[PATCH] Notify page fault call chain

With this patch Kprobes now registers for page fault notifications only when
their is an active probe registered.  Once all the active probes are
unregistered their is no need to be notified of page faults and kprobes
unregisters itself from the page fault notifications.  Hence we will have ZERO
side effects when no probes are active.

Signed-off-by: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-i386/kprobes.h

@@ -44,6 +44,7 @@ typedef u8 kprobe_opcode_t;
 
 #define JPROBE_ENTRY(pentry)	(kprobe_opcode_t *)pentry
 #define ARCH_SUPPORTS_KRETPROBES
+#define  ARCH_INACTIVE_KPROBE_COUNT 0
 
 void arch_remove_kprobe(struct kprobe *p);
 void kretprobe_trampoline(void);

include/asm-ia64/kprobes.h

@@ -82,6 +82,7 @@ struct kprobe_ctlblk {
 #define JPROBE_ENTRY(pentry)	(kprobe_opcode_t *)pentry
 
 #define ARCH_SUPPORTS_KRETPROBES
+#define  ARCH_INACTIVE_KPROBE_COUNT 1
 
 #define SLOT0_OPCODE_SHIFT	(37)
 #define SLOT1_p1_OPCODE_SHIFT	(37 - (64-46))

include/asm-powerpc/kprobes.h

@@ -50,6 +50,8 @@ typedef unsigned int kprobe_opcode_t;
 			IS_TWI(instr) || IS_TDI(instr))
 
 #define ARCH_SUPPORTS_KRETPROBES
+#define  ARCH_INACTIVE_KPROBE_COUNT 1
+
 void kretprobe_trampoline(void);
 extern void arch_remove_kprobe(struct kprobe *p);
 

include/asm-sparc64/kprobes.h

@@ -12,6 +12,7 @@ typedef u32 kprobe_opcode_t;
 
 #define JPROBE_ENTRY(pentry)	(kprobe_opcode_t *)pentry
 #define arch_remove_kprobe(p)	do {} while (0)
+#define  ARCH_INACTIVE_KPROBE_COUNT 0
 
 /* Architecture specific copy of original instruction*/
 struct arch_specific_insn {

include/asm-x86_64/kprobes.h

@@ -43,6 +43,7 @@ typedef u8 kprobe_opcode_t;
 
 #define JPROBE_ENTRY(pentry)	(kprobe_opcode_t *)pentry
 #define ARCH_SUPPORTS_KRETPROBES
+#define  ARCH_INACTIVE_KPROBE_COUNT 1
 
 void kretprobe_trampoline(void);
 extern void arch_remove_kprobe(struct kprobe *p);

kernel/kprobes.c

@@ -47,11 +47,17 @@
 
 static struct hlist_head kprobe_table[KPROBE_TABLE_SIZE];
 static struct hlist_head kretprobe_inst_table[KPROBE_TABLE_SIZE];
+static atomic_t kprobe_count;
 
 DEFINE_MUTEX(kprobe_mutex);		/* Protects kprobe_table */
 DEFINE_SPINLOCK(kretprobe_lock);	/* Protects kretprobe_inst_table */
 static DEFINE_PER_CPU(struct kprobe *, kprobe_instance) = NULL;
 
+static struct notifier_block kprobe_page_fault_nb = {
+	.notifier_call = kprobe_exceptions_notify,
+	.priority = 0x7fffffff /* we need to notified first */
+};
+
 #ifdef __ARCH_WANT_KPROBES_INSN_SLOT
 /*
  * kprobe->ainsn.insn points to the copy of the instruction to be
@@ -465,6 +471,8 @@ static int __kprobes __register_kprobe(struct kprobe *p,
 	old_p = get_kprobe(p->addr);
 	if (old_p) {
 		ret = register_aggr_kprobe(old_p, p);
+		if (!ret)
+			atomic_inc(&kprobe_count);
 		goto out;
 	}
 
@@ -475,6 +483,10 @@ static int __kprobes __register_kprobe(struct kprobe *p,
 	hlist_add_head_rcu(&p->hlist,
 		       &kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);
 
+	if (atomic_add_return(1, &kprobe_count) == \
+				(ARCH_INACTIVE_KPROBE_COUNT + 1))
+		register_page_fault_notifier(&kprobe_page_fault_nb);
+
   	arch_arm_kprobe(p);
 
 out:
@@ -553,6 +565,16 @@ valid_p:
 		}
 		mutex_unlock(&kprobe_mutex);
 	}
+
+	/* Call unregister_page_fault_notifier()
+	 * if no probes are active
+	 */
+	mutex_lock(&kprobe_mutex);
+	if (atomic_add_return(-1, &kprobe_count) == \
+				ARCH_INACTIVE_KPROBE_COUNT)
+		unregister_page_fault_notifier(&kprobe_page_fault_nb);
+	mutex_unlock(&kprobe_mutex);
+	return;
 }
 
 static struct notifier_block kprobe_exceptions_nb = {
@@ -560,10 +582,6 @@ static struct notifier_block kprobe_exceptions_nb = {
 	.priority = 0x7fffffff /* we need to be notified first */
 };
 
-static struct notifier_block kprobe_page_fault_nb = {
-	.notifier_call = kprobe_exceptions_notify,
-	.priority = 0x7fffffff /* we need to notified first */
-};
 
 int __kprobes register_jprobe(struct jprobe *jp)
 {
@@ -673,14 +691,12 @@ static int __init init_kprobes(void)
 		INIT_HLIST_HEAD(&kprobe_table[i]);
 		INIT_HLIST_HEAD(&kretprobe_inst_table[i]);
 	}
+	atomic_set(&kprobe_count, 0);
 
 	err = arch_init_kprobes();
 	if (!err)
 		err = register_die_notifier(&kprobe_exceptions_nb);
 
-	if (!err)
-		err = register_page_fault_notifier(&kprobe_page_fault_nb);
-
 	return err;
 }