[PATCH] x86_64 specific function return probes
The following patch adds the x86_64 architecture specific implementation
for function return probes.
Function return probes is a mechanism built on top of kprobes that allows
a caller to register a handler to be called when a given function exits.
For example, to instrument the return path of sys_mkdir:
static int sys_mkdir_exit(struct kretprobe_instance *i, struct pt_regs *regs)
{
printk("sys_mkdir exited\n");
return 0;
}
static struct kretprobe return_probe = {
.handler = sys_mkdir_exit,
};
<inside setup function>
return_probe.kp.addr = (kprobe_opcode_t *) kallsyms_lookup_name("sys_mkdir");
if (register_kretprobe(&return_probe)) {
printk(KERN_DEBUG "Unable to register return probe!\n");
/* do error path */
}
<inside cleanup function>
unregister_kretprobe(&return_probe);
The way this works is that:
* At system initialization time, kernel/kprobes.c installs a kprobe
on a function called kretprobe_trampoline() that is implemented in
the arch/x86_64/kernel/kprobes.c (More on this later)
* When a return probe is registered using register_kretprobe(),
kernel/kprobes.c will install a kprobe on the first instruction of the
targeted function with the pre handler set to arch_prepare_kretprobe()
which is implemented in arch/x86_64/kernel/kprobes.c.
* arch_prepare_kretprobe() will prepare a kretprobe instance that stores:
- nodes for hanging this instance in an empty or free list
- a pointer to the return probe
- the original return address
- a pointer to the stack address
With all this stowed away, arch_prepare_kretprobe() then sets the return
address for the targeted function to a special trampoline function called
kretprobe_trampoline() implemented in arch/x86_64/kernel/kprobes.c
* The kprobe completes as normal, with control passing back to the target
function that executes as normal, and eventually returns to our trampoline
function.
* Since a kprobe was installed on kretprobe_trampoline() during system
initialization, control passes back to kprobes via the architecture
specific function trampoline_probe_handler() which will lookup the
instance in an hlist maintained by kernel/kprobes.c, and then call
the handler function.
* When trampoline_probe_handler() is done, the kprobes infrastructure
single steps the original instruction (in this case just a top), and
then calls trampoline_post_handler(). trampoline_post_handler() then
looks up the instance again, puts the instance back on the free list,
and then makes a long jump back to the original return instruction.
So to recap, to instrument the exit path of a function this implementation
will cause four interruptions:
- A breakpoint at the very beginning of the function allowing us to
switch out the return address
- A single step interruption to execute the original instruction that
we replaced with the break instruction (normal kprobe flow)
- A breakpoint in the trampoline function where our instrumented function
returned to
- A single step interruption to execute the original instruction that
we replaced with the break instruction (normal kprobe flow)
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index dce8bab..e59d1f9 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -34,6 +34,7 @@
#include <linux/ptrace.h>
#include <linux/utsname.h>
#include <linux/random.h>
+#include <linux/kprobes.h>
#include <asm/uaccess.h>
#include <asm/pgtable.h>
@@ -293,6 +294,14 @@
{
struct task_struct *me = current;
struct thread_struct *t = &me->thread;
+
+ /*
+ * Remove function-return probe instances associated with this task
+ * and put them back on the free list. Do not insert an exit probe for
+ * this function, it will be disabled by kprobe_flush_task if you do.
+ */
+ kprobe_flush_task(me);
+
if (me->thread.io_bitmap_ptr) {
struct tss_struct *tss = &per_cpu(init_tss, get_cpu());
@@ -312,6 +321,13 @@
struct task_struct *tsk = current;
struct thread_info *t = current_thread_info();
+ /*
+ * Remove function-return probe instances associated with this task
+ * and put them back on the free list. Do not insert an exit probe for
+ * this function, it will be disabled by kprobe_flush_task if you do.
+ */
+ kprobe_flush_task(tsk);
+
if (t->flags & _TIF_ABI_PENDING)
t->flags ^= (_TIF_ABI_PENDING | _TIF_IA32);