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: Abhishek Sagar <sagar.abhishek@gmail.com>
Cc: Prasanna S Panchamukhi <prasanna@in.ibm.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Acked-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 94 insertions, 15 deletions

diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
index 53a63890aea4..30c101761d0d 100644
--- a/Documentation/kprobes.txt
+++ b/Documentation/kprobes.txt
@@ -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;
+
+        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)
@@ -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", my_kretprobe.kp.addr);
+        printk("Kretprobe active on %s\n", my_kretprobe.kp.symbol_name);
         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);
 }
 
 module_init(kretprobe_init)
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 6168c0a44172..4a6ce82ba039 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -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 {
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index d0493eafea3e..7a86e6432338 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -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)
         INIT_HLIST_HEAD(&rp->used_instances);
         INIT_HLIST_HEAD(&rp->free_instances);
         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) {
                         free_rp_inst(rp);
                         return -ENOMEM;
diff --git a/kernel/test_kprobes.c b/kernel/test_kprobes.c
index 88cdb109e13c..06b6395b45b2 100644
--- a/kernel/test_kprobes.c
+++ b/kernel/test_kprobes.c
@@ -135,6 +135,12 @@ static int test_jprobe(void)
 #ifdef CONFIG_KRETPROBES
 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) {
+                handler_errors++;
+                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);
         unregister_kretprobe(&rp);
-        if (krph_val == 0) {
+        if (krph_val != rand1) {
                 printk(KERN_ERR "Kprobe smoke test failed: "
                                 "kretprobe handler not called\n");
                 handler_errors++;