Commit f47cd9b5 authored by Abhishek Sagar's avatar Abhishek Sagar Committed by Linus Torvalds

kprobes: kretprobe user entry-handler

Provide support to add an optional user defined callback to be run at
function entry of a kretprobe'd function.  Also modify the kprobe smoke
tests to include an entry-handler during the kretprobe sanity test.
Signed-off-by: default avatarAbhishek Sagar <>
Cc: Prasanna S Panchamukhi <>
Cc: Ananth N Mavinakayanahalli <>
Cc: Anil S Keshavamurthy <>
Acked-by: default avatarJim Keniston <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 5beec4aa
......@@ -96,7 +96,9 @@ 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?
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
......@@ -107,9 +109,9 @@ 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.
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
......@@ -131,6 +133,30 @@ 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.
2. Architectures Supported
Kprobes, jprobes, and return probes are implemented on the following
......@@ -274,6 +300,8 @@ of interest:
- 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
......@@ -556,23 +584,52 @@ report failed calls to sys_open().
#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;
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)
......@@ -584,7 +641,7 @@ static int __init kretprobe_init(void)
printk("register_kretprobe failed, returned %d\n", ret);
return -1;
printk("Planted return probe at %p\n",;
printk("Kretprobe active on %s\n",;
return 0;
......@@ -594,7 +651,7 @@ static void __exit kretprobe_exit(void)
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);
......@@ -152,8 +152,10 @@ static inline int arch_trampoline_kprobe(struct kprobe *p)
struct kretprobe {
struct kprobe kp;
kretprobe_handler_t handler;
kretprobe_handler_t entry_handler;
int maxactive;
int nmissed;
size_t data_size;
struct hlist_head free_instances;
struct hlist_head used_instances;
......@@ -164,6 +166,7 @@ struct kretprobe_instance {
struct kretprobe *rp;
kprobe_opcode_t *ret_addr;
struct task_struct *task;
char data[0];
struct kretprobe_blackpoint {
......@@ -699,6 +699,12 @@ static int __kprobes pre_handler_kretprobe(struct kprobe *p,
struct kretprobe_instance, uflist);
ri->rp = rp;
ri->task = current;
if (rp->entry_handler && rp->entry_handler(ri, regs)) {
spin_unlock_irqrestore(&kretprobe_lock, flags);
return 0;
arch_prepare_kretprobe(ri, regs);
/* XXX(hch): why is there no hlist_move_head? */
......@@ -745,7 +751,8 @@ int __kprobes register_kretprobe(struct kretprobe *rp)
for (i = 0; i < rp->maxactive; i++) {
inst = kmalloc(sizeof(struct kretprobe_instance), GFP_KERNEL);
inst = kmalloc(sizeof(struct kretprobe_instance) +
rp->data_size, GFP_KERNEL);
if (inst == NULL) {
return -ENOMEM;
......@@ -135,6 +135,12 @@ static int test_jprobe(void)
static u32 krph_val;
static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
krph_val = (rand1 / div_factor);
return 0;
static int return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
unsigned long ret = regs_return_value(regs);
......@@ -144,13 +150,19 @@ static int return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
printk(KERN_ERR "Kprobe smoke test failed: "
"incorrect value in kretprobe handler\n");
if (krph_val == 0) {
printk(KERN_ERR "Kprobe smoke test failed: "
"call to kretprobe entry handler failed\n");
krph_val = (rand1 / div_factor);
krph_val = rand1;
return 0;
static struct kretprobe rp = {
.handler = return_handler,
.entry_handler = entry_handler,
.kp.symbol_name = "kprobe_target"
......@@ -167,7 +179,7 @@ static int test_kretprobe(void)
ret = kprobe_target(rand1);
if (krph_val == 0) {
if (krph_val != rand1) {
printk(KERN_ERR "Kprobe smoke test failed: "
"kretprobe handler not called\n");
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