Kernel Probes (Kprobes)

Author:Jim Keniston <jkenisto@us.ibm.com>
Author:Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
Author:Masami Hiramatsu <mhiramat@redhat.com>

Concepts: Kprobes and Return Probes

Kprobes enables you to dynamically break into any kernel routine andcollect debugging and performance information non-disruptively. Youcan trap at almost any kernel code address[1], specifying a handlerroutine to be invoked when the breakpoint is hit.

[1]some parts of the kernel code can not be trapped, seeBlacklist)

There are currently two types of probes: kprobes, and kretprobes(also called return probes). A kprobe can be inserted on virtuallyany instruction in the kernel. A return probe fires when a specifiedfunction returns.

In the typical case, Kprobes-based instrumentation is packaged asa kernel module. The module’s init function installs (“registers”)one or more probes, and the exit function unregisters them. Aregistration function such as register_kprobe() specifies wherethe probe is to be inserted and what handler is to be called whenthe probe is hit.

There are alsoregister_/unregister_*probes() functions for batchregistration/unregistration of a group of*probes. These functionscan speed up unregistration process when you have to unregistera lot of probes at once.

The next four subsections explain how the different types ofprobes work and how jump optimization works. They explain certainthings that you’ll need to know in order to make the best use ofKprobes – e.g., the difference between a pre_handler anda post_handler, and how to use the maxactive and nmissed fields ofa kretprobe. But if you’re in a hurry to start using Kprobes, youcan skip ahead toArchitectures Supported.

How Does a Kprobe Work?

When a kprobe is registered, Kprobes makes a copy of the probedinstruction and replaces the first byte(s) of the probed instructionwith a breakpoint instruction (e.g., int3 on i386 and x86_64).

When a CPU hits the breakpoint instruction, a trap occurs, the CPU’sregisters are saved, and control passes to Kprobes via thenotifier_call_chain mechanism. Kprobes executes the “pre_handler”associated with the kprobe, passing the handler the addresses of thekprobe 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 breakpointinstruction. This would open a small time window when another CPUcould 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 theregister set, including instruction pointer. This operation requiresmaximum care, such as keeping the stack frame, recovering the executionpath etc. Since it operates on a running kernel and needs deep knowledgeof computer architecture and concurrent computing, you can easily shootyour foot.

If you change the instruction pointer (and set up other relatedregisters) in pre_handler, you must return !0 so that kprobes stopssingle 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 useTOC (Table of Contents) for function call, since you have to setup a newTOC for your function in your module, and recover the old one afterreturning from it.

Return Probes

How Does a Return Probe Work?

When you call register_kretprobe(), Kprobes establishes a kprobe atthe entry to the function. When the probed function is called and thisprobe is hit, Kprobes saves a copy of the return address, and replacesthe return address with the address of a “trampoline.” The trampolineis 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, controlpasses to the trampoline and that probe is hit. Kprobes’ trampolinehandler calls the user-specified return handler associated with thekretprobe, then sets the saved instruction pointer to the saved returnaddress, and that’s where execution resumes upon return from the trap.

While the probed function is executing, its return address isstored in an object of type kretprobe_instance. Before callingregister_kretprobe(), the user sets the maxactive field of thekretprobe struct to specify how many instances of the specifiedfunction 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 aspinlock held, maxactive = 1 should be enough. If the function isnon-recursive and can never relinquish the CPU (e.g., via a semaphoreor preemption), NR_CPUS should be enough. If maxactive <= 0, it isset to a default value. If CONFIG_PREEMPT is enabled, the defaultis max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.

It’s not a disaster if you set maxactive too low; you’ll just misssome probes. In the kretprobe struct, the nmissed field is set tozero when the return probe is registered, and is incremented everytime the probed function is entered but there is no kretprobe_instanceobject available for establishing the return probe.

Kretprobe entry-handler

Kretprobes also provides an optional user-specified handler which runson function entry. This handler is specified by setting the entry_handlerfield of the kretprobe struct. Whenever the kprobe placed by kretprobe at thefunction entry is hit, the user-defined entry_handler, if any, is invoked.If the entry_handler returns 0 (success) then a corresponding return handleris guaranteed to be called upon function return. If the entry_handlerreturns a non-zero error then Kprobes leaves the return address as is, andthe kretprobe has no further effect for that particular function instance.

Multiple entry and return handler invocations are matched using the uniquekretprobe_instance object associated with them. Additionally, a usermay also specify per return-instance private data to be part of eachkretprobe_instance object. This is especially useful when sharing privatedata between corresponding user entry and return handlers. The size of eachprivate data object can be specified at kretprobe registration time bysetting the data_size field of the kretprobe struct. This data can beaccessed through the data field of each kretprobe_instance object.

In case probed function is entered but there is no kretprobe_instanceobject available, then in addition to incrementing the nmissed count,the user entry_handler invocation is also skipped.

How Does Jump Optimization Work?

If your kernel is built with CONFIG_OPTPROBES=y (currently this flagis automatically set ‘y’ on x86/x86-64, non-preemptive kernel) andthe “debug.kprobes_optimization” kernel parameter is set to 1 (seesysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jumpinstruction 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 specifiedaddress. So, even if it’s not possible to optimize this particularprobepoint, 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 jumpinstruction (the “optimized region”) lies entirely within one function.(A jump instruction is multiple bytes, and so may overlay multipleinstructions.)
  • Kprobes analyzes the entire function and verifies that there is nojump into the optimized region. Specifically:
    • the function contains no indirect jump;
    • the function contains no instruction that causes an exception (sincethe fixup code triggered by the exception could jump back into theoptimized region – Kprobes checks the exception tables to verify this);
    • there is no near jump to the optimized region (other than to the firstbyte).
  • For each instruction in the optimized region, Kprobes verifies thatthe instruction can be executed out of line.

Preparing Detour Buffer

Next, Kprobes prepares a “detour” buffer, which contains the followinginstruction 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 thefollowing 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 startoptimizing it again if the situation is changed.

If the kprobe can be optimized, Kprobes enqueues the kprobe to anoptimizing list, and kicks the kprobe-optimizer workqueue to optimizeit. If the to-be-optimized probepoint is hit before being optimized,Kprobes returns control to the original instruction path by settingthe 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 callssynchronize_rcu() for safety first, because it’spossible for a CPU to be interrupted in the middle of executing theoptimized region[3]. As you know,synchronize_rcu() can ensurethat all interruptions that were active whensynchronize_rcu()was called are done, but only if CONFIG_PREEMPT=n. So, this versionof kprobe optimization supports only kernels with CONFIG_PREEMPT=n[4].

After that, the Kprobe-optimizer calls stop_machine() to replacethe optimized region with a jump instruction to the detour buffer,using text_poke_smp().

Unoptimization

When an optimized kprobe is unregistered, disabled, or blocked byanother kprobe, it will be unoptimized. If this happens beforethe optimization is complete, the kprobe is just dequeued from theoptimized list. If the optimization has been done, the jump isreplaced with the original code (except for an int3 breakpoint inthe first byte) by using text_poke_smp().

[3]Please imagine that the 2nd instruction is interrupted and thenthe optimizer replaces the 2nd instruction with the jumpaddresswhile the interrupt handler is running. When the interruptreturns to original address, there is no valid instruction,and it causes an unexpected result.
[4]This optimization-safety checking may be replaced with thestop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=ykernel.

NOTE for geeks:The jump optimization changes the kprobe’s pre_handler behavior.Without optimization, the pre_handler can change the kernel’s executionpath by changing regs->ip and returning 1. However, when the probeis optimized, that modification is ignored. Thus, if you want totweak 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’

Blacklist

Kprobes can probe most of the kernel except itself. This meansthat there are some functions where kprobes cannot probe. Probing(trapping) such functions can cause a recursive trap (e.g. doublefault) 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 needto (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macroto specify a blacklisted function.Kprobes checks the given probe address against the blacklist andrejects registering it, if the given address is in the blacklist.

Architectures Supported

Kprobes and return probes are implemented on the followingarchitectures:

  • i386 (Supports jump optimization)
  • x86_64 (AMD-64, EM64T) (Supports jump optimization)
  • ppc64
  • ia64 (Does not support probes on instruction slot1.)
  • sparc64 (Return probes not yet implemented.)
  • arm
  • ppc
  • mips
  • s390
  • parisc

Configuring Kprobes

When configuring the kernel using make menuconfig/xconfig/oldconfig,ensure that CONFIG_KPROBES is set to “y”. Under “General setup”, lookfor “Kprobes”.

So that you can load and unload Kprobes-based instrumentation modules,make sure “Loadable module support” (CONFIG_MODULES) and “Moduleunloading” (CONFIG_MODULE_UNLOAD) are set to “y”.

Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALLare set to “y”, since kallsyms_lookup_name() is used by the in-kernelkprobe address resolution code.

If you need to insert a probe in the middle of a function, you may findit useful to “Compile the kernel with debug info” (CONFIG_DEBUG_INFO),so you can use “objdump -d -l vmlinux” to see the source-to-objectcode 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 andthe associated probe handlers that you’ll write. See the files in thesamples/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 ishit, Kprobes calls kp->pre_handler. After the probed instructionis single-stepped, Kprobe calls kp->post_handler. If a faultoccurs during execution of kp->pre_handler or kp->post_handler,or during single-stepping of the probed instruction, Kprobes callskp->fault_handler. Any or all handlers can be NULL. If kp->flagsis 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 handledtransparently)

  2. Use the “offset” field of struct kprobe if the offset into the symbolto install a probepoint is known. This field is used to calculate theprobepoint.

  3. Specify either the kprobe “symbol_name” OR the “addr”. If both arespecified, kprobe registration will fail with -EINVAL.

  4. With CISC architectures (such as i386 and x86_64), the kprobes codedoes 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 whenthe 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 seemsto be zero.

User’s fault-handler (kp->fault_handler):

#include <linux/kprobes.h>#include <linux/ptrace.h>int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);

p and regs are as described for the pre_handler. trapnr is thearchitecture-specific trap number associated with the fault (e.g.,on i386, 13 for a general protection fault or 14 for a page fault).Returns 1 if it successfully handled the exception.

register_kretprobe

#include <linux/kprobes.h>int register_kretprobe(struct kretprobe *rp);

Establishes a return probe for the function whose address isrp->kp.addr. When that function returns, Kprobes calls rp->handler.You must set rp->maxactive appropriately before you callregister_kretprobe(); see “How Does a Return Probe Work?” for details.

register_kretprobe() returns 0 on success, or a negative errnootherwise.

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 thekretprobe_instance object, of which the following fields may beof 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 toextract the return value from the appropriate register as defined bythe 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 calledat 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 anyerror occurs during registration, all probes in the array, up tothe bad probe, are safely unregistered before the register_*probesfunction 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 allof 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. unregisteredprobes) in the specified array, they clear the addr field of thoseincorrect probes. However, other probes in the array areunregistered 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 usingenable_*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 specifythe 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 optimizedprobepoint, 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 exceptionsare discussed in this section.

The register_*probe functions will return -EINVAL if you attemptto install a probe in the code that implements Kprobes (mostlykernel/kprobes.c andarch/*/kernel/kprobes.c, but also functions suchas do_page_fault and notifier_call_chain).

If you install a probe in an inline-able function, Kprobes makesno attempt to chase down all inline instances of the function andinstall 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 thecontents of the pt_regs struct (which are restored to the registersupon return from the breakpoint). So Kprobes can be used, for example,to install a bug fix or to inject faults for testing. Kprobes, ofcourse, has no way to distinguish the deliberately injected faultsfrom the accidental ones. Don’t drink and probe.

Kprobes makes no attempt to prevent probe handlers from stepping oneach other – e.g., probingprintk() and then callingprintk() from aprobe handler. If a probe handler hits a probe, that second probe’shandlers won’t be run in that instance, and the kprobe.nmissed memberof the second probe will be incremented.

As of Linux v2.6.15-rc1, multiple handlers (or multiple instances ofthe same handler) may run concurrently on different CPUs.

Kprobes does not use mutexes or allocate memory except duringregistration 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 interruptdisabled on x86/x86-64). In any case, your handler should not yieldthe CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).

Since a return probe is implemented by replacing the returnaddress with the trampoline’s address, stack backtraces and callsto __builtin_return_address() will typically yield the trampoline’saddress instead of the real return address for kretprobed functions.(As far as we can tell, __builtin_return_address() is used onlyfor instrumentation and error reporting.)

If the number of times a function is called does not match the numberof times it returns, registering a return probe on that function mayproduce undesirable results. In such a case, a line:kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48cgets printed. With this information, one will be able to correlate theexact instance of the kretprobe that caused the problem. We have thedo_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 ona stack other than that of the current task, registering a returnprobe on that function may produce undesirable results. For thisreason, Kprobes doesn’t support return probes (or kprobes)on the x86_64 version of __switch_to(); the registration functionsreturn -EINVAL.

On x86/x86-64, since the Jump Optimization of Kprobes modifiesinstructions widely, there are some limitations to optimization. Toexplain it, we introduce some terminology. Imagine a 3-instructionsequence consisting of a two 2-byte instructions and one 3-byteinstruction.

        IA        |[-2][-1][0][1][2][3][4][5][6][7]        [ins1][ins2][  ins3 ]        [<-     DCR       ->]        [<- JTPR ->]ins1: 1st Instructionins2: 2nd Instructionins3: 3rd InstructionIA:  Insertion AddressJTPR: Jump Target Prohibition RegionDCR: Detoured Code Region

The instructions in DCR are copied to the out-of-line bufferof the kprobe, because the bytes in DCR are replaced bya 5-byte jump instruction. So there are several limitations.

  1. The instructions in DCR must be relocatable.
  2. The instructions in DCR must not include a call instruction.
  3. JTPR must not be targeted by any jump or call instruction.
  4. DCR must not straddle the border between functions.

Anyway, these limitations are checked by the in-kernel instructiondecoder, 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 1.0microseconds to process. Specifically, a benchmark that hits the sameprobepoint repeatedly, firing a simple handler each time, reports 1-2million hits per second, depending on the architecture. A return-probehit typically takes 50-75% longer than a kprobe hit.When you have a return probe set on a function, adding a kprobe atthe 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 probeon same functioni386: Intel Pentium M, 1495 MHz, 2957.31 bogomipsk = 0.57 usec; r = 0.92; kr = 0.99x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomipsk = 0.49 usec; r = 0.80; kr = 0.82ppc64: 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 toprocess. 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 bogomipsk = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomipsk = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30

TODO

  1. SystemTap (http://sourceware.org/systemtap): Provides a simplifiedprogramming interface for probe-based instrumentation. Try it out.
  2. Kernel return probes for sparc64.
  3. Support for other architectures.
  4. User-space probes.
  5. 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 shouldmigrate to other tracing features or use older kernels. Please consider tomigrate 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 itis off) statically defined event interface. You can define new eventsand trace it via ftrace or any other tracing tools.

    See the following urls:

  • Use ftrace dynamic events (kprobe event) with perf-probe.

    If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you canfind which register/stack is assigned to which local variable or argumentsby 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 visibleunder 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+0x0c03dedc5  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 alsospecified. Following columns show probe status. If the probe is ona virtual address that is no longer valid (module init sections, modulevirtual 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 ismarked 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, allregistered probes will be disarmed, till such time a “1” is echoed to thisfile. Note that this knob just disarms and arms all kprobes and doesn’tchange 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 providesa knob to globally and forcibly turn jump optimization (see sectionHow Does Jump Optimization Work?) ON or OFF. By default, jump optimizationis allowed (ON). If you echo “0” to this file or set“debug.kprobes_optimization” to 0 via sysctl, all optimized probes will beunoptimized, and any new probes registered after that will not be optimized.

Note that this knobchanges the optimized state. This means that optimizedprobes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will beremoved). If the knob is turned on, they will be optimized again.