TOMOYO Linux Cross Reference
Linux/Documentation/trace/kprobes.rst

   =======================
   Kernel Probes (Kprobes)
   =======================
   
   :Author: Jim Keniston <jkenisto@us.ibm.com>
   :Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
   :Author: Masami Hiramatsu <mhiramat@kernel.org>
   
   .. CONTENTS
  
    1. Concepts: Kprobes, and Return Probes
    2. Architectures Supported
    3. Configuring Kprobes
    4. API Reference
    5. Kprobes Features and Limitations
    6. Probe Overhead
    7. TODO
    8. Kprobes Example
    9. Kretprobes Example
    10. Deprecated Features
    Appendix A: The kprobes debugfs interface
    Appendix B: The kprobes sysctl interface
    Appendix C: References
  
  Concepts: Kprobes and Return Probes
  =========================================
  
  Kprobes enables you to dynamically break into any kernel routine and
  collect debugging and performance information non-disruptively. You
  can trap at almost any kernel code address [1]_, specifying a handler
  routine to be invoked when the breakpoint is hit.
  
  .. [1] some parts of the kernel code can not be trapped, see
         :ref:`kprobes_blacklist`)
  
  There are currently two types of probes: kprobes, and kretprobes
  (also called return probes).  A kprobe can be inserted on virtually
  any instruction in the kernel.  A return probe fires when a specified
  function returns.
  
  In the typical case, Kprobes-based instrumentation is packaged as
  a kernel module.  The module's init function installs ("registers")
  one or more probes, and the exit function unregisters them.  A
  registration function such as register_kprobe() specifies where
  the probe is to be inserted and what handler is to be called when
  the probe is hit.
  
  There are also ``register_/unregister_*probes()`` functions for batch
  registration/unregistration of a group of ``*probes``. These functions
  can speed up unregistration process when you have to unregister
  a lot of probes at once.
  
  The next four subsections explain how the different types of
  probes work and how jump optimization works.  They explain certain
  things that you'll need to know in order to make the best use of
  Kprobes -- e.g., the difference between a pre_handler and
  a post_handler, and how to use the maxactive and nmissed fields of
  a kretprobe.  But if you're in a hurry to start using Kprobes, you
  can skip ahead to :ref:`kprobes_archs_supported`.
  
  How Does a Kprobe Work?
  -----------------------
  
  When a kprobe is registered, Kprobes makes a copy of the probed
  instruction and replaces the first byte(s) of the probed instruction
  with a breakpoint instruction (e.g., int3 on i386 and x86_64).
  
  When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
  registers are saved, and control passes to Kprobes via the
  notifier_call_chain mechanism.  Kprobes executes the "pre_handler"
  associated with the kprobe, passing the handler the addresses of the
  kprobe struct and the saved registers.
  
  Next, Kprobes single-steps its copy of the probed instruction.
  (It would be simpler to single-step the actual instruction in place,
  but then Kprobes would have to temporarily remove the breakpoint
  instruction.  This would open a small time window when another CPU
  could sail right past the probepoint.)
  
  After the instruction is single-stepped, Kprobes executes the
  "post_handler," if any, that is associated with the kprobe.
  Execution then continues with the instruction following the probepoint.
  
  Changing Execution Path
  -----------------------
  
  Since kprobes can probe into a running kernel code, it can change the
  register set, including instruction pointer. This operation requires
  maximum care, such as keeping the stack frame, recovering the execution
  path etc. Since it operates on a running kernel and needs deep knowledge
  of computer architecture and concurrent computing, you can easily shoot
  your foot.
  
  If you change the instruction pointer (and set up other related
  registers) in pre_handler, you must return !0 so that kprobes stops
  single stepping and just returns to the given address.
  This also means post_handler should not be called anymore.
  
  Note that this operation may be harder on some architectures which use
 TOC (Table of Contents) for function call, since you have to setup a new
 TOC for your function in your module, and recover the old one after
 returning from it.
 
 Return Probes
 -------------
 
 How Does a Return Probe Work?
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 When you call register_kretprobe(), Kprobes establishes a kprobe at
 the entry to the function.  When the probed function is called and this
 probe is hit, Kprobes saves a copy of the return address, and replaces
 the return address with the address of a "trampoline."  The trampoline
 is an arbitrary piece of code -- typically just a nop instruction.
 At boot time, Kprobes registers a kprobe at the trampoline.
 
 When the probed function executes its return instruction, control
 passes to the trampoline and that probe is hit.  Kprobes' trampoline
 handler calls the user-specified return handler associated with the
 kretprobe, then sets the saved instruction pointer to the saved return
 address, and that's where execution resumes upon return from the trap.
 
 While the probed function is executing, its return address is
 stored in an object of type kretprobe_instance.  Before calling
 register_kretprobe(), the user sets the maxactive field of the
 kretprobe struct to specify how many instances of the specified
 function can be probed simultaneously.  register_kretprobe()
 pre-allocates the indicated number of kretprobe_instance objects.
 
 For example, if the function is non-recursive and is called with a
 spinlock held, maxactive = 1 should be enough.  If the function is
 non-recursive and can never relinquish the CPU (e.g., via a semaphore
 or preemption), NR_CPUS should be enough.  If maxactive <= 0, it is
 set to a default value: max(10, 2*NR_CPUS).
 
 It's not a disaster if you set maxactive too low; you'll just miss
 some probes.  In the kretprobe struct, the nmissed field is set to
 zero when the return probe is registered, and is incremented every
 time the probed function is entered but there is no kretprobe_instance
 object available for establishing the return probe.
 
 Kretprobe entry-handler
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 Kretprobes also provides an optional user-specified handler which runs
 on function entry. This handler is specified by setting the entry_handler
 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
 function entry is hit, the user-defined entry_handler, if any, is invoked.
 If the entry_handler returns 0 (success) then a corresponding return handler
 is guaranteed to be called upon function return. If the entry_handler
 returns a non-zero error then Kprobes leaves the return address as is, and
 the kretprobe has no further effect for that particular function instance.
 
 Multiple entry and return handler invocations are matched using the unique
 kretprobe_instance object associated with them. Additionally, a user
 may also specify per return-instance private data to be part of each
 kretprobe_instance object. This is especially useful when sharing private
 data between corresponding user entry and return handlers. The size of each
 private data object can be specified at kretprobe registration time by
 setting the data_size field of the kretprobe struct. This data can be
 accessed through the data field of each kretprobe_instance object.
 
 In case probed function is entered but there is no kretprobe_instance
 object available, then in addition to incrementing the nmissed count,
 the user entry_handler invocation is also skipped.
 
 .. _kprobes_jump_optimization:
 
 How Does Jump Optimization Work?
 --------------------------------
 
 If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
 is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
 the "debug.kprobes_optimization" kernel parameter is set to 1 (see
 sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
 instruction instead of a breakpoint instruction at each probepoint.
 
 Init a Kprobe
 ^^^^^^^^^^^^^
 
 When a probe is registered, before attempting this optimization,
 Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
 address. So, even if it's not possible to optimize this particular
 probepoint, there'll be a probe there.
 
 Safety Check
 ^^^^^^^^^^^^
 
 Before optimizing a probe, Kprobes performs the following safety checks:
 
 - Kprobes verifies that the region that will be replaced by the jump
   instruction (the "optimized region") lies entirely within one function.
   (A jump instruction is multiple bytes, and so may overlay multiple
   instructions.)
 
 - Kprobes analyzes the entire function and verifies that there is no
   jump into the optimized region.  Specifically:
 
   - the function contains no indirect jump;
   - the function contains no instruction that causes an exception (since
     the fixup code triggered by the exception could jump back into the
     optimized region -- Kprobes checks the exception tables to verify this);
   - there is no near jump to the optimized region (other than to the first
     byte).
 
 - For each instruction in the optimized region, Kprobes verifies that
   the instruction can be executed out of line.
 
 Preparing Detour Buffer
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 Next, Kprobes prepares a "detour" buffer, which contains the following
 instruction sequence:
 
 - code to push the CPU's registers (emulating a breakpoint trap)
 - a call to the trampoline code which calls user's probe handlers.
 - code to restore registers
 - the instructions from the optimized region
 - a jump back to the original execution path.
 
 Pre-optimization
 ^^^^^^^^^^^^^^^^
 
 After preparing the detour buffer, Kprobes verifies that none of the
 following situations exist:
 
 - The probe has a post_handler.
 - Other instructions in the optimized region are probed.
 - The probe is disabled.
 
 In any of the above cases, Kprobes won't start optimizing the probe.
 Since these are temporary situations, Kprobes tries to start
 optimizing it again if the situation is changed.
 
 If the kprobe can be optimized, Kprobes enqueues the kprobe to an
 optimizing list, and kicks the kprobe-optimizer workqueue to optimize
 it.  If the to-be-optimized probepoint is hit before being optimized,
 Kprobes returns control to the original instruction path by setting
 the CPU's instruction pointer to the copied code in the detour buffer
 -- thus at least avoiding the single-step.
 
 Optimization
 ^^^^^^^^^^^^
 
 The Kprobe-optimizer doesn't insert the jump instruction immediately;
 rather, it calls synchronize_rcu() for safety first, because it's
 possible for a CPU to be interrupted in the middle of executing the
 optimized region [3]_.  As you know, synchronize_rcu() can ensure
 that all interruptions that were active when synchronize_rcu()
 was called are done, but only if CONFIG_PREEMPT=n.  So, this version
 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
 
 After that, the Kprobe-optimizer calls stop_machine() to replace
 the optimized region with a jump instruction to the detour buffer,
 using text_poke_smp().
 
 Unoptimization
 ^^^^^^^^^^^^^^
 
 When an optimized kprobe is unregistered, disabled, or blocked by
 another kprobe, it will be unoptimized.  If this happens before
 the optimization is complete, the kprobe is just dequeued from the
 optimized list.  If the optimization has been done, the jump is
 replaced with the original code (except for an int3 breakpoint in
 the first byte) by using text_poke_smp().
 
 .. [3] Please imagine that the 2nd instruction is interrupted and then
    the optimizer replaces the 2nd instruction with the jump *address*
    while the interrupt handler is running. When the interrupt
    returns to original address, there is no valid instruction,
    and it causes an unexpected result.
 
 .. [4] This optimization-safety checking may be replaced with the
    stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
    kernel.
 
 NOTE for geeks:
 The jump optimization changes the kprobe's pre_handler behavior.
 Without optimization, the pre_handler can change the kernel's execution
 path by changing regs->ip and returning 1.  However, when the probe
 is optimized, that modification is ignored.  Thus, if you want to
 tweak the kernel's execution path, you need to suppress optimization,
 using one of the following techniques:
 
 - Specify an empty function for the kprobe's post_handler.
 
 or
 
 - Execute 'sysctl -w debug.kprobes_optimization=n'
 
 .. _kprobes_blacklist:
 
 Blacklist
 ---------
 
 Kprobes can probe most of the kernel except itself. This means
 that there are some functions where kprobes cannot probe. Probing
 (trapping) such functions can cause a recursive trap (e.g. double
 fault) or the nested probe handler may never be called.
 Kprobes manages such functions as a blacklist.
 If you want to add a function into the blacklist, you just need
 to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
 to specify a blacklisted function.
 Kprobes checks the given probe address against the blacklist and
 rejects registering it, if the given address is in the blacklist.
 
 .. _kprobes_archs_supported:
 
 Architectures Supported
 =======================
 
 Kprobes and return probes are implemented on the following
 architectures:
 
 - i386 (Supports jump optimization)
 - x86_64 (AMD-64, EM64T) (Supports jump optimization)
 - ppc64
 - sparc64 (Return probes not yet implemented.)
 - arm
 - ppc
 - mips
 - s390
 - parisc
 - loongarch
 - riscv
 
 Configuring Kprobes
 ===================
 
 When configuring the kernel using make menuconfig/xconfig/oldconfig,
 ensure that CONFIG_KPROBES is set to "y", look for "Kprobes" under
 "General architecture-dependent options".
 
 So that you can load and unload Kprobes-based instrumentation modules,
 make sure "Loadable module support" (CONFIG_MODULES) and "Module
 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
 
 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
 kprobe address resolution code.
 
 If you need to insert a probe in the middle of a function, you may find
 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
 so you can use "objdump -d -l vmlinux" to see the source-to-object
 code mapping.
 
 API Reference
 =============
 
 The Kprobes API includes a "register" function and an "unregister"
 function for each type of probe. The API also includes "register_*probes"
 and "unregister_*probes" functions for (un)registering arrays of probes.
 Here are terse, mini-man-page specifications for these functions and
 the associated probe handlers that you'll write. See the files in the
 samples/kprobes/ sub-directory for examples.
 
 register_kprobe
 ---------------
 
 ::
 
         #include <linux/kprobes.h>
         int register_kprobe(struct kprobe *kp);
 
 Sets a breakpoint at the address kp->addr.  When the breakpoint is hit, Kprobes
 calls kp->pre_handler.  After the probed instruction is single-stepped, Kprobe
 calls kp->post_handler.  Any or all handlers can be NULL. If kp->flags is set
 KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers
 aren't hit until calling enable_kprobe(kp).
 
 .. note::
 
    1. With the introduction of the "symbol_name" field to struct kprobe,
       the probepoint address resolution will now be taken care of by the kernel.
       The following will now work::
 
         kp.symbol_name = "symbol_name";
 
       (64-bit powerpc intricacies such as function descriptors are handled
       transparently)
 
    2. Use the "offset" field of struct kprobe if the offset into the symbol
       to install a probepoint is known. This field is used to calculate the
       probepoint.
 
    3. Specify either the kprobe "symbol_name" OR the "addr". If both are
       specified, kprobe registration will fail with -EINVAL.
 
    4. With CISC architectures (such as i386 and x86_64), the kprobes code
       does not validate if the kprobe.addr is at an instruction boundary.
       Use "offset" with caution.
 
 register_kprobe() returns 0 on success, or a negative errno otherwise.
 
 User's pre-handler (kp->pre_handler)::
 
         #include <linux/kprobes.h>
         #include <linux/ptrace.h>
         int pre_handler(struct kprobe *p, struct pt_regs *regs);
 
 Called with p pointing to the kprobe associated with the breakpoint,
 and regs pointing to the struct containing the registers saved when
 the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
 
 User's post-handler (kp->post_handler)::
 
         #include <linux/kprobes.h>
         #include <linux/ptrace.h>
         void post_handler(struct kprobe *p, struct pt_regs *regs,
                           unsigned long flags);
 
 p and regs are as described for the pre_handler.  flags always seems
 to be zero.
 
 register_kretprobe
 ------------------
 
 ::
 
         #include <linux/kprobes.h>
         int register_kretprobe(struct kretprobe *rp);
 
 Establishes a return probe for the function whose address is
 rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
 You must set rp->maxactive appropriately before you call
 register_kretprobe(); see "How Does a Return Probe Work?" for details.
 
 register_kretprobe() returns 0 on success, or a negative errno
 otherwise.
 
 User's return-probe handler (rp->handler)::
 
         #include <linux/kprobes.h>
         #include <linux/ptrace.h>
         int kretprobe_handler(struct kretprobe_instance *ri,
                               struct pt_regs *regs);
 
 regs is as described for kprobe.pre_handler.  ri points to the
 kretprobe_instance object, of which the following fields may be
 of interest:
 
 - ret_addr: the return address
 - rp: points to the corresponding kretprobe object
 - task: points to the corresponding task struct
 - data: points to per return-instance private data; see "Kretprobe
         entry-handler" for details.
 
 The regs_return_value(regs) macro provides a simple abstraction to
 extract the return value from the appropriate register as defined by
 the architecture's ABI.
 
 The handler's return value is currently ignored.
 
 unregister_*probe
 ------------------
 
 ::
 
         #include <linux/kprobes.h>
         void unregister_kprobe(struct kprobe *kp);
         void unregister_kretprobe(struct kretprobe *rp);
 
 Removes the specified probe.  The unregister function can be called
 at any time after the probe has been registered.
 
 .. note::
 
    If the functions find an incorrect probe (ex. an unregistered probe),
    they clear the addr field of the probe.
 
 register_*probes
 ----------------
 
 ::
 
         #include <linux/kprobes.h>
         int register_kprobes(struct kprobe **kps, int num);
         int register_kretprobes(struct kretprobe **rps, int num);
 
 Registers each of the num probes in the specified array.  If any
 error occurs during registration, all probes in the array, up to
 the bad probe, are safely unregistered before the register_*probes
 function returns.
 
 - kps/rps: an array of pointers to ``*probe`` data structures
 - num: the number of the array entries.
 
 .. note::
 
    You have to allocate(or define) an array of pointers and set all
    of the array entries before using these functions.
 
 unregister_*probes
 ------------------
 
 ::
 
         #include <linux/kprobes.h>
         void unregister_kprobes(struct kprobe **kps, int num);
         void unregister_kretprobes(struct kretprobe **rps, int num);
 
 Removes each of the num probes in the specified array at once.
 
 .. note::
 
    If the functions find some incorrect probes (ex. unregistered
    probes) in the specified array, they clear the addr field of those
    incorrect probes. However, other probes in the array are
    unregistered correctly.
 
 disable_*probe
 --------------
 
 ::
 
         #include <linux/kprobes.h>
         int disable_kprobe(struct kprobe *kp);
         int disable_kretprobe(struct kretprobe *rp);
 
 Temporarily disables the specified ``*probe``. You can enable it again by using
 enable_*probe(). You must specify the probe which has been registered.
 
 enable_*probe
 -------------
 
 ::
 
         #include <linux/kprobes.h>
         int enable_kprobe(struct kprobe *kp);
         int enable_kretprobe(struct kretprobe *rp);
 
 Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
 the probe which has been registered.
 
 Kprobes Features and Limitations
 ================================
 
 Kprobes allows multiple probes at the same address. Also,
 a probepoint for which there is a post_handler cannot be optimized.
 So if you install a kprobe with a post_handler, at an optimized
 probepoint, the probepoint will be unoptimized automatically.
 
 In general, you can install a probe anywhere in the kernel.
 In particular, you can probe interrupt handlers.  Known exceptions
 are discussed in this section.
 
 The register_*probe functions will return -EINVAL if you attempt
 to install a probe in the code that implements Kprobes (mostly
 kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
 as do_page_fault and notifier_call_chain).
 
 If you install a probe in an inline-able function, Kprobes makes
 no attempt to chase down all inline instances of the function and
 install probes there.  gcc may inline a function without being asked,
 so keep this in mind if you're not seeing the probe hits you expect.
 
 A probe handler can modify the environment of the probed function
 -- e.g., by modifying kernel data structures, or by modifying the
 contents of the pt_regs struct (which are restored to the registers
 upon return from the breakpoint).  So Kprobes can be used, for example,
 to install a bug fix or to inject faults for testing.  Kprobes, of
 course, has no way to distinguish the deliberately injected faults
 from the accidental ones.  Don't drink and probe.
 
 Kprobes makes no attempt to prevent probe handlers from stepping on
 each other -- e.g., probing printk() and then calling printk() from a
 probe handler.  If a probe handler hits a probe, that second probe's
 handlers won't be run in that instance, and the kprobe.nmissed member
 of the second probe will be incremented.
 
 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
 the same handler) may run concurrently on different CPUs.
 
 Kprobes does not use mutexes or allocate memory except during
 registration and unregistration.
 
 Probe handlers are run with preemption disabled or interrupt disabled,
 which depends on the architecture and optimization state.  (e.g.,
 kretprobe handlers and optimized kprobe handlers run without interrupt
 disabled on x86/x86-64).  In any case, your handler should not yield
 the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
 
 Since a return probe is implemented by replacing the return
 address with the trampoline's address, stack backtraces and calls
 to __builtin_return_address() will typically yield the trampoline's
 address instead of the real return address for kretprobed functions.
 (As far as we can tell, __builtin_return_address() is used only
 for instrumentation and error reporting.)
 
 If the number of times a function is called does not match the number
 of times it returns, registering a return probe on that function may
 produce undesirable results. In such a case, a line:
 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
 gets printed. With this information, one will be able to correlate the
 exact instance of the kretprobe that caused the problem. We have the
 do_exit() case covered. do_execve() and do_fork() are not an issue.
 We're unaware of other specific cases where this could be a problem.
 
 If, upon entry to or exit from a function, the CPU is running on
 a stack other than that of the current task, registering a return
 probe on that function may produce undesirable results.  For this
 reason, Kprobes doesn't support return probes (or kprobes)
 on the x86_64 version of __switch_to(); the registration functions
 return -EINVAL.
 
 On x86/x86-64, since the Jump Optimization of Kprobes modifies
 instructions widely, there are some limitations to optimization. To
 explain it, we introduce some terminology. Imagine a 3-instruction
 sequence consisting of a two 2-byte instructions and one 3-byte
 instruction.
 
 ::
 
                 IA
                 |
         [-2][-1][0][1][2][3][4][5][6][7]
                 [ins1][ins2][  ins3 ]
                 [<-     DCR       ->]
                 [<- JTPR ->]
 
         ins1: 1st Instruction
         ins2: 2nd Instruction
         ins3: 3rd Instruction
         IA:  Insertion Address
         JTPR: Jump Target Prohibition Region
         DCR: Detoured Code Region
 
 The instructions in DCR are copied to the out-of-line buffer
 of the kprobe, because the bytes in DCR are replaced by
 a 5-byte jump instruction. So there are several limitations.
 
 a) The instructions in DCR must be relocatable.
 b) The instructions in DCR must not include a call instruction.
 c) JTPR must not be targeted by any jump or call instruction.
 d) DCR must not straddle the border between functions.
 
 Anyway, these limitations are checked by the in-kernel instruction
 decoder, so you don't need to worry about that.
 
 Probe Overhead
 ==============
 
 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
 microseconds to process.  Specifically, a benchmark that hits the same
 probepoint repeatedly, firing a simple handler each time, reports 1-2
 million hits per second, depending on the architecture.  A return-probe
 hit typically takes 50-75% longer than a kprobe hit.
 When you have a return probe set on a function, adding a kprobe at
 the entry to that function adds essentially no overhead.
 
 Here are sample overhead figures (in usec) for different architectures::
 
   k = kprobe; r = return probe; kr = kprobe + return probe
   on same function
 
   i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
   k = 0.57 usec; r = 0.92; kr = 0.99
 
   x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
   k = 0.49 usec; r = 0.80; kr = 0.82
 
   ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
   k = 0.77 usec; r = 1.26; kr = 1.45
 
 Optimized Probe Overhead
 ------------------------
 
 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
 process. Here are sample overhead figures (in usec) for x86 architectures::
 
   k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
   r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
 
   i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
   k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
 
   x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
   k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
 
 TODO
 ====
 
 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
    programming interface for probe-based instrumentation.  Try it out.
 b. Kernel return probes for sparc64.
 c. Support for other architectures.
 d. User-space probes.
 e. Watchpoint probes (which fire on data references).
 
 Kprobes Example
 ===============
 
 See samples/kprobes/kprobe_example.c
 
 Kretprobes Example
 ==================
 
 See samples/kprobes/kretprobe_example.c
 
 Deprecated Features
 ===================
 
 Jprobes is now a deprecated feature. People who are depending on it should
 migrate to other tracing features or use older kernels. Please consider to
 migrate your tool to one of the following options:
 
 - Use trace-event to trace target function with arguments.
 
   trace-event is a low-overhead (and almost no visible overhead if it
   is off) statically defined event interface. You can define new events
   and trace it via ftrace or any other tracing tools.
 
   See the following urls:
 
     - https://lwn.net/Articles/379903/
     - https://lwn.net/Articles/381064/
     - https://lwn.net/Articles/383362/
 
 - Use ftrace dynamic events (kprobe event) with perf-probe.
 
   If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
   find which register/stack is assigned to which local variable or arguments
   by using perf-probe and set up new event to trace it.
 
   See following documents:
 
   - Documentation/trace/kprobetrace.rst
   - Documentation/trace/events.rst
   - tools/perf/Documentation/perf-probe.txt
 
 
 The kprobes debugfs interface
 =============================
 
 
 With recent kernels (> 2.6.20) the list of registered kprobes is visible
 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
 
 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
 
         c015d71a  k  vfs_read+0x0
         c03dedc5  r  tcp_v4_rcv+0x0
 
 The first column provides the kernel address where the probe is inserted.
 The second column identifies the type of probe (k - kprobe and r - kretprobe)
 while the third column specifies the symbol+offset of the probe.
 If the probed function belongs to a module, the module name is also
 specified. Following columns show probe status. If the probe is on
 a virtual address that is no longer valid (module init sections, module
 virtual addresses that correspond to modules that've been unloaded),
 such probes are marked with [GONE]. If the probe is temporarily disabled,
 such probes are marked with [DISABLED]. If the probe is optimized, it is
 marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
 [FTRACE].
 
 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
 
 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
 By default, all kprobes are enabled. By echoing "0" to this file, all
 registered probes will be disarmed, till such time a "1" is echoed to this
 file. Note that this knob just disarms and arms all kprobes and doesn't
 change each probe's disabling state. This means that disabled kprobes (marked
 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
 
 
 The kprobes sysctl interface
 ============================
 
 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
 
 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
 a knob to globally and forcibly turn jump optimization (see section
 :ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
 is allowed (ON). If you echo "0" to this file or set
 "debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
 unoptimized, and any new probes registered after that will not be optimized.
 
 Note that this knob *changes* the optimized state. This means that optimized
 probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
 removed). If the knob is turned on, they will be optimized again.
 
 References
 ==========
 
 For additional information on Kprobes, refer to the following URLs:
 
 - https://lwn.net/Articles/132196/
 - https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf
 

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