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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. 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 you configured your kernel with CONFIG_OPTPROBES=y (currently this option is supported 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.


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