790 строки
30 KiB
ReStructuredText
790 строки
30 KiB
ReStructuredText
=======================
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Kernel Probes (Kprobes)
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=======================
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:Author: Jim Keniston <jkenisto@us.ibm.com>
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:Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
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:Author: Masami Hiramatsu <mhiramat@redhat.com>
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.. CONTENTS
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1. Concepts: Kprobes, and Return Probes
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2. Architectures Supported
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3. Configuring Kprobes
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4. API Reference
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5. Kprobes Features and Limitations
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6. Probe Overhead
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7. TODO
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8. Kprobes Example
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9. Kretprobes Example
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10. Deprecated Features
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Appendix A: The kprobes debugfs interface
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Appendix B: The kprobes sysctl interface
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Appendix C: References
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Concepts: Kprobes and Return Probes
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=========================================
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Kprobes enables you to dynamically break into any kernel routine and
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collect debugging and performance information non-disruptively. You
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can trap at almost any kernel code address [1]_, specifying a handler
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routine to be invoked when the breakpoint is hit.
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.. [1] some parts of the kernel code can not be trapped, see
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:ref:`kprobes_blacklist`)
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There are currently two types of probes: kprobes, and kretprobes
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(also called return probes). A kprobe can be inserted on virtually
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any instruction in the kernel. A return probe fires when a specified
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function returns.
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In the typical case, Kprobes-based instrumentation is packaged as
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a kernel module. The module's init function installs ("registers")
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one or more probes, and the exit function unregisters them. A
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registration function such as register_kprobe() specifies where
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the probe is to be inserted and what handler is to be called when
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the probe is hit.
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There are also ``register_/unregister_*probes()`` functions for batch
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registration/unregistration of a group of ``*probes``. These functions
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can speed up unregistration process when you have to unregister
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a lot of probes at once.
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The next four subsections explain how the different types of
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probes work and how jump optimization works. They explain certain
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things that you'll need to know in order to make the best use of
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Kprobes -- e.g., the difference between a pre_handler and
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a post_handler, and how to use the maxactive and nmissed fields of
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a kretprobe. But if you're in a hurry to start using Kprobes, you
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can skip ahead to :ref:`kprobes_archs_supported`.
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How Does a Kprobe Work?
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-----------------------
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When a kprobe is registered, Kprobes makes a copy of the probed
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instruction and replaces the first byte(s) of the probed instruction
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with a breakpoint instruction (e.g., int3 on i386 and x86_64).
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When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
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registers are saved, and control passes to Kprobes via the
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notifier_call_chain mechanism. Kprobes executes the "pre_handler"
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associated with the kprobe, passing the handler the addresses of the
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kprobe struct and the saved registers.
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Next, Kprobes single-steps its copy of the probed instruction.
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(It would be simpler to single-step the actual instruction in place,
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but then Kprobes would have to temporarily remove the breakpoint
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instruction. This would open a small time window when another CPU
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could sail right past the probepoint.)
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After the instruction is single-stepped, Kprobes executes the
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"post_handler," if any, that is associated with the kprobe.
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Execution then continues with the instruction following the probepoint.
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Changing Execution Path
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-----------------------
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Since kprobes can probe into a running kernel code, it can change the
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register set, including instruction pointer. This operation requires
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maximum care, such as keeping the stack frame, recovering the execution
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path etc. Since it operates on a running kernel and needs deep knowledge
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of computer architecture and concurrent computing, you can easily shoot
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your foot.
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If you change the instruction pointer (and set up other related
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registers) in pre_handler, you must return !0 so that kprobes stops
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single stepping and just returns to the given address.
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This also means post_handler should not be called anymore.
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Note that this operation may be harder on some architectures which use
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TOC (Table of Contents) for function call, since you have to setup a new
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TOC for your function in your module, and recover the old one after
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returning from it.
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Return Probes
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-------------
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How Does a Return Probe Work?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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When you call register_kretprobe(), Kprobes establishes a kprobe at
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the entry to the function. When the probed function is called and this
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probe is hit, Kprobes saves a copy of the return address, and replaces
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the return address with the address of a "trampoline." The trampoline
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is an arbitrary piece of code -- typically just a nop instruction.
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At boot time, Kprobes registers a kprobe at the trampoline.
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When the probed function executes its return instruction, control
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passes to the trampoline and that probe is hit. Kprobes' trampoline
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handler calls the user-specified return handler associated with the
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kretprobe, then sets the saved instruction pointer to the saved return
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address, and that's where execution resumes upon return from the trap.
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While the probed function is executing, its return address is
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stored in an object of type kretprobe_instance. Before calling
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register_kretprobe(), the user sets the maxactive field of the
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kretprobe struct to specify how many instances of the specified
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function can be probed simultaneously. register_kretprobe()
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pre-allocates the indicated number of kretprobe_instance objects.
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For example, if the function is non-recursive and is called with a
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spinlock held, maxactive = 1 should be enough. If the function is
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non-recursive and can never relinquish the CPU (e.g., via a semaphore
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or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
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set to a default value. If CONFIG_PREEMPT is enabled, the default
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is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
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It's not a disaster if you set maxactive too low; you'll just miss
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some probes. In the kretprobe struct, the nmissed field is set to
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zero when the return probe is registered, and is incremented every
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time the probed function is entered but there is no kretprobe_instance
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object available for establishing the return probe.
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Kretprobe entry-handler
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^^^^^^^^^^^^^^^^^^^^^^^
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Kretprobes also provides an optional user-specified handler which runs
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on function entry. This handler is specified by setting the entry_handler
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field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
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function entry is hit, the user-defined entry_handler, if any, is invoked.
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If the entry_handler returns 0 (success) then a corresponding return handler
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is guaranteed to be called upon function return. If the entry_handler
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returns a non-zero error then Kprobes leaves the return address as is, and
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the kretprobe has no further effect for that particular function instance.
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Multiple entry and return handler invocations are matched using the unique
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kretprobe_instance object associated with them. Additionally, a user
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may also specify per return-instance private data to be part of each
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kretprobe_instance object. This is especially useful when sharing private
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data between corresponding user entry and return handlers. The size of each
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private data object can be specified at kretprobe registration time by
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setting the data_size field of the kretprobe struct. This data can be
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accessed through the data field of each kretprobe_instance object.
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In case probed function is entered but there is no kretprobe_instance
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object available, then in addition to incrementing the nmissed count,
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the user entry_handler invocation is also skipped.
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.. _kprobes_jump_optimization:
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How Does Jump Optimization Work?
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--------------------------------
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If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
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is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
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the "debug.kprobes_optimization" kernel parameter is set to 1 (see
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sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
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instruction instead of a breakpoint instruction at each probepoint.
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Init a Kprobe
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^^^^^^^^^^^^^
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When a probe is registered, before attempting this optimization,
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Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
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address. So, even if it's not possible to optimize this particular
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probepoint, there'll be a probe there.
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Safety Check
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^^^^^^^^^^^^
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Before optimizing a probe, Kprobes performs the following safety checks:
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- Kprobes verifies that the region that will be replaced by the jump
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instruction (the "optimized region") lies entirely within one function.
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(A jump instruction is multiple bytes, and so may overlay multiple
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instructions.)
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- Kprobes analyzes the entire function and verifies that there is no
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jump into the optimized region. Specifically:
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- the function contains no indirect jump;
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- the function contains no instruction that causes an exception (since
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the fixup code triggered by the exception could jump back into the
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optimized region -- Kprobes checks the exception tables to verify this);
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- there is no near jump to the optimized region (other than to the first
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byte).
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- For each instruction in the optimized region, Kprobes verifies that
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the instruction can be executed out of line.
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Preparing Detour Buffer
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^^^^^^^^^^^^^^^^^^^^^^^
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Next, Kprobes prepares a "detour" buffer, which contains the following
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instruction sequence:
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- code to push the CPU's registers (emulating a breakpoint trap)
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- a call to the trampoline code which calls user's probe handlers.
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- code to restore registers
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- the instructions from the optimized region
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- a jump back to the original execution path.
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Pre-optimization
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^^^^^^^^^^^^^^^^
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After preparing the detour buffer, Kprobes verifies that none of the
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following situations exist:
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- The probe has a post_handler.
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- Other instructions in the optimized region are probed.
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- The probe is disabled.
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In any of the above cases, Kprobes won't start optimizing the probe.
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Since these are temporary situations, Kprobes tries to start
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optimizing it again if the situation is changed.
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If the kprobe can be optimized, Kprobes enqueues the kprobe to an
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optimizing list, and kicks the kprobe-optimizer workqueue to optimize
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it. If the to-be-optimized probepoint is hit before being optimized,
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Kprobes returns control to the original instruction path by setting
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the CPU's instruction pointer to the copied code in the detour buffer
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-- thus at least avoiding the single-step.
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Optimization
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^^^^^^^^^^^^
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The Kprobe-optimizer doesn't insert the jump instruction immediately;
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rather, it calls synchronize_rcu() for safety first, because it's
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possible for a CPU to be interrupted in the middle of executing the
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optimized region [3]_. As you know, synchronize_rcu() can ensure
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that all interruptions that were active when synchronize_rcu()
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was called are done, but only if CONFIG_PREEMPT=n. So, this version
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of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
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After that, the Kprobe-optimizer calls stop_machine() to replace
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the optimized region with a jump instruction to the detour buffer,
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using text_poke_smp().
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Unoptimization
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^^^^^^^^^^^^^^
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When an optimized kprobe is unregistered, disabled, or blocked by
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another kprobe, it will be unoptimized. If this happens before
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the optimization is complete, the kprobe is just dequeued from the
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optimized list. If the optimization has been done, the jump is
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replaced with the original code (except for an int3 breakpoint in
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the first byte) by using text_poke_smp().
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.. [3] Please imagine that the 2nd instruction is interrupted and then
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the optimizer replaces the 2nd instruction with the jump *address*
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while the interrupt handler is running. When the interrupt
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returns to original address, there is no valid instruction,
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and it causes an unexpected result.
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.. [4] This optimization-safety checking may be replaced with the
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stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
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kernel.
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NOTE for geeks:
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The jump optimization changes the kprobe's pre_handler behavior.
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Without optimization, the pre_handler can change the kernel's execution
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path by changing regs->ip and returning 1. However, when the probe
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is optimized, that modification is ignored. Thus, if you want to
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tweak the kernel's execution path, you need to suppress optimization,
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using one of the following techniques:
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- Specify an empty function for the kprobe's post_handler.
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or
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- Execute 'sysctl -w debug.kprobes_optimization=n'
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.. _kprobes_blacklist:
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Blacklist
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---------
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Kprobes can probe most of the kernel except itself. This means
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that there are some functions where kprobes cannot probe. Probing
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(trapping) such functions can cause a recursive trap (e.g. double
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fault) or the nested probe handler may never be called.
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Kprobes manages such functions as a blacklist.
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If you want to add a function into the blacklist, you just need
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to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
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to specify a blacklisted function.
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Kprobes checks the given probe address against the blacklist and
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rejects registering it, if the given address is in the blacklist.
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.. _kprobes_archs_supported:
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Architectures Supported
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=======================
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Kprobes and return probes are implemented on the following
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architectures:
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- i386 (Supports jump optimization)
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- x86_64 (AMD-64, EM64T) (Supports jump optimization)
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- ppc64
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- ia64 (Does not support probes on instruction slot1.)
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- sparc64 (Return probes not yet implemented.)
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- arm
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- ppc
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- mips
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- s390
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- parisc
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Configuring Kprobes
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===================
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When configuring the kernel using make menuconfig/xconfig/oldconfig,
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ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
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for "Kprobes".
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So that you can load and unload Kprobes-based instrumentation modules,
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make sure "Loadable module support" (CONFIG_MODULES) and "Module
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unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
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Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
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are set to "y", since kallsyms_lookup_name() is used by the in-kernel
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kprobe address resolution code.
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If you need to insert a probe in the middle of a function, you may find
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it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
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so you can use "objdump -d -l vmlinux" to see the source-to-object
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code mapping.
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API Reference
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=============
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The Kprobes API includes a "register" function and an "unregister"
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function for each type of probe. The API also includes "register_*probes"
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and "unregister_*probes" functions for (un)registering arrays of probes.
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Here are terse, mini-man-page specifications for these functions and
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the associated probe handlers that you'll write. See the files in the
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samples/kprobes/ sub-directory for examples.
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register_kprobe
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---------------
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::
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#include <linux/kprobes.h>
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int register_kprobe(struct kprobe *kp);
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Sets a breakpoint at the address kp->addr. When the breakpoint is hit, Kprobes
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calls kp->pre_handler. After the probed instruction is single-stepped, Kprobe
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calls kp->post_handler. Any or all handlers can be NULL. If kp->flags is set
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KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers
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aren't hit until calling enable_kprobe(kp).
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.. note::
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1. With the introduction of the "symbol_name" field to struct kprobe,
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the probepoint address resolution will now be taken care of by the kernel.
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The following will now work::
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kp.symbol_name = "symbol_name";
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(64-bit powerpc intricacies such as function descriptors are handled
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transparently)
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2. Use the "offset" field of struct kprobe if the offset into the symbol
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to install a probepoint is known. This field is used to calculate the
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probepoint.
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3. Specify either the kprobe "symbol_name" OR the "addr". If both are
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specified, kprobe registration will fail with -EINVAL.
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4. With CISC architectures (such as i386 and x86_64), the kprobes code
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does not validate if the kprobe.addr is at an instruction boundary.
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Use "offset" with caution.
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register_kprobe() returns 0 on success, or a negative errno otherwise.
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User's pre-handler (kp->pre_handler)::
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int pre_handler(struct kprobe *p, struct pt_regs *regs);
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Called with p pointing to the kprobe associated with the breakpoint,
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and regs pointing to the struct containing the registers saved when
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the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
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User's post-handler (kp->post_handler)::
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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void post_handler(struct kprobe *p, struct pt_regs *regs,
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unsigned long flags);
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p and regs are as described for the pre_handler. flags always seems
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to be zero.
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register_kretprobe
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------------------
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::
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#include <linux/kprobes.h>
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int register_kretprobe(struct kretprobe *rp);
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Establishes a return probe for the function whose address is
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rp->kp.addr. When that function returns, Kprobes calls rp->handler.
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You must set rp->maxactive appropriately before you call
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register_kretprobe(); see "How Does a Return Probe Work?" for details.
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register_kretprobe() returns 0 on success, or a negative errno
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otherwise.
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User's return-probe handler (rp->handler)::
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int kretprobe_handler(struct kretprobe_instance *ri,
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struct pt_regs *regs);
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regs is as described for kprobe.pre_handler. ri points to the
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kretprobe_instance object, of which the following fields may be
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of interest:
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- ret_addr: the return address
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- rp: points to the corresponding kretprobe object
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- task: points to the corresponding task struct
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- data: points to per return-instance private data; see "Kretprobe
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entry-handler" for details.
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The regs_return_value(regs) macro provides a simple abstraction to
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extract the return value from the appropriate register as defined by
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the architecture's ABI.
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The handler's return value is currently ignored.
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unregister_*probe
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------------------
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::
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#include <linux/kprobes.h>
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void unregister_kprobe(struct kprobe *kp);
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void unregister_kretprobe(struct kretprobe *rp);
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Removes the specified probe. The unregister function can be called
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at any time after the probe has been registered.
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.. note::
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If the functions find an incorrect probe (ex. an unregistered probe),
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they clear the addr field of the probe.
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register_*probes
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----------------
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::
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#include <linux/kprobes.h>
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int register_kprobes(struct kprobe **kps, int num);
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int register_kretprobes(struct kretprobe **rps, int num);
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Registers each of the num probes in the specified array. If any
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error occurs during registration, all probes in the array, up to
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the bad probe, are safely unregistered before the register_*probes
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function returns.
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- kps/rps: an array of pointers to ``*probe`` data structures
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- num: the number of the array entries.
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.. note::
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You have to allocate(or define) an array of pointers and set all
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of the array entries before using these functions.
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unregister_*probes
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------------------
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::
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#include <linux/kprobes.h>
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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
|
|
|