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Documentation/kprobes.txt

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+Title	: Kernel Probes (Kprobes)
+Authors	: Jim Keniston <jkenisto@us.ibm.com>
+	: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
+	: Masami Hiramatsu <mhiramat@redhat.com>
+
+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
+Appendix B: The kprobes sysctl 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.
+(*: some parts of the kernel code can not be trapped, see 1.5 Blacklist)
+
+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 four subsections explain how the different types of
+probes work and how jump optimization works.  They explain certain
+things that you'll need to know in order to make the best use of
+Kprobes -- e.g., the difference between a pre_handler and
+a post_handler, and how to use the maxactive and nmissed fields of
+a kretprobe.  But if you're in a hurry to start using Kprobes, you
+can skip ahead to 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.
+
+1.4 How Does Jump Optimization Work?
+
+If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
+is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
+the "debug.kprobes_optimization" kernel parameter is set to 1 (see
+sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
+instruction instead of a breakpoint instruction at each probepoint.
+
+1.4.1 Init a Kprobe
+
+When a probe is registered, before attempting this optimization,
+Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
+address. So, even if it's not possible to optimize this particular
+probepoint, there'll be a probe there.
+
+1.4.2 Safety Check
+
+Before optimizing a probe, Kprobes performs the following safety checks:
+
+- Kprobes verifies that the region that will be replaced by the jump
+instruction (the "optimized region") lies entirely within one function.
+(A jump instruction is multiple bytes, and so may overlay multiple
+instructions.)
+
+- Kprobes analyzes the entire function and verifies that there is no
+jump into the optimized region.  Specifically:
+  - the function contains no indirect jump;
+  - the function contains no instruction that causes an exception (since
+  the fixup code triggered by the exception could jump back into the
+  optimized region -- Kprobes checks the exception tables to verify this);
+  and
+  - there is no near jump to the optimized region (other than to the first
+  byte).
+
+- For each instruction in the optimized region, Kprobes verifies that
+the instruction can be executed out of line.
+
+1.4.3 Preparing Detour Buffer
+
+Next, Kprobes prepares a "detour" buffer, which contains the following
+instruction sequence:
+- code to push the CPU's registers (emulating a breakpoint trap)
+- a call to the trampoline code which calls user's probe handlers.
+- code to restore registers
+- the instructions from the optimized region
+- a jump back to the original execution path.
+
+1.4.4 Pre-optimization
+
+After preparing the detour buffer, Kprobes verifies that none of the
+following situations exist:
+- The probe has either a break_handler (i.e., it's a jprobe) or a
+post_handler.
+- Other instructions in the optimized region are probed.
+- The probe is disabled.
+In any of the above cases, Kprobes won't start optimizing the probe.
+Since these are temporary situations, Kprobes tries to start
+optimizing it again if the situation is changed.
+
+If the kprobe can be optimized, Kprobes enqueues the kprobe to an
+optimizing list, and kicks the kprobe-optimizer workqueue to optimize
+it.  If the to-be-optimized probepoint is hit before being optimized,
+Kprobes returns control to the original instruction path by setting
+the CPU's instruction pointer to the copied code in the detour buffer
+-- thus at least avoiding the single-step.
+
+1.4.5 Optimization
+
+The Kprobe-optimizer doesn't insert the jump instruction immediately;
+rather, it calls synchronize_sched() for safety first, because it's
+possible for a CPU to be interrupted in the middle of executing the
+optimized region(*).  As you know, synchronize_sched() can ensure
+that all interruptions that were active when synchronize_sched()
+was called are done, but only if CONFIG_PREEMPT=n.  So, this version
+of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
+
+After that, the Kprobe-optimizer calls stop_machine() to replace
+the optimized region with a jump instruction to the detour buffer,
+using text_poke_smp().
+
+1.4.6 Unoptimization
+
+When an optimized kprobe is unregistered, disabled, or blocked by
+another kprobe, it will be unoptimized.  If this happens before
+the optimization is complete, the kprobe is just dequeued from the
+optimized list.  If the optimization has been done, the jump is
+replaced with the original code (except for an int3 breakpoint in
+the first byte) by using text_poke_smp().
+
+(*)Please imagine that the 2nd instruction is interrupted and then
+the optimizer replaces the 2nd instruction with the jump *address*
+while the interrupt handler is running. When the interrupt
+returns to original address, there is no valid instruction,
+and it causes an unexpected result.
+
+(**)This optimization-safety checking may be replaced with the
+stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
+kernel.
+
+NOTE for geeks:
+The jump optimization changes the kprobe's pre_handler behavior.
+Without optimization, the pre_handler can change the kernel's execution
+path by changing regs->ip and returning 1.  However, when the probe
+is optimized, that modification is ignored.  Thus, if you want to
+tweak the kernel's execution path, you need to suppress optimization,
+using one of the following techniques:
+- Specify an empty function for the kprobe's post_handler or break_handler.
+ or
+- Execute 'sysctl -w debug.kprobes_optimization=n'
+
+1.5 Blacklist
+
+Kprobes can probe most of the kernel except itself. This means
+that there are some functions where kprobes cannot probe. Probing
+(trapping) such functions can cause a recursive trap (e.g. double
+fault) or the nested probe handler may never be called.
+Kprobes manages such functions as a blacklist.
+If you want to add a function into the blacklist, you just need
+to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
+to specify a blacklisted function.
+Kprobes checks the given probe address against the blacklist and
+rejects registering it, if the given address is in the blacklist.
+
+2. Architectures Supported
+
+Kprobes, jprobes, and return probes are implemented on the following
+architectures:
+
+- i386 (Supports jump optimization)
+- x86_64 (AMD-64, EM64T) (Supports jump optimization)
+- ppc64
+- ia64 (Does not support probes on instruction slot1.)
+- sparc64 (Return probes not yet implemented.)
+- arm
+- ppc
+- mips
+- s390
+
+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 <linux/kprobes.h>
+int register_kprobe(struct kprobe *kp);
+
+Sets a breakpoint at the address kp->addr.  When the breakpoint is
+hit, Kprobes calls kp->pre_handler.  After the probed instruction
+is single-stepped, Kprobe calls kp->post_handler.  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, its handlers aren't hit until calling enable_kprobe(kp).
+
+NOTE:
+1. With the introduction of the "symbol_name" field to struct kprobe,
+the probepoint address resolution will now be taken care of by the kernel.
+The following will now work:
+
+	kp.symbol_name = "symbol_name";
+
+(64-bit powerpc intricacies such as function descriptors are handled
+transparently)
+
+2. Use the "offset" field of struct kprobe if the offset into the symbol
+to install a probepoint is known. This field is used to calculate the
+probepoint.
+
+3. Specify either the kprobe "symbol_name" OR the "addr". If both are
+specified, kprobe registration will fail with -EINVAL.
+
+4. With CISC architectures (such as i386 and x86_64), the kprobes code
+does not validate if the kprobe.addr is at an instruction boundary.
+Use "offset" with caution.
+
+register_kprobe() returns 0 on success, or a negative errno otherwise.
+
+User's pre-handler (kp->pre_handler):
+#include <linux/kprobes.h>
+#include <linux/ptrace.h>
+int pre_handler(struct kprobe *p, struct pt_regs *regs);
+
+Called with p pointing to the kprobe associated with the breakpoint,
+and regs pointing to the struct containing the registers saved when
+the breakpoint was hit.  Return 0 here unless you're a Kprobes geek.
+
+User's post-handler (kp->post_handler):
+#include <linux/kprobes.h>
+#include <linux/ptrace.h>
+void post_handler(struct kprobe *p, struct pt_regs *regs,
+	unsigned long flags);
+
+p and regs are as described for the pre_handler.  flags always seems
+to be zero.
+
+User's fault-handler (kp->fault_handler):
+#include <linux/kprobes.h>
+#include <linux/ptrace.h>
+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 <linux/kprobes.h>
+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 <linux/kprobes.h>
+int register_kretprobe(struct kretprobe *rp);
+
+Establishes a return probe for the function whose address is
+rp->kp.addr.  When that function returns, Kprobes calls rp->handler.
+You must set rp->maxactive appropriately before you call
+register_kretprobe(); see "How Does a Return Probe Work?" for details.
+
+register_kretprobe() returns 0 on success, or a negative errno
+otherwise.
+
+User's return-probe handler (rp->handler):
+#include <linux/kprobes.h>
+#include <linux/ptrace.h>
+int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
+
+regs is as described for kprobe.pre_handler.  ri points to the
+kretprobe_instance object, of which the following fields may be
+of interest:
+- ret_addr: the return address
+- rp: points to the corresponding kretprobe object
+- task: points to the corresponding task struct
+- data: points to per return-instance private data; see "Kretprobe
+	entry-handler" for details.
+
+The regs_return_value(regs) macro provides a simple abstraction to
+extract the return value from the appropriate register as defined by
+the architecture's ABI.
+
+The handler's return value is currently ignored.
+
+4.4 unregister_*probe
+
+#include <linux/kprobes.h>
+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 <linux/kprobes.h>
+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 <linux/kprobes.h>
+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 <linux/kprobes.h>
+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 <linux/kprobes.h>
+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.  Also, a probepoint for which there is a jprobe or
+a post_handler cannot be optimized.  So if you install a jprobe,
+or 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.  Depending on the
+architecture and optimization state, handlers may also run with
+interrupts disabled (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).
+
+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.
+
+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.
+
+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
+
+6.1 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
+
+7. 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).
+
+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
+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 /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]. If the probe is optimized, it is
+marked with [OPTIMIZED].
+
+/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.
+
+
+Appendix B: 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
+1.4) 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.
+