kprobe.txt

	Title	: Kernel Probes (Kprobes)
	Authors	: Jim Keniston 
		: Prasanna S Panchamukhi 
	
	CONTENTS
	
	1. Concepts: Kprobes, Jprobes, 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. Jprobes Example
	10. Kretprobes Example
	Appendix A: The kprobes debugfs interface
	
	1. Concepts: Kprobes, Jprobes, 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, specifying a handler
	routine to be invoked when the breakpoint is hit.
	
	There are currently three types of probes: kprobes, jprobes, and
	kretprobes (also called return probes).  A kprobe can be inserted
	on virtually any instruction in the kernel.  A jprobe is inserted at
	the entry to a kernel function, and provides convenient access to the
	function's arguments.  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 three subsections explain how the different types of
	probes work.  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 section 2.
	
	1.1 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.
	
	1.2 How Does a Jprobe Work?
	
	A jprobe is implemented using a kprobe that is placed on a function's
	entry point.  It employs a simple mirroring principle to allow
	seamless access to the probed function's arguments.  The jprobe
	handler routine should have the same signature (arg list and return
	type) as the function being probed, and must always end by calling
	the Kprobes function jprobe_return().
	
	Here's how it works.  When the probe is hit, Kprobes makes a copy of
	the saved registers and a generous portion of the stack (see below).
	Kprobes then points the saved instruction pointer at the jprobe's
	handler routine, and returns from the trap.  As a result, control
	passes to the handler, which is presented with the same register and
	stack contents as the probed function.  When it is done, the handler
	calls jprobe_return(), which traps again to restore the original stack
	contents and processor state and switch to the probed function.
	
	By convention, the callee owns its arguments, so gcc may produce code
	that unexpectedly modifies that portion of the stack.  This is why
	Kprobes saves a copy of the stack and restores it after the jprobe
	handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
	64 bytes on i386.
	
	Note that the probed function's args may be passed on the stack
	or in registers.  The jprobe will work in either case, so long as the
	handler's prototype matches that of the probed function.
	
	1.3 Return Probes
	
	1.3.1 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.
	
	1.3.2 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.
	
	2. Architectures Supported
	
	Kprobes, jprobes, and return probes are implemented on the following
	architectures:
	
	- i386
	- x86_64 (AMD-64, EM64T)
	- ppc64
	- ia64 (Does not support probes on instruction slot1.)
	- sparc64 (Return probes not yet implemented.)
	- arm
	- ppc
	
	3. Configuring Kprobes
	
	When configuring the kernel using make menuconfig/xconfig/oldconfig,
	ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation
	Support", 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.
	
	4. 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.
	
	4.1 register_kprobe
	
	#include 
	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.  If a fault
	occurs during execution of kp->pre_handler or kp->post_handler,
	or during single-stepping of the probed instruction, Kprobes calls
	kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
	is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
	so, it's 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 
	#include 
	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 
	#include 
	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.
	
	User's fault-handler (kp->fault_handler):
	#include 
	#include 
	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
	
	p and regs are as described for the pre_handler.  trapnr is the
	architecture-specific trap number associated with the fault (e.g.,
	on i386, 13 for a general protection fault or 14 for a page fault).
	Returns 1 if it successfully handled the exception.
	
	4.2 register_jprobe
	
	#include 
	int register_jprobe(struct jprobe *jp)
	
	Sets a breakpoint at the address jp->kp.addr, which must be the address
	of the first instruction of a function.  When the breakpoint is hit,
	Kprobes runs the handler whose address is jp->entry.
	
	The handler should have the same arg list and return type as the probed
	function; and just before it returns, it must call jprobe_return().
	(The handler never actually returns, since jprobe_return() returns
	control to Kprobes.)  If the probed function is declared asmlinkage
	or anything else that affects how args are passed, the handler's
	declaration must match.
	
	register_jprobe() returns 0 on success, or a negative errno otherwise.
	
	4.3 register_kretprobe
	
	#include 
	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 
	#include 
	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.
	
	4.4 unregister_*probe
	
	#include 
	void unregister_kprobe(struct kprobe *kp);
	void unregister_jprobe(struct jprobe *jp);
	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.
	
	4.5 register_*probes
	
	#include 
	int register_kprobes(struct kprobe **kps, int num);
	int register_kretprobes(struct kretprobe **rps, int num);
	int register_jprobes(struct jprobe **jps, 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/jps: 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.
	
	4.6 unregister_*probes
	
	#include 
	void unregister_kprobes(struct kprobe **kps, int num);
	void unregister_kretprobes(struct kretprobe **rps, int num);
	void unregister_jprobes(struct jprobe **jps, 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.
	
	4.7 disable_*probe
	
	#include 
	int disable_kprobe(struct kprobe *kp);
	int disable_kretprobe(struct kretprobe *rp);
	int disable_jprobe(struct jprobe *jp);
	
	Temporarily disables the specified *probe. You can enable it again by using
	enable_*probe(). You must specify the probe which has been registered.
	
	4.8 enable_*probe
	
	#include 
	int enable_kprobe(struct kprobe *kp);
	int enable_kretprobe(struct kretprobe *rp);
	int enable_jprobe(struct jprobe *jp);
	
	Enables *probe which has been disabled by disable_*probe(). You must specify
	the probe which has been registered.
	
	5. Kprobes Features and Limitations
	
	Kprobes allows multiple probes at the same address.  Currently,
	however, there cannot be multiple jprobes on the same function at
	the same time.
	
	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.  Depending on the
	architecture, handlers may also run with interrupts disabled.  In any
	case, your handler should not yield the CPU (e.g., by attempting to
	acquire a semaphore).
	
	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 or jprobes)
	on the x86_64 version of __switch_to(); the registration functions
	return -EINVAL.
	
	6. 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 jprobe or
	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; j = jprobe; r = return probe; kr = kprobe + return probe
	on same function; jr = jprobe + return probe on same function
	
	i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
	k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
	
	x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
	k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
	
	ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
	k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
	
	7. TODO
	
	a. 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).
	
	8. Kprobes Example
	
	See samples/kprobes/kprobe_example.c
	
	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
	
	
	 (pages 101-115)
	
	
	Appendix A: 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
	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. 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].
	
	/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.