8. Kprobes Example
9. Jprobes Example
10. Kretprobes Example
+Appendix A: The kprobes debugfs interface
1. Concepts: Kprobes, Jprobes, Return Probes
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.
+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 How Does a Return Probe Work?
+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
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.
+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
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.
+
2. Architectures Supported
Kprobes, jprobes, and return probes are implemented on the following
- ppc64
- ia64 (Does not support probes on instruction slot1.)
- sparc64 (Return probes not yet implemented.)
+- arm
3. Configuring Kprobes
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.
-
-NOTE: A macro JPROBE_ENTRY is provided to handle architecture-specific
-aliasing of jp->entry. In the interest of portability, it is advised
-to use:
-
- jp->entry = JPROBE_ENTRY(handler);
+control to Kprobes.) If the probed function is declared asmlinkage
+or anything else that affects how args are passed, the handler's
+declaration must match.
register_jprobe() returns 0 on success, or a negative errno otherwise.
- ret_addr: the return address
- rp: points to the corresponding kretprobe object
- task: points to the corresponding task struct
+- data: points to per return-instance private data; see "Kretprobe
+ entry-handler" for details.
The regs_return_value(regs) macro provides a simple abstraction to
extract the return value from the appropriate register as defined by
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() case covered.
-do_execve() and do_fork() are not an issue. We're unaware of other
-specific cases where this could be a problem.
+produce undesirable results. In such a case, a line:
+kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
+gets printed. With this information, one will be able to correlate the
+exact instance of the kretprobe that caused the problem. We have the
+do_exit() case covered. do_execve() and do_fork() are not an issue.
+We're unaware of other specific cases where this could be a problem.
If, upon entry to or exit from a function, the CPU is running on
a stack other than that of the current task, registering a return
kp.fault_handler = handler_fault;
kp.symbol_name = "do_fork";
- if ((ret = register_kprobe(&kp) < 0)) {
+ ret = register_kprobe(&kp);
+ if (ret < 0) {
printk("register_kprobe failed, returned %d\n", ret);
- return -1;
+ return ret;
}
printk("kprobe registered\n");
return 0;
}
static struct jprobe my_jprobe = {
- .entry = JPROBE_ENTRY(jdo_fork)
+ .entry = jdo_fork
};
static int __init jprobe_init(void)
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
+#include <linux/ktime.h>
+
+/* per-instance private data */
+struct my_data {
+ ktime_t entry_stamp;
+};
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)
+/* Timestamp function entry. */
+static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+ struct my_data *data;
+
+ if(!current->mm)
+ return 1; /* skip kernel threads */
+
+ data = (struct my_data *)ri->data;
+ data->entry_stamp = ktime_get();
+ return 0;
+}
+
+/* If the probed function failed, log the return value and duration.
+ * Duration may turn out to be zero consistently, depending upon the
+ * granularity of time accounting on the platform. */
+static int return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
{
int retval = regs_return_value(regs);
+ struct my_data *data = (struct my_data *)ri->data;
+ s64 delta;
+ ktime_t now;
+
if (retval < 0) {
- printk("%s returns %d\n", probed_func, retval);
+ now = ktime_get();
+ delta = ktime_to_ns(ktime_sub(now, data->entry_stamp));
+ printk("%s: return val = %d (duration = %lld ns)\n",
+ probed_func, retval, delta);
}
return 0;
}
static struct kretprobe my_kretprobe = {
- .handler = ret_handler,
- /* Probe up to 20 instances concurrently. */
- .maxactive = 20
+ .handler = return_handler,
+ .entry_handler = entry_handler,
+ .data_size = sizeof(struct my_data),
+ .maxactive = 20, /* probe up to 20 instances concurrently */
};
static int __init kretprobe_init(void)
printk("register_kretprobe failed, returned %d\n", ret);
return -1;
}
- printk("Planted return probe at %p\n", my_kretprobe.kp.addr);
+ printk("Kretprobe active on %s\n", my_kretprobe.kp.symbol_name);
return 0;
}
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);
+ my_kretprobe.nmissed, probed_func);
}
module_init(kretprobe_init)
http://www.redhat.com/magazine/005mar05/features/kprobes/
http://www-users.cs.umn.edu/~boutcher/kprobes/
http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
+
+
+Appendix A: The kprobes debugfs interface
+
+With recent kernels (> 2.6.20) the list of registered kprobes is visible
+under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
+
+/debug/kprobes/list: Lists all registered probes on the system
+
+c015d71a k vfs_read+0x0
+c011a316 j do_fork+0x0
+c03dedc5 r tcp_v4_rcv+0x0
+
+The first column provides the kernel address where the probe is inserted.
+The second column identifies the type of probe (k - kprobe, r - kretprobe
+and j - jprobe), while the third column specifies the symbol+offset of
+the probe. If the probed function belongs to a module, the module name
+is also specified.
+
+/debug/kprobes/enabled: Turn kprobes ON/OFF
+
+Provides a knob to globally turn registered kprobes ON or OFF. By default,
+all kprobes are enabled. By echoing "0" to this file, all registered probes
+will be disarmed, till such time a "1" is echoed to this file.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff