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diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt new file mode 100644 index 000000000000..0541fe1de704 --- /dev/null +++ b/Documentation/kprobes.txt @@ -0,0 +1,588 @@ +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/ |
