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Add the description of batch registration interfaces to Documentation/kprobes.txt. Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com> Cc: Jim Keniston <jkenisto@us.ibm.com> Cc: Prasanna S Panchamukhi <prasanna@in.ibm.com> Cc: Shaohua Li <shaohua.li@intel.com> Cc: David Miller <davem@davemloft.net> Cc: "Frank Ch. Eigler" <fche@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
505 lines
21 KiB
Text
505 lines
21 KiB
Text
Title : Kernel Probes (Kprobes)
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Authors : Jim Keniston <jkenisto@us.ibm.com>
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: Prasanna S Panchamukhi <prasanna@in.ibm.com>
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CONTENTS
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1. Concepts: Kprobes, Jprobes, 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. Jprobes Example
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10. Kretprobes Example
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Appendix A: The kprobes debugfs interface
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1. Concepts: Kprobes, Jprobes, Return Probes
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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, specifying a handler
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routine to be invoked when the breakpoint is hit.
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There are currently three types of probes: kprobes, jprobes, and
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kretprobes (also called return probes). A kprobe can be inserted
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on virtually any instruction in the kernel. A jprobe is inserted at
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the entry to a kernel function, and provides convenient access to the
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function's arguments. A return probe fires when a specified function
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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 three subsections explain how the different types of
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probes work. They explain certain things that you'll need to
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know in order to make the best use of Kprobes -- e.g., the
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difference between a pre_handler and a post_handler, and how
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to use the maxactive and nmissed fields of a kretprobe. But
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if you're in a hurry to start using Kprobes, you can skip ahead
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to section 2.
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1.1 How Does a Kprobe Work?
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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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1.2 How Does a Jprobe Work?
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A jprobe is implemented using a kprobe that is placed on a function's
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entry point. It employs a simple mirroring principle to allow
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seamless access to the probed function's arguments. The jprobe
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handler routine should have the same signature (arg list and return
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type) as the function being probed, and must always end by calling
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the Kprobes function jprobe_return().
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Here's how it works. When the probe is hit, Kprobes makes a copy of
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the saved registers and a generous portion of the stack (see below).
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Kprobes then points the saved instruction pointer at the jprobe's
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handler routine, and returns from the trap. As a result, control
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passes to the handler, which is presented with the same register and
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stack contents as the probed function. When it is done, the handler
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calls jprobe_return(), which traps again to restore the original stack
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contents and processor state and switch to the probed function.
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By convention, the callee owns its arguments, so gcc may produce code
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that unexpectedly modifies that portion of the stack. This is why
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Kprobes saves a copy of the stack and restores it after the jprobe
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handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
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64 bytes on i386.
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Note that the probed function's args may be passed on the stack
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or in registers. The jprobe will work in either case, so long as the
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handler's prototype matches that of the probed function.
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1.3 Return Probes
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1.3.1 How Does a Return Probe Work?
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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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1.3.2 Kretprobe entry-handler
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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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2. Architectures Supported
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Kprobes, jprobes, and return probes are implemented on the following
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architectures:
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- i386
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- x86_64 (AMD-64, EM64T)
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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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3. Configuring Kprobes
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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 "Instrumentation
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Support", look 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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4. API Reference
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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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4.1 register_kprobe
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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
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hit, Kprobes calls kp->pre_handler. After the probed instruction
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is single-stepped, Kprobe calls kp->post_handler. If a fault
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occurs during execution of kp->pre_handler or kp->post_handler,
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or during single-stepping of the probed instruction, Kprobes calls
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kp->fault_handler. Any or all handlers can be NULL.
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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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User's fault-handler (kp->fault_handler):
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
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p and regs are as described for the pre_handler. trapnr is the
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architecture-specific trap number associated with the fault (e.g.,
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on i386, 13 for a general protection fault or 14 for a page fault).
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Returns 1 if it successfully handled the exception.
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4.2 register_jprobe
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#include <linux/kprobes.h>
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int register_jprobe(struct jprobe *jp)
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Sets a breakpoint at the address jp->kp.addr, which must be the address
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of the first instruction of a function. When the breakpoint is hit,
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Kprobes runs the handler whose address is jp->entry.
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The handler should have the same arg list and return type as the probed
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function; and just before it returns, it must call jprobe_return().
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(The handler never actually returns, since jprobe_return() returns
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control to Kprobes.) If the probed function is declared asmlinkage
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or anything else that affects how args are passed, the handler's
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declaration must match.
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register_jprobe() returns 0 on success, or a negative errno otherwise.
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4.3 register_kretprobe
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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, 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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4.4 unregister_*probe
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#include <linux/kprobes.h>
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void unregister_kprobe(struct kprobe *kp);
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void unregister_jprobe(struct jprobe *jp);
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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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4.5 register_*probes
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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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int register_jprobes(struct jprobe **jps, 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/jps: 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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4.6 unregister_*probes
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#include <linux/kprobes.h>
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void unregister_kprobes(struct kprobe **kps, int num);
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void unregister_kretprobes(struct kretprobe **rps, int num);
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void unregister_jprobes(struct jprobe **jps, int num);
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Removes each of the num probes in the specified array at once.
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NOTE:
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If the functions find some incorrect probes (ex. unregistered
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probes) in the specified array, they clear the addr field of those
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incorrect probes. However, other probes in the array are
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unregistered correctly.
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5. Kprobes Features and Limitations
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Kprobes allows multiple probes at the same address. Currently,
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however, there cannot be multiple jprobes on the same function at
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the same time.
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In general, you can install a probe anywhere in the kernel.
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In particular, you can probe interrupt handlers. Known exceptions
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are discussed in this section.
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The register_*probe functions will return -EINVAL if you attempt
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to install a probe in the code that implements Kprobes (mostly
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kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
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as do_page_fault and notifier_call_chain).
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If you install a probe in an inline-able function, Kprobes makes
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no attempt to chase down all inline instances of the function and
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install probes there. gcc may inline a function without being asked,
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so keep this in mind if you're not seeing the probe hits you expect.
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A probe handler can modify the environment of the probed function
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-- e.g., by modifying kernel data structures, or by modifying the
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contents of the pt_regs struct (which are restored to the registers
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upon return from the breakpoint). So Kprobes can be used, for example,
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to install a bug fix or to inject faults for testing. Kprobes, of
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course, has no way to distinguish the deliberately injected faults
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from the accidental ones. Don't drink and probe.
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Kprobes makes no attempt to prevent probe handlers from stepping on
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each other -- e.g., probing printk() and then calling printk() from a
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probe handler. If a probe handler hits a probe, that second probe's
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handlers won't be run in that instance, and the kprobe.nmissed member
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of the second probe will be incremented.
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As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
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the same handler) may run concurrently on different CPUs.
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Kprobes does not use mutexes or allocate memory except during
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registration and unregistration.
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Probe handlers are run with preemption disabled. Depending on the
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architecture, handlers may also run with interrupts disabled. In any
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case, your handler should not yield the CPU (e.g., by attempting to
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acquire a semaphore).
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Since a return probe is implemented by replacing the return
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address with the trampoline's address, stack backtraces and calls
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to __builtin_return_address() will typically yield the trampoline's
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address instead of the real return address for kretprobed functions.
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(As far as we can tell, __builtin_return_address() is used only
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for instrumentation and error reporting.)
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If the number of times a function is called does not match the number
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of times it returns, registering a return probe on that function may
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produce undesirable results. In such a case, a line:
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kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
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gets printed. With this information, one will be able to correlate the
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exact instance of the kretprobe that caused the problem. We have the
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do_exit() case covered. do_execve() and do_fork() are not an issue.
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We're unaware of other specific cases where this could be a problem.
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If, upon entry to or exit from a function, the CPU is running on
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a stack other than that of the current task, registering a return
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probe on that function may produce undesirable results. For this
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reason, Kprobes doesn't support return probes (or kprobes or jprobes)
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on the x86_64 version of __switch_to(); the registration functions
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return -EINVAL.
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6. Probe Overhead
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On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
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microseconds to process. Specifically, a benchmark that hits the same
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probepoint repeatedly, firing a simple handler each time, reports 1-2
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million hits per second, depending on the architecture. A jprobe or
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return-probe hit typically takes 50-75% longer than a kprobe hit.
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When you have a return probe set on a function, adding a kprobe at
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the entry to that function adds essentially no overhead.
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Here are sample overhead figures (in usec) for different architectures.
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k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
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on same function; jr = jprobe + return probe on same function
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i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
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k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
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x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
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k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
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ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
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k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
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7. TODO
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|
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a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
|
|
programming interface for probe-based instrumentation. Try it out.
|
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b. Kernel return probes for sparc64.
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|
c. Support for other architectures.
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d. User-space probes.
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e. Watchpoint probes (which fire on data references).
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|
|
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8. Kprobes Example
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|
|
|
See samples/kprobes/kprobe_example.c
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|
|
|
9. Jprobes Example
|
|
|
|
See samples/kprobes/jprobe_example.c
|
|
|
|
10. Kretprobes Example
|
|
|
|
See samples/kprobes/kretprobe_example.c
|
|
|
|
For additional information on Kprobes, refer to the following URLs:
|
|
http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
|
|
http://www.redhat.com/magazine/005mar05/features/kprobes/
|
|
http://www-users.cs.umn.edu/~boutcher/kprobes/
|
|
http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
|
|
|
|
|
|
Appendix A: The kprobes debugfs interface
|
|
|
|
With recent kernels (> 2.6.20) the list of registered kprobes is visible
|
|
under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
|
|
|
|
/debug/kprobes/list: Lists all registered probes on the system
|
|
|
|
c015d71a k vfs_read+0x0
|
|
c011a316 j do_fork+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, r - kretprobe
|
|
and j - jprobe), 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.
|
|
|
|
/debug/kprobes/enabled: Turn kprobes ON/OFF
|
|
|
|
Provides a knob to globally 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.
|