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.
 216
 217NOTE:
 2181. With the introduction of the "symbol_name" field to struct kprobe,
 219the probepoint address resolution will now be taken care of by the kernel.
 220The following will now work:
 221
 222        kp.symbol_name = "symbol_name";
 223
 224(64-bit powerpc intricacies such as function descriptors are handled
 225transparently)
 226
 2272. Use the "offset" field of struct kprobe if the offset into the symbol
 228to install a probepoint is known. This field is used to calculate the
 229probepoint.
 230
 2313. Specify either the kprobe "symbol_name" OR the "addr". If both are
 232specified, kprobe registration will fail with -EINVAL.
 233
 2344. With CISC architectures (such as i386 and x86_64), the kprobes code
 235does not validate if the kprobe.addr is at an instruction boundary.
 236Use "offset" with caution.
 237
 238register_kprobe() returns 0 on success, or a negative errno otherwise.
 239
 240User's pre-handler (kp->pre_handler):
 241#include <linux/kprobes.h>
 242#include <linux/ptrace.h>
 243int pre_handler(struct kprobe *p, struct pt_regs *regs);
 244
 245Called with p pointing to the kprobe associated with the breakpoint,
 246and regs pointing to the struct containing the registers saved when
 247the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
 248
 249User's post-handler (kp->post_handler):
 250#include <linux/kprobes.h>
 251#include <linux/ptrace.h>
 252void post_handler(struct kprobe *p, struct pt_regs *regs,
 253        unsigned long flags);
 254
 255p and regs are as described for the pre_handler.  flags always seems
 256to be zero.
 257
 258User's fault-handler (kp->fault_handler):
 259#include <linux/kprobes.h>
 260#include <linux/ptrace.h>
 261int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
 262
 263p and regs are as described for the pre_handler.  trapnr is the
 264architecture-specific trap number associated with the fault (e.g.,
 265on i386, 13 for a general protection fault or 14 for a page fault).
 266Returns 1 if it successfully handled the exception.
 267
 2684.2 register_jprobe
 269
 270#include <linux/kprobes.h>
 271int register_jprobe(struct jprobe *jp)
 272
 273Sets a breakpoint at the address jp->kp.addr, which must be the address
 274of the first instruction of a function.  When the breakpoint is hit,
 275Kprobes runs the handler whose address is jp->entry.
 276
 277The handler should have the same arg list and return type as the probed
 278function; and just before it returns, it must call jprobe_return().
 279(The handler never actually returns, since jprobe_return() returns
 280control to Kprobes.)  If the probed function is declared asmlinkage
 281or anything else that affects how args are passed, the handler's
 282declaration must match.
 283
 284register_jprobe() returns 0 on success, or a negative errno otherwise.
 285
 2864.3 register_kretprobe
 287
 288#include <linux/kprobes.h>
 289int register_kretprobe(struct kretprobe *rp);
 290
 291Establishes a return probe for the function whose address is
 292rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
 293You must set rp->maxactive appropriately before you call
 294register_kretprobe(); see "How Does a Return Probe Work?" for details.
 295
 296register_kretprobe() returns 0 on success, or a negative errno
 297otherwise.
 298
 299User's return-probe handler (rp->handler):
 300#include <linux/kprobes.h>
 301#include <linux/ptrace.h>
 302int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
 303
 304regs is as described for kprobe.pre_handler.  ri points to the
 305kretprobe_instance object, of which the following fields may be
 306of interest:
 307- ret_addr: the return address
 308- rp: points to the corresponding kretprobe object
 309- task: points to the corresponding task struct
 310- data: points to per return-instance private data; see "Kretprobe
 311        entry-handler" for details.
 312
 313The regs_return_value(regs) macro provides a simple abstraction to
 314extract the return value from the appropriate register as defined by
 315the architecture's ABI.
 316
 317The handler's return value is currently ignored.
 318
 3194.4 unregister_*probe
 320
 321#include <linux/kprobes.h>
 322void unregister_kprobe(struct kprobe *kp);
 323void unregister_jprobe(struct jprobe *jp);
 324void unregister_kretprobe(struct kretprobe *rp);
 325
 326Removes the specified probe.  The unregister function can be called
 327at any time after the probe has been registered.
 328
 329NOTE:
 330If the functions find an incorrect probe (ex. an unregistered probe),
 331they clear the addr field of the probe.
 332
 3334.5 register_*probes
 334
 335#include <linux/kprobes.h>
 336int register_kprobes(struct kprobe **kps, int num);
 337int register_kretprobes(struct kretprobe **rps, int num);
 338int register_jprobes(struct jprobe **jps, int num);
 339
 340Registers each of the num probes in the specified array.  If any
 341error occurs during registration, all probes in the array, up to
 342the bad probe, are safely unregistered before the register_*probes
 343function returns.
 344- kps/rps/jps: an array of pointers to *probe data structures
 345- num: the number of the array entries.
 346
 347NOTE:
 348You have to allocate(or define) an array of pointers and set all
 349of the array entries before using these functions.
 350
 3514.6 unregister_*probes
 352
 353#include <linux/kprobes.h>
 354void unregister_kprobes(struct kprobe **kps, int num);
 355void unregister_kretprobes(struct kretprobe **rps, int num);
 356void unregister_jprobes(struct jprobe **jps, int num);
 357
 358Removes each of the num probes in the specified array at once.
 359
 360NOTE:
 361If the functions find some incorrect probes (ex. unregistered
 362probes) in the specified array, they clear the addr field of those
 363incorrect probes. However, other probes in the array are
 364unregistered correctly.
 365
 3665. Kprobes Features and Limitations
 367
 368Kprobes allows multiple probes at the same address.  Currently,
 369however, there cannot be multiple jprobes on the same function at
 370the same time.
 371
 372In general, you can install a probe anywhere in the kernel.
 373In particular, you can probe interrupt handlers.  Known exceptions
 374are discussed in this section.
 375
 376The register_*probe functions will return -EINVAL if you attempt
 377to install a probe in the code that implements Kprobes (mostly
 378kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
 379as do_page_fault and notifier_call_chain).
 380
 381If you install a probe in an inline-able function, Kprobes makes
 382no attempt to chase down all inline instances of the function and
 383install probes there.  gcc may inline a function without being asked,
 384so keep this in mind if you're not seeing the probe hits you expect.
 385
 386A probe handler can modify the environment of the probed function
 387-- e.g., by modifying kernel data structures, or by modifying the
 388contents of the pt_regs struct (which are restored to the registers
 389upon return from the breakpoint).  So Kprobes can be used, for example,
 390to install a bug fix or to inject faults for testing.  Kprobes, of
 391course, has no way to distinguish the deliberately injected faults
 392from the accidental ones.  Don't drink and probe.
 393
 394Kprobes makes no attempt to prevent probe handlers from stepping on
 395each other -- e.g., probing printk() and then calling printk() from a
 396probe handler.  If a probe handler hits a probe, that second probe's
 397handlers won't be run in that instance, and the kprobe.nmissed member
 398of the second probe will be incremented.
 399
 400As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
 401the same handler) may run concurrently on different CPUs.
 402
 403Kprobes does not use mutexes or allocate memory except during
 404registration and unregistration.
 405
 406Probe handlers are run with preemption disabled.  Depending on the
 407architecture, handlers may also run with interrupts disabled.  In any
 408case, your handler should not yield the CPU (e.g., by attempting to
 409acquire a semaphore).
 410
 411Since a return probe is implemented by replacing the return
 412address with the trampoline's address, stack backtraces and calls
 413to __builtin_return_address() will typically yield the trampoline's
 414address instead of the real return address for kretprobed functions.
 415(As far as we can tell, __builtin_return_address() is used only
 416for instrumentation and error reporting.)
 417
 418If the number of times a function is called does not match the number
 419of times it returns, registering a return probe on that function may
 420produce undesirable results. In such a case, a line:
 421kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
 422gets printed. With this information, one will be able to correlate the
 423exact instance of the kretprobe that caused the problem. We have the
 424do_exit() case covered. do_execve() and do_fork() are not an issue.
 425We're unaware of other specific cases where this could be a problem.
 426
 427If, upon entry to or exit from a function, the CPU is running on
 428a stack other than that of the current task, registering a return
 429probe on that function may produce undesirable results.  For this
 430reason, Kprobes doesn't support return probes (or kprobes or jprobes)
 431on the x86_64 version of __switch_to(); the registration functions
 432return -EINVAL.
 433
 4346. Probe Overhead
 435
 436On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
 437microseconds to process.  Specifically, a benchmark that hits the same
 438probepoint repeatedly, firing a simple handler each time, reports 1-2
 439million hits per second, depending on the architecture.  A jprobe or
 440return-probe hit typically takes 50-75% longer than a kprobe hit.
 441When you have a return probe set on a function, adding a kprobe at
 442the entry to that function adds essentially no overhead.
 443
 444Here are sample overhead figures (in usec) for different architectures.
 445k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
 446on same function; jr = jprobe + return probe on same function
 447
 448i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
 449k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
 450
 451x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
 452k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
 453
 454ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
 455k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
 456
 4577. TODO
 458
 459a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
 460programming interface for probe-based instrumentation.  Try it out.
 461b. Kernel return probes for sparc64.
 462c. Support for other architectures.
 463d. User-space probes.
 464e. Watchpoint probes (which fire on data references).
 465
 4668. Kprobes Example
 467
 468See samples/kprobes/kprobe_example.c
 469
 4709. Jprobes Example
 471
 472See samples/kprobes/jprobe_example.c
 473
 47410. Kretprobes Example
 475
 476See samples/kprobes/kretprobe_example.c
 477
 478For additional information on Kprobes, refer to the following URLs:
 479http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
 480http://www.redhat.com/magazine/005mar05/features/kprobes/
 481http://www-users.cs.umn.edu/~boutcher/kprobes/
 482http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
 483
 484
 485Appendix A: The kprobes debugfs interface
 486
 487With recent kernels (> 2.6.20) the list of registered kprobes is visible
 488under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
 489
 490/debug/kprobes/list: Lists all registered probes on the system
 491
 492c015d71a  k  vfs_read+0x0
 493c011a316  j  do_fork+0x0
 494c03dedc5  r  tcp_v4_rcv+0x0
 495
 496The first column provides the kernel address where the probe is inserted.
 497The second column identifies the type of probe (k - kprobe, r - kretprobe
 498and j - jprobe), while the third column specifies the symbol+offset of
 499the probe. If the probed function belongs to a module, the module name
 500is also specified. Following columns show probe status. If the probe is on
 501a virtual address that is no longer valid (module init sections, module
 502virtual addresses that correspond to modules that've been unloaded),
 503such probes are marked with [GONE].
 504
 505/debug/kprobes/enabled: Turn kprobes ON/OFF
 506
 507Provides a knob to globally turn registered kprobes ON or OFF. By default,
 508all kprobes are enabled. By echoing "0" to this file, all registered probes
 509will be disarmed, till such time a "1" is echoed to this file.
 510