forked from Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
789 lines
30 KiB
789 lines
30 KiB
======================= |
|
Kernel Probes (Kprobes) |
|
======================= |
|
|
|
:Author: Jim Keniston <[email protected]> |
|
:Author: Prasanna S Panchamukhi <[email protected]> |
|
:Author: Masami Hiramatsu <[email protected]> |
|
|
|
.. 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". Under "General setup", look |
|
for "Kprobes". |
|
|
|
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://www.ibm.com/developerworks/library/l-kprobes/index.html |
|
- https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf |
|
|
|
|