[PATCH] Add Documentation/kprobes.txt

Acked-by: Prasanna S Panchamukhi <prasanna@in.ibm.com>
Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/kprobes.txt

+Title	: Kernel Probes (Kprobes)
+Authors	: Jim Keniston <jkenisto@us.ibm.com>
+	: Prasanna S Panchamukhi <prasanna@in.ibm.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
+
+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.
+
+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 (e.g., for x86_64 or for an i386 fastcall function).
+The jprobe will work in either case, so long as the handler's
+prototype matches that of the probed function.
+
+1.3 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 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.
+
+2. Architectures Supported
+
+Kprobes, jprobes, and return probes are implemented on the following
+architectures:
+
+- i386
+- x86_64 (AMD-64, E64MT)
+- ppc64
+- ia64 (Support for probes on certain instruction types is still in progress.)
+- sparc64 (Return probes not yet implemented.)
+
+3. Configuring Kprobes
+
+When configuring the kernel using make menuconfig/xconfig/oldconfig,
+ensure that CONFIG_KPROBES is set to "y".  Under "Kernel hacking",
+look for "Kprobes".  You may have to enable "Kernel debugging"
+(CONFIG_DEBUG_KERNEL) before you can enable Kprobes.
+
+You may also want to ensure that CONFIG_KALLSYMS and perhaps even
+CONFIG_KALLSYMS_ALL are set to "y", since kallsyms_lookup_name()
+is a handy, version-independent way to find a function's address.
+
+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.  Here are terse, mini-man-page
+specifications for these functions and the associated probe handlers
+that you'll write.  See the latter half of this document 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.
+
+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,
+fastcall, 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
+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.
+
+5. Kprobes Features and Limitations
+
+As of Linux v2.6.12, 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.
+
+For obvious reasons, it's a bad idea to install a probe in
+the code that implements Kprobes (mostly kernel/kprobes.c and
+arch/*/kernel/kprobes.c).  A patch in the v2.6.13 timeframe instructs
+Kprobes to reject such requests.
+
+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.  As of Linux v2.6.12, if a probe handler hits a probe,
+that second probe's handlers won't be run in that instance.
+
+In Linux v2.6.12 and previous versions, Kprobes' data structures are
+protected by a single lock that is held during probe registration and
+unregistration and while handlers are run.  Thus, no two handlers
+can run simultaneously.  To improve scalability on SMP systems,
+this restriction will probably be removed soon, in which case
+multiple handlers (or multiple instances of the same handler) may
+run concurrently on different CPUs.  Code your handlers accordingly.
+
+Kprobes does not use semaphores 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.  We have the do_exit()
+and do_execve() cases covered.  do_fork() is not an issue.  We're
+unaware of other specific cases where this could be a problem.
+
+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 (http://sourceware.org/systemtap): Work in progress
+to provide a simplified programming interface for probe-based
+instrumentation.
+b. Improved SMP scalability: Currently, work is in progress to handle
+multiple kprobes in parallel.
+c. Kernel return probes for sparc64.
+d. Support for other architectures.
+e. User-space probes.
+
+8. Kprobes Example
+
+Here's a sample kernel module showing the use of kprobes to dump a
+stack trace and selected i386 registers when do_fork() is called.
+----- cut here -----
+/*kprobe_example.c*/
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/kprobes.h>
+#include <linux/kallsyms.h>
+#include <linux/sched.h>
+
+/*For each probe you need to allocate a kprobe structure*/
+static struct kprobe kp;
+
+/*kprobe pre_handler: called just before the probed instruction is executed*/
+int handler_pre(struct kprobe *p, struct pt_regs *regs)
+{
+	printk("pre_handler: p->addr=0x%p, eip=%lx, eflags=0x%lx\n",
+		p->addr, regs->eip, regs->eflags);
+	dump_stack();
+	return 0;
+}
+
+/*kprobe post_handler: called after the probed instruction is executed*/
+void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long flags)
+{
+	printk("post_handler: p->addr=0x%p, eflags=0x%lx\n",
+		p->addr, regs->eflags);
+}
+
+/* fault_handler: this is called if an exception is generated for any
+ * instruction within the pre- or post-handler, or when Kprobes
+ * single-steps the probed instruction.
+ */
+int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
+{
+	printk("fault_handler: p->addr=0x%p, trap #%dn",
+		p->addr, trapnr);
+	/* Return 0 because we don't handle the fault. */
+	return 0;
+}
+
+int init_module(void)
+{
+	int ret;
+	kp.pre_handler = handler_pre;
+	kp.post_handler = handler_post;
+	kp.fault_handler = handler_fault;
+	kp.addr = (kprobe_opcode_t*) kallsyms_lookup_name("do_fork");
+	/* register the kprobe now */
+	if (!kp.addr) {
+		printk("Couldn't find %s to plant kprobe\n", "do_fork");
+		return -1;
+	}
+	if ((ret = register_kprobe(&kp) < 0)) {
+		printk("register_kprobe failed, returned %d\n", ret);
+		return -1;
+	}
+	printk("kprobe registered\n");
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	unregister_kprobe(&kp);
+	printk("kprobe unregistered\n");
+}
+
+MODULE_LICENSE("GPL");
+----- cut here -----
+
+You can build the kernel module, kprobe-example.ko, using the following
+Makefile:
+----- cut here -----
+obj-m := kprobe-example.o
+KDIR := /lib/modules/$(shell uname -r)/build
+PWD := $(shell pwd)
+default:
+	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
+clean:
+	rm -f *.mod.c *.ko *.o
+----- cut here -----
+
+$ make
+$ su -
+...
+# insmod kprobe-example.ko
+
+You will see the trace data in /var/log/messages and on the console
+whenever do_fork() is invoked to create a new process.
+
+9. Jprobes Example
+
+Here's a sample kernel module showing the use of jprobes to dump
+the arguments of do_fork().
+----- cut here -----
+/*jprobe-example.c */
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/uio.h>
+#include <linux/kprobes.h>
+#include <linux/kallsyms.h>
+
+/*
+ * Jumper probe for do_fork.
+ * Mirror principle enables access to arguments of the probed routine
+ * from the probe handler.
+ */
+
+/* Proxy routine having the same arguments as actual do_fork() routine */
+long jdo_fork(unsigned long clone_flags, unsigned long stack_start,
+	      struct pt_regs *regs, unsigned long stack_size,
+	      int __user * parent_tidptr, int __user * child_tidptr)
+{
+	printk("jprobe: clone_flags=0x%lx, stack_size=0x%lx, regs=0x%p\n",
+	       clone_flags, stack_size, regs);
+	/* Always end with a call to jprobe_return(). */
+	jprobe_return();
+	/*NOTREACHED*/
+	return 0;
+}
+
+static struct jprobe my_jprobe = {
+	.entry = (kprobe_opcode_t *) jdo_fork
+};
+
+int init_module(void)
+{
+	int ret;
+	my_jprobe.kp.addr = (kprobe_opcode_t *) kallsyms_lookup_name("do_fork");
+	if (!my_jprobe.kp.addr) {
+		printk("Couldn't find %s to plant jprobe\n", "do_fork");
+		return -1;
+	}
+
+	if ((ret = register_jprobe(&my_jprobe)) <0) {
+		printk("register_jprobe failed, returned %d\n", ret);
+		return -1;
+	}
+	printk("Planted jprobe at %p, handler addr %p\n",
+	       my_jprobe.kp.addr, my_jprobe.entry);
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	unregister_jprobe(&my_jprobe);
+	printk("jprobe unregistered\n");
+}
+
+MODULE_LICENSE("GPL");
+----- cut here -----
+
+Build and insert the kernel module as shown in the above kprobe
+example.  You will see the trace data in /var/log/messages and on
+the console whenever do_fork() is invoked to create a new process.
+(Some messages may be suppressed if syslogd is configured to
+eliminate duplicate messages.)
+
+10. Kretprobes Example
+
+Here's a sample kernel module showing the use of return probes to
+report failed calls to sys_open().
+----- cut here -----
+/*kretprobe-example.c*/
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/kprobes.h>
+#include <linux/kallsyms.h>
+
+static const char *probed_func = "sys_open";
+
+/* Return-probe handler: If the probed function fails, log the return value. */
+static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	// Substitute the appropriate register name for your architecture --
+	// e.g., regs->rax for x86_64, regs->gpr[3] for ppc64.
+	int retval = (int) regs->eax;
+	if (retval < 0) {
+		printk("%s returns %d\n", probed_func, retval);
+	}
+	return 0;
+}
+
+static struct kretprobe my_kretprobe = {
+	.handler = ret_handler,
+	/* Probe up to 20 instances concurrently. */
+	.maxactive = 20
+};
+
+int init_module(void)
+{
+	int ret;
+	my_kretprobe.kp.addr =
+		(kprobe_opcode_t *) kallsyms_lookup_name(probed_func);
+	if (!my_kretprobe.kp.addr) {
+		printk("Couldn't find %s to plant return probe\n", probed_func);
+		return -1;
+	}
+	if ((ret = register_kretprobe(&my_kretprobe)) < 0) {
+		printk("register_kretprobe failed, returned %d\n", ret);
+		return -1;
+	}
+	printk("Planted return probe at %p\n", my_kretprobe.kp.addr);
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	unregister_kretprobe(&my_kretprobe);
+	printk("kretprobe unregistered\n");
+	/* nmissed > 0 suggests that maxactive was set too low. */
+	printk("Missed probing %d instances of %s\n",
+		my_kretprobe.nmissed, probed_func);
+}
+
+MODULE_LICENSE("GPL");
+----- cut here -----
+
+Build and insert the kernel module as shown in the above kprobe
+example.  You will see the trace data in /var/log/messages and on the
+console whenever sys_open() returns a negative value.  (Some messages
+may be suppressed if syslogd is configured to eliminate duplicate
+messages.)
+
+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/