| ======================= |
| Kernel Probes (Kprobes) |
| ======================= |
| |
| :Author: Jim Keniston <jkenisto@us.ibm.com> |
| :Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com> |
| :Author: Masami Hiramatsu <mhiramat@redhat.com> |
| |
| .. 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. If CONFIG_PREEMPT is enabled, the default |
| is 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 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 |
| - 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", 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 |
| |