[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>
This commit is contained in:
Jim Keniston
Linus Torvalds
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Title : Kernel Probes (Kprobes)
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Authors : Jim Keniston <jkenisto@us.ibm.com>
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: Prasanna S Panchamukhi <prasanna@in.ibm.com>
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CONTENTS
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1. Concepts: Kprobes, Jprobes, Return Probes
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2. Architectures Supported
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3. Configuring Kprobes
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4. API Reference
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5. Kprobes Features and Limitations
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6. Probe Overhead
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7. TODO
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8. Kprobes Example
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9. Jprobes Example
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10. Kretprobes Example
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1. Concepts: Kprobes, Jprobes, Return Probes
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Kprobes enables you to dynamically break into any kernel routine and
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collect debugging and performance information non-disruptively. You
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can trap at almost any kernel code address, specifying a handler
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routine to be invoked when the breakpoint is hit.
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There are currently three types of probes: kprobes, jprobes, and
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kretprobes (also called return probes). A kprobe can be inserted
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on virtually any instruction in the kernel. A jprobe is inserted at
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the entry to a kernel function, and provides convenient access to the
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function's arguments. A return probe fires when a specified function
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returns.
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In the typical case, Kprobes-based instrumentation is packaged as
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a kernel module. The module's init function installs ("registers")
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one or more probes, and the exit function unregisters them. A
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registration function such as register_kprobe() specifies where
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the probe is to be inserted and what handler is to be called when
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the probe is hit.
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The next three subsections explain how the different types of
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probes work. They explain certain things that you'll need to
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know in order to make the best use of Kprobes -- e.g., the
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difference between a pre_handler and a post_handler, and how
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to use the maxactive and nmissed fields of a kretprobe. But
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if you're in a hurry to start using Kprobes, you can skip ahead
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to section 2.
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1.1 How Does a Kprobe Work?
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When a kprobe is registered, Kprobes makes a copy of the probed
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instruction and replaces the first byte(s) of the probed instruction
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with a breakpoint instruction (e.g., int3 on i386 and x86_64).
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When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
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registers are saved, and control passes to Kprobes via the
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notifier_call_chain mechanism. Kprobes executes the "pre_handler"
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associated with the kprobe, passing the handler the addresses of the
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kprobe struct and the saved registers.
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Next, Kprobes single-steps its copy of the probed instruction.
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(It would be simpler to single-step the actual instruction in place,
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but then Kprobes would have to temporarily remove the breakpoint
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instruction. This would open a small time window when another CPU
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could sail right past the probepoint.)
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After the instruction is single-stepped, Kprobes executes the
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"post_handler," if any, that is associated with the kprobe.
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Execution then continues with the instruction following the probepoint.
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1.2 How Does a Jprobe Work?
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A jprobe is implemented using a kprobe that is placed on a function's
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entry point. It employs a simple mirroring principle to allow
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seamless access to the probed function's arguments. The jprobe
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handler routine should have the same signature (arg list and return
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type) as the function being probed, and must always end by calling
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the Kprobes function jprobe_return().
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Here's how it works. When the probe is hit, Kprobes makes a copy of
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the saved registers and a generous portion of the stack (see below).
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Kprobes then points the saved instruction pointer at the jprobe's
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handler routine, and returns from the trap. As a result, control
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passes to the handler, which is presented with the same register and
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stack contents as the probed function. When it is done, the handler
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calls jprobe_return(), which traps again to restore the original stack
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contents and processor state and switch to the probed function.
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By convention, the callee owns its arguments, so gcc may produce code
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that unexpectedly modifies that portion of the stack. This is why
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Kprobes saves a copy of the stack and restores it after the jprobe
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handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
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64 bytes on i386.
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Note that the probed function's args may be passed on the stack
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or in registers (e.g., for x86_64 or for an i386 fastcall function).
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The jprobe will work in either case, so long as the handler's
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prototype matches that of the probed function.
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1.3 How Does a Return Probe Work?
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When you call register_kretprobe(), Kprobes establishes a kprobe at
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the entry to the function. When the probed function is called and this
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probe is hit, Kprobes saves a copy of the return address, and replaces
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the return address with the address of a "trampoline." The trampoline
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is an arbitrary piece of code -- typically just a nop instruction.
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At boot time, Kprobes registers a kprobe at the trampoline.
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When the probed function executes its return instruction, control
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passes to the trampoline and that probe is hit. Kprobes' trampoline
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handler calls the user-specified handler associated with the kretprobe,
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then sets the saved instruction pointer to the saved return address,
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and that's where execution resumes upon return from the trap.
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While the probed function is executing, its return address is
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stored in an object of type kretprobe_instance. Before calling
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register_kretprobe(), the user sets the maxactive field of the
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kretprobe struct to specify how many instances of the specified
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function can be probed simultaneously. register_kretprobe()
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pre-allocates the indicated number of kretprobe_instance objects.
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For example, if the function is non-recursive and is called with a
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spinlock held, maxactive = 1 should be enough. If the function is
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non-recursive and can never relinquish the CPU (e.g., via a semaphore
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or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
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set to a default value. If CONFIG_PREEMPT is enabled, the default
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is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
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It's not a disaster if you set maxactive too low; you'll just miss
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some probes. In the kretprobe struct, the nmissed field is set to
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zero when the return probe is registered, and is incremented every
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time the probed function is entered but there is no kretprobe_instance
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object available for establishing the return probe.
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2. Architectures Supported
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Kprobes, jprobes, and return probes are implemented on the following
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architectures:
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- i386
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- x86_64 (AMD-64, E64MT)
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- ppc64
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- ia64 (Support for probes on certain instruction types is still in progress.)
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- sparc64 (Return probes not yet implemented.)
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3. Configuring Kprobes
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When configuring the kernel using make menuconfig/xconfig/oldconfig,
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ensure that CONFIG_KPROBES is set to "y". Under "Kernel hacking",
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look for "Kprobes". You may have to enable "Kernel debugging"
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(CONFIG_DEBUG_KERNEL) before you can enable Kprobes.
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You may also want to ensure that CONFIG_KALLSYMS and perhaps even
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CONFIG_KALLSYMS_ALL are set to "y", since kallsyms_lookup_name()
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is a handy, version-independent way to find a function's address.
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If you need to insert a probe in the middle of a function, you may find
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it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
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so you can use "objdump -d -l vmlinux" to see the source-to-object
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code mapping.
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4. API Reference
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The Kprobes API includes a "register" function and an "unregister"
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function for each type of probe. Here are terse, mini-man-page
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specifications for these functions and the associated probe handlers
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that you'll write. See the latter half of this document for examples.
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4.1 register_kprobe
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#include <linux/kprobes.h>
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int register_kprobe(struct kprobe *kp);
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Sets a breakpoint at the address kp->addr. When the breakpoint is
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hit, Kprobes calls kp->pre_handler. After the probed instruction
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is single-stepped, Kprobe calls kp->post_handler. If a fault
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occurs during execution of kp->pre_handler or kp->post_handler,
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or during single-stepping of the probed instruction, Kprobes calls
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kp->fault_handler. Any or all handlers can be NULL.
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register_kprobe() returns 0 on success, or a negative errno otherwise.
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User's pre-handler (kp->pre_handler):
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int pre_handler(struct kprobe *p, struct pt_regs *regs);
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Called with p pointing to the kprobe associated with the breakpoint,
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and regs pointing to the struct containing the registers saved when
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the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
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User's post-handler (kp->post_handler):
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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void post_handler(struct kprobe *p, struct pt_regs *regs,
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unsigned long flags);
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p and regs are as described for the pre_handler. flags always seems
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to be zero.
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User's fault-handler (kp->fault_handler):
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
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p and regs are as described for the pre_handler. trapnr is the
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architecture-specific trap number associated with the fault (e.g.,
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on i386, 13 for a general protection fault or 14 for a page fault).
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Returns 1 if it successfully handled the exception.
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4.2 register_jprobe
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#include <linux/kprobes.h>
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int register_jprobe(struct jprobe *jp)
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Sets a breakpoint at the address jp->kp.addr, which must be the address
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of the first instruction of a function. When the breakpoint is hit,
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Kprobes runs the handler whose address is jp->entry.
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The handler should have the same arg list and return type as the probed
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function; and just before it returns, it must call jprobe_return().
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(The handler never actually returns, since jprobe_return() returns
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control to Kprobes.) If the probed function is declared asmlinkage,
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fastcall, or anything else that affects how args are passed, the
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handler's declaration must match.
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register_jprobe() returns 0 on success, or a negative errno otherwise.
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4.3 register_kretprobe
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#include <linux/kprobes.h>
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int register_kretprobe(struct kretprobe *rp);
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Establishes a return probe for the function whose address is
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rp->kp.addr. When that function returns, Kprobes calls rp->handler.
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You must set rp->maxactive appropriately before you call
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register_kretprobe(); see "How Does a Return Probe Work?" for details.
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register_kretprobe() returns 0 on success, or a negative errno
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otherwise.
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User's return-probe handler (rp->handler):
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#include <linux/kprobes.h>
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#include <linux/ptrace.h>
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int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
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regs is as described for kprobe.pre_handler. ri points to the
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kretprobe_instance object, of which the following fields may be
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of interest:
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- ret_addr: the return address
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- rp: points to the corresponding kretprobe object
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- task: points to the corresponding task struct
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The handler's return value is currently ignored.
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4.4 unregister_*probe
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#include <linux/kprobes.h>
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void unregister_kprobe(struct kprobe *kp);
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void unregister_jprobe(struct jprobe *jp);
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void unregister_kretprobe(struct kretprobe *rp);
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Removes the specified probe. The unregister function can be called
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at any time after the probe has been registered.
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5. Kprobes Features and Limitations
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As of Linux v2.6.12, Kprobes allows multiple probes at the same
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address. Currently, however, there cannot be multiple jprobes on
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the same function at the same time.
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In general, you can install a probe anywhere in the kernel.
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In particular, you can probe interrupt handlers. Known exceptions
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are discussed in this section.
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For obvious reasons, it's a bad idea to install a probe in
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the code that implements Kprobes (mostly kernel/kprobes.c and
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arch/*/kernel/kprobes.c). A patch in the v2.6.13 timeframe instructs
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Kprobes to reject such requests.
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If you install a probe in an inline-able function, Kprobes makes
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no attempt to chase down all inline instances of the function and
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install probes there. gcc may inline a function without being asked,
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so keep this in mind if you're not seeing the probe hits you expect.
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A probe handler can modify the environment of the probed function
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-- e.g., by modifying kernel data structures, or by modifying the
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contents of the pt_regs struct (which are restored to the registers
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upon return from the breakpoint). So Kprobes can be used, for example,
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to install a bug fix or to inject faults for testing. Kprobes, of
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course, has no way to distinguish the deliberately injected faults
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from the accidental ones. Don't drink and probe.
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Kprobes makes no attempt to prevent probe handlers from stepping on
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each other -- e.g., probing printk() and then calling printk() from a
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probe handler. As of Linux v2.6.12, if a probe handler hits a probe,
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that second probe's handlers won't be run in that instance.
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In Linux v2.6.12 and previous versions, Kprobes' data structures are
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protected by a single lock that is held during probe registration and
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unregistration and while handlers are run. Thus, no two handlers
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can run simultaneously. To improve scalability on SMP systems,
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this restriction will probably be removed soon, in which case
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multiple handlers (or multiple instances of the same handler) may
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run concurrently on different CPUs. Code your handlers accordingly.
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Kprobes does not use semaphores or allocate memory except during
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registration and unregistration.
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Probe handlers are run with preemption disabled. Depending on the
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architecture, handlers may also run with interrupts disabled. In any
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case, your handler should not yield the CPU (e.g., by attempting to
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acquire a semaphore).
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Since a return probe is implemented by replacing the return
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address with the trampoline's address, stack backtraces and calls
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to __builtin_return_address() will typically yield the trampoline's
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address instead of the real return address for kretprobed functions.
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(As far as we can tell, __builtin_return_address() is used only
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for instrumentation and error reporting.)
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If the number of times a function is called does not match the
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number of times it returns, registering a return probe on that
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function may produce undesirable results. We have the do_exit()
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and do_execve() cases covered. do_fork() is not an issue. We're
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unaware of other specific cases where this could be a problem.
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6. Probe Overhead
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On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
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microseconds to process. Specifically, a benchmark that hits the same
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probepoint repeatedly, firing a simple handler each time, reports 1-2
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million hits per second, depending on the architecture. A jprobe or
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return-probe hit typically takes 50-75% longer than a kprobe hit.
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When you have a return probe set on a function, adding a kprobe at
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the entry to that function adds essentially no overhead.
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Here are sample overhead figures (in usec) for different architectures.
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k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
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on same function; jr = jprobe + return probe on same function
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i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
|
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k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
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x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
|
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k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
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ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
|
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k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
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7. TODO
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a. SystemTap (http://sourceware.org/systemtap): Work in progress
|
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to provide a simplified programming interface for probe-based
|
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instrumentation.
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b. Improved SMP scalability: Currently, work is in progress to handle
|
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multiple kprobes in parallel.
|
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c. Kernel return probes for sparc64.
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d. Support for other architectures.
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e. User-space probes.
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8. Kprobes Example
|
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Here's a sample kernel module showing the use of kprobes to dump a
|
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stack trace and selected i386 registers when do_fork() is called.
|
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----- cut here -----
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/*kprobe_example.c*/
|
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/kprobes.h>
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#include <linux/kallsyms.h>
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#include <linux/sched.h>
|
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|
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/*For each probe you need to allocate a kprobe structure*/
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static struct kprobe kp;
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|
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/*kprobe pre_handler: called just before the probed instruction is executed*/
|
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int handler_pre(struct kprobe *p, struct pt_regs *regs)
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{
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printk("pre_handler: p->addr=0x%p, eip=%lx, eflags=0x%lx\n",
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p->addr, regs->eip, regs->eflags);
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dump_stack();
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return 0;
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}
|
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|
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/*kprobe post_handler: called after the probed instruction is executed*/
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void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long flags)
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{
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printk("post_handler: p->addr=0x%p, eflags=0x%lx\n",
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p->addr, regs->eflags);
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}
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/* fault_handler: this is called if an exception is generated for any
|
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* instruction within the pre- or post-handler, or when Kprobes
|
||||
* single-steps the probed instruction.
|
||||
*/
|
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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/
|
||||
Загрузка…
Ссылка в новой задаче