively. You
  23can trap at almost any kernel code address, specifying a handler
  24routine to be invoked when the breakpoint is hit.
  25
  26There are currently three types of probes: kprobes, jprobes, and
  27kretprobes (also called return probes).  A kprobe can be inserted
  28on virtually any instruction in the kernel.  A jprobe is inserted at
  29the entry to a kernel function, and provides convenient access to the
  30function's arguments.  A return probe fires when a specified function
  31returns.
  32
  33In the typical case, Kprobes-based instrumentation is packaged as
  34a kernel module.  The module's init function installs ("registers")
  35one or more probes, and the exit function unregisters them.  A
  36registration function such as register_kprobe() specifies where
  37the probe is to be inserted and what handler is to be called when
  38the probe is hit.
  39
  40There are also register_/unregister_*probes() functions for batch
  41registration/unregistration of a group of *probes. These functions
  42can speed up unregistration process when you have to unregister
  43a lot of probes at once.
  44
  45The next three subsections explain how the different types of
  46probes work.  They explain certain things that you'll need to
  47know in order to make the best use of Kprobes -- e.g., the
  48difference between a pre_handler and a post_handler, and how
  49to use the maxactive and nmissed fields of a kretprobe.  But
  50if you're in a hurry to start using Kprobes, you can skip ahead
  51to section 2.
  52
  531.1 How Does a Kprobe Work?
  54
  55When a kprobe is registered, Kprobes makes a copy of the probed
  56instruction and replaces the first byte(s) of the probed instruction
  57with a breakpoint instruction (e.g., int3 on i386 and x86_64).
  58
  59When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
  60registers are saved, and control passes to Kprobes via the
  61notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
  62associated with the kprobe, passing the handler the addresses of the
  63kprobe struct and the saved registers.
  64
  65Next, Kprobes single-steps its copy of the probed instruction.
  66(It would be simpler to single-step the actual instruction in place,
  67but then Kprobes would have to temporarily remove the breakpoint
  68instruction.  This would open a small time window when another CPU
  69could sail right past the probepoint.)
  70
  71After the instruction is single-stepped, Kprobes executes the
  72"post_handler," if any, that is associated with the kprobe.
  73Execution then continues with the instruction following the probepoint.
  74
  751.2 How Does a Jprobe Work?
  76
  77A jprobe is implemented using a kprobe that is placed on a function's
  78entry point.  It employs a simple mirroring principle to allow
  79seamless access to the probed function's arguments.  The jprobe
  80handler routine should have the same signature (arg list and return
  81type) as the function being probed, and must always end by calling
  82the Kprobes function jprobe_return().
  83
  84Here's how it works.  When the probe is hit, Kprobes makes a copy of
  85the saved registers and a generous portion of the stack (see below).
  86Kprobes then points the saved instruction pointer at the jprobe's
  87handler routine, and returns from the trap.  As a result, control
  88passes to the handler, which is presented with the same register and
  89stack contents as the probed function.  When it is done, the handler
  90calls jprobe_return(), which traps again to restore the original stack
  91contents and processor state and switch to the probed function.
  92
  93By convention, the callee owns its arguments, so gcc may produce code
  94that unexpectedly modifies that portion of the stack.  This is why
  95Kprobes saves a copy of the stack and restores it after the jprobe
  96handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
  9764 bytes on i386.
  98
  99Note that the probed function's args may be passed on the stack
 100or in registers.  The jprobe will work in either case, so long as the
 101handler's prototype matches that of the probed function.
 102
 1031.3 Return Probes
 104
 1051.3.1 How Does a Return Probe Work?
 106
 107When you call register_kretprobe(), Kprobes establishes a kprobe at
 108the entry to the function.  When the probed function is called and this
 109probe is hit, Kprobes saves a copy of the return address, and replaces
 110the return address with the address of a "trampoline."  The trampoline
 111is an arbitrary piece of code -- typically just a nop instruction.
 112At boot time, Kprobes registers a kprobe at the trampoline.
 113
 114When the probed function executes its return instruction, control
 115passes to the trampoline and that probe is hit.  Kprobes' trampoline
 116handler calls the user-specified return handler associated with the
 117kretprobe, then sets the saved instruction pointer to the saved return
 118address, and that's where execution resumes upon return from the trap.
 119
 120While the probed function is executing, its return address is
 121stored in an object of type kretprobe_instance.  Before calling
 122register_kretprobe(), the user sets the maxactive field of the
 123kretprobe struct to specify how many instances of the specified
 124function can be probed simultaneously.  register_kretprobe()
 125pre-allocates the indicated number of kretprobe_instance objects.
 126
 127For example, if the function is non-recursive and is called with a
 128spinlock held, maxactive = 1 should be enough.  If the function is
 129non-recursive and can never relinquish the CPU (e.g., via a semaphore
 130or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
 131set to a default value.  If CONFIG_PREEMPT is enabled, the default
 132is max(10, 2*NR_CPUS).  Otherwise, the default is NR_CPUS.
 133
 134It's not a disaster if you set maxactive too low; you'll just miss
 135some probes.  In the kretprobe struct, the nmissed field is set to
 136zero when the return probe is registered, and is incremented every
 137time the probed function is entered but there is no kretprobe_instance
 138object available for establishing the return probe.
 139
 1401.3.2 Kretprobe entry-handler
 141
 142Kretprobes also provides an optional user-specified handler which runs
 143on function entry. This handler is specified by setting the entry_handler
 144field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
 145function entry is hit, the user-defined entry_handler, if any, is invoked.
 146If the entry_handler returns 0 (success) then a corresponding return handler
 147is guaranteed to be called upon function return. If the entry_handler
 148returns a non-zero error then Kprobes leaves the return address as is, and
 149the kretprobe has no further effect for that particular function instance.
 150
 151Multiple entry and return handler invocations are matched using the unique
 152kretprobe_instance object associated with them. Additionally, a user
 153may also specify per return-instance private data to be part of each
 154kretprobe_instance object. This is especially useful when sharing private
 155data between corresponding user entry and return handlers. The size of each
 156private data object can be specified at kretprobe registration time by
 157setting the data_size field of the kretprobe struct. This data can be
 158accessed through the data field of each kretprobe_instance object.
 159
 160In case probed function is entered but there is no kretprobe_instance
 161object available, then in addition to incrementing the nmissed count,
 162the user entry_handler invocation is also skipped.
 163
 1642. Architectures Supported
 165
 166Kprobes, jprobes, and return probes are implemented on the following
 167architectures:
 168
 169- i386
 170- x86_64 (AMD-64, EM64T)
 171- ppc64
 172- ia64 (Does not support probes on instruction slot1.)
 173- sparc64 (Return probes not yet implemented.)
 174- arm
 175- ppc
 176
 1773. Configuring Kprobes
 178
 179When configuring the kernel using make menuconfig/xconfig/oldconfig,
 180ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
 181Support", look for "Kprobes".
 182
 183So that you can load and unload Kprobes-based instrumentation modules,
 184make sure "Loadable module support" (CONFIG_MODULES) and "Module
 185unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
 186
 187Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
 188are set to "y", since kallsyms_lookup_name() is used by the in-kernel
 189kprobe address resolution code.
 190
 191If you need to insert a probe in the middle of a function, you may find
 192it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
 193so you can use "objdump -d -l vmlinux" to see the source-to-object
 194code mapping.
 195
 1964. API Reference
 197
 198The Kprobes API includes a "register" function and an "unregister"
 199function for each type of probe. The API also includes "register_*probes"
 200and "unregister_*probes" functions for (un)registering arrays of probes.
 201Here are terse, mini-man-page specifications for these functions and
 202the associated probe handlers that you'll write. See the files in the
 203samples/kprobes/ sub-directory for examples.
 204
 2054.1 register_kprobe
 206
 207#include <linux/kprobes.h>
 208int register_kprobe(struct kprobe *kp);
 209
 210Sets a breakpoint at the address kp->addr.  When the breakpoint is
 211hit, Kprobes calls kp->pre_handler.  After the probed instruction
 212is single-stepped, Kprobe calls kp->post_handler.  If a fault
 213occurs during execution of kp->pre_handler or kp->post_handler,
 214or during single-stepping of the probed instruction, Kprobes calls
 215kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
 216is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
 217so, it's handlers aren't hit until calling enable_kprobe(kp).
 218
 219NOTE:
 2201. With the introduction of the "symbol_name" field to struct kprobe,
 221the probepoint address resolution will now be taken care of by the kernel.
 222The following will now work:
 223
 224        kp.symbol_name = "symbol_name";
 225
 226(64-bit powerpc intricacies such as function descriptors are handled
 227transparently)
 228
 2292. Use the "offset" field of struct kprobe if the offset into the symbol
 230to install a probepoint is known. This field is used to calculate the
 231probepoint.
 232
 2333. Specify either the kprobe "symbol_name" OR the "addr". If both are
 234specified, kprobe registration will fail with -EINVAL.
 235
 2364. With CISC architectures (such as i386 and x86_64), the kprobes code
 237does not validate if the kprobe.addr is at an instruction boundary.
 238Use "offset" with caution.
 239
 240register_kprobe() returns 0 on success, or a negative errno otherwise.
 241
 242User's pre-handler (kp->pre_handler):
 243#include <linux/kprobes.h>
 244#include <linux/ptrace.h>
 245int pre_handler(struct kprobe *p, struct pt_regs *regs);
 246
 247Called with p pointing to the kprobe associated with the breakpoint,
 248and regs pointing to the struct containing the registers saved when
 249the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
 250
 251User's post-handler (kp->post_handler):
 252#include <linux/kprobes.h>
 253#include <linux/ptrace.h>
 254void post_handler(struct kprobe *p, struct pt_regs *regs,
 255        unsigned long flags);
 256
 257p and regs are as described for the pre_handler.  flags always seems
 258to be zero.
 259
 260User's fault-handler (kp->fault_handler):
 261#include <linux/kprobes.h>
 262#include <linux/ptrace.h>
 263int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
 264
 265p and regs are as described for the pre_handler.  trapnr is the
 266architecture-specific trap number associated with the fault (e.g.,
 267on i386, 13 for a general protection fault or 14 for a page fault).
 268Returns 1 if it successfully handled the exception.
 269
 2704.2 register_jprobe
 271
 272#include <linux/kprobes.h>
 273int register_jprobe(struct jprobe *jp)
 274
 275Sets a breakpoint at the address jp->kp.addr, which must be the address
 276of the first instruction of a function.  When the breakpoint is hit,
 277Kprobes runs the handler whose address is jp->entry.
 278
 279The handler should have the same arg list and return type as the probed
 280function; and just before it returns, it must call jprobe_return().
 281(The handler never actually returns, since jprobe_return() returns
 282control to Kprobes.)  If the probed function is declared asmlinkage
 283or anything else that affects how args are passed, the handler's
 284declaration must match.
 285
 286register_jprobe() returns 0 on success, or a negative errno otherwise.
 287
 2884.3 register_kretprobe
 289
 290#include <linux/kprobes.h>
 291int register_kretprobe(struct kretprobe *rp);
 292
 293Establishes a return probe for the function whose address is
 294rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
 295You must set rp->maxactive appropriately before you call
 296register_kretprobe(); see "How Does a Return Probe Work?" for details.
 297
 298register_kretprobe() returns 0 on success, or a negative errno
 299otherwise.
 300
 301User's return-probe handler (rp->handler):
 302#include <linux/kprobes.h>
 303#include <linux/ptrace.h>
 304int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
 305
 306regs is as described for kprobe.pre_handler.  ri points to the
 307kretprobe_instance object, of which the following fields may be
 308of interest:
 309- ret_addr: the return address
 310- rp: points to the corresponding kretprobe object
 311- task: points to the corresponding task struct
 312- data: points to per return-instance private data; see "Kretprobe
 313        entry-handler" for details.
 314
 315The regs_return_value(regs) macro provides a simple abstraction to
 316extract the return value from the appropriate register as defined by
 317the architecture's ABI.
 318
 319The handler's return value is currently ignored.
 320
 3214.4 unregister_*probe
 322
 323#include <linux/kprobes.h>
 324void unregister_kprobe(struct kprobe *kp);
 325void unregister_jprobe(struct jprobe *jp);
 326void unregister_kretprobe(struct kretprobe *rp);
 327
 328Removes the specified probe.  The unregister function can be called
 329at any time after the probe has been registered.
 330
 331NOTE:
 332If the functions find an incorrect probe (ex. an unregistered probe),
 333they clear the addr field of the probe.
 334
 3354.5 register_*probes
 336
 337#include <linux/kprobes.h>
 338int register_kprobes(struct kprobe **kps, int num);
 339int register_kretprobes(struct kretprobe **rps, int num);
 340int register_jprobes(struct jprobe **jps, int num);
 341
 342Registers each of the num probes in the specified array.  If any
 343error occurs during registration, all probes in the array, up to
 344the bad probe, are safely unregistered before the register_*probes
 345function returns.
 346- kps/rps/jps: an array of pointers to *probe data structures
 347- num: the number of the array entries.
 348
 349NOTE:
 350You have to allocate(or define) an array of pointers and set all
 351of the array entries before using these functions.
 352
 3534.6 unregister_*probes
 354
 355#include <linux/kprobes.h>
 356void unregister_kprobes(struct kprobe **kps, int num);
 357void unregister_kretprobes(struct kretprobe **rps, int num);
 358void unregister_jprobes(struct jprobe **jps, int num);
 359
 360Removes each of the num probes in the specified array at once.
 361
 362NOTE:
 363If the functions find some incorrect probes (ex. unregistered
 364probes) in the specified array, they clear the addr field of those
 365incorrect probes. However, other probes in the array are
 366unregistered correctly.
 367
 3684.7 disable_*probe
 369
 370#include <linux/kprobes.h>
 371int disable_kprobe(struct kprobe *kp);
 372int disable_kretprobe(struct kretprobe *rp);
 373int disable_jprobe(struct jprobe *jp);
 374
 375Temporarily disables the specified *probe. You can enable it again by using
 376enable_*probe(). You must specify the probe which has been registered.
 377
 3784.8 enable_*probe
 379
 380#include <linux/kprobes.h>
 381int enable_kprobe(struct kprobe *kp);
 382int enable_kretprobe(struct kretprobe *rp);
 383int enable_jprobe(struct jprobe *jp);
 384
 385Enables *probe which has been disabled by disable_*probe(). You must specify
 386the probe which has been registered.
 387
 3885. Kprobes Features and Limitations
 389
 390Kprobes allows multiple probes at the same address.  Currently,
 391however, there cannot be multiple jprobes on the same function at
 392the same time.
 393
 394In general, you can install a probe anywhere in the kernel.
 395In particular, you can probe interrupt handlers.  Known exceptions
 396are discussed in this section.
 397
 398The register_*probe functions will return -EINVAL if you attempt
 399to install a probe in the code that implements Kprobes (mostly
 400kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
 401as do_page_fault and notifier_call_chain).
 402
 403If you install a probe in an inline-able function, Kprobes makes
 404no attempt to chase down all inline instances of the function and
 405install probes there.  gcc may inline a function without being asked,
 406so keep this in mind if you're not seeing the probe hits you expect.
 407
 408A probe handler can modify the environment of the probed function
 409-- e.g., by modifying kernel data structures, or by modifying the
 410contents of the pt_regs struct (which are restored to the registers
 411upon return from the breakpoint).  So Kprobes can be used, for example,
 412to install a bug fix or to inject faults for testing.  Kprobes, of
 413course, has no way to distinguish the deliberately injected faults
 414from the accidental ones.  Don't drink and probe.
 415
 416Kprobes makes no attempt to prevent probe handlers from stepping on
 417each other -- e.g., probing printk() and then calling printk() from a
 418probe handler.  If a probe handler hits a probe, that second probe's
 419handlers won't be run in that instance, and the kprobe.nmissed member
 420of the second probe will be incremented.
 421
 422As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
 423the same handler) may run concurrently on different CPUs.
 424
 425Kprobes does not use mutexes or allocate memory except during
 426registration and unregistration.
 427
 428Probe handlers are run with preemption disabled.  Depending on the
 429architecture, handlers may also run with interrupts disabled.  In any
 430case, your handler should not yield the CPU (e.g., by attempting to
 431acquire a semaphore).
 432
 433Since a return probe is implemented by replacing the return
 434address with the trampoline's address, stack backtraces and calls
 435to __builtin_return_address() will typically yield the trampoline's
 436address instead of the real return address for kretprobed functions.
 437(As far as we can tell, __builtin_return_address() is used only
 438for instrumentation and error reporting.)
 439
 440If the number of times a function is called does not match the number
 441of times it returns, registering a return probe on that function may
 442produce undesirable results. In such a case, a line:
 443kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
 444gets printed. With this information, one will be able to correlate the
 445exact instance of the kretprobe that caused the problem. We have the
 446do_exit() case covered. do_execve() and do_fork() are not an issue.
 447We're unaware of other specific cases where this could be a problem.
 448
 449If, upon entry to or exit from a function, the CPU is running on
 450a stack other than that of the current task, registering a return
 451probe on that function may produce undesirable results.  For this
 452reason, Kprobes doesn't support return probes (or kprobes or jprobes)
 453on the x86_64 version of __switch_to(); the registration functions
 454return -EINVAL.
 455
 4566. Probe Overhead
 457
 458On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
 459microseconds to process.  Specifically, a benchmark that hits the same
 460probepoint repeatedly, firing a simple handler each time, reports 1-2
 461million hits per second, depending on the architecture.  A jprobe or
 462return-probe hit typically takes 50-75% longer than a kprobe hit.
 463When you have a return probe set on a function, adding a kprobe at
 464the entry to that function adds essentially no overhead.
 465
 466Here are sample overhead figures (in usec) for different architectures.
 467k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
 468on same function; jr = jprobe + return probe on same function
 469
 470i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
 471k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
 472
 473x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
 474k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
 475
 476ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
 477k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
 478
 4797. TODO
 480
 481a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
 482programming interface for probe-based instrumentation.  Try it out.
 483b. Kernel return probes for sparc64.
 484c. Support for other architectures.
 485d. User-space probes.
 486e. Watchpoint probes (which fire on data references).
 487
 4888. Kprobes Example
 489
 490See samples/kprobes/kprobe_example.c
 491
 4929. Jprobes Example
 493
 494See samples/kprobes/jprobe_example.c
 495
 49610. Kretprobes Example
 497
 498See samples/kprobes/kretprobe_example.c
 499
 500For additional information on Kprobes, refer to the following URLs:
 501http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
 502http://www.redhat.com/magazine/005mar05/features/kprobes/
 503http://www-users.cs.umn.edu/~boutcher/kprobes/
 504http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
 505
 506
 507Appendix A: The kprobes debugfs interface
 508
 509With recent kernels (> 2.6.20) the list of registered kprobes is visible
 510under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
 511
 512/debug/kprobes/list: Lists all registered probes on the system
 513
 514c015d71a  k  vfs_read+0x0
 515c011a316  j  do_fork+0x0
 516c03dedc5  r  tcp_v4_rcv+0x0
 517
 518The first column provides the kernel address where the probe is inserted.
 519The second column identifies the type of probe (k - kprobe, r - kretprobe
 520and j - jprobe), while the third column specifies the symbol+offset of
 521the probe. If the probed function belongs to a module, the module name
 522is also specified. Following columns show probe status. If the probe is on
 523a virtual address that is no longer valid (module init sections, module
 524virtual addresses that correspond to modules that've been unloaded),
 525such probes are marked with [GONE]. If the probe is temporarily disabled,
 526such probes are marked with [DISABLED].
 527
 528/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
 529
 530Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
 531By default, all kprobes are enabled. By echoing "0" to this file, all
 532registered probes will be disarmed, till such time a "1" is echoed to this
 533file. Note that this knob just disarms and arms all kprobes and doesn't
 534change each probe's disabling state. This means that disabled kprobes (marked
 535[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
 536
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