[PATCH] kprobes: Allow multiple kprobes at the same address

Allow registration of multiple kprobes at an address in an architecture
agnostic way. Corresponding handlers will be invoked in a sequence. But,
a kprobe and a jprobe can't (yet) co-exist at the same address.

Signed-off-by: Ananth N Mavinakayanahalli <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/kprobes.h

···
 struct kprobe {
 	struct hlist_node hlist;
 
+	/* list of kprobes for multi-handler support */
+	struct list_head list;
+
 	/* location of the probe point */
 	kprobe_opcode_t *addr;
 

kernel/kprobes.c

···
 
 unsigned int kprobe_cpu = NR_CPUS;
 static DEFINE_SPINLOCK(kprobe_lock);
+static struct kprobe *curr_kprobe;
 
 /* Locks kprobe: irqs must be disabled */
 void lock_kprobes(void)
···
 	return NULL;
 }
 
+/*
+ * Aggregate handlers for multiple kprobes support - these handlers
+ * take care of invoking the individual kprobe handlers on p->list
+ */
+int aggr_pre_handler(struct kprobe *p, struct pt_regs *regs)
+{
+	struct kprobe *kp;
+
+	list_for_each_entry(kp, &p->list, list) {
+		if (kp->pre_handler) {
+			curr_kprobe = kp;
+			kp->pre_handler(kp, regs);
+			curr_kprobe = NULL;
+		}
+	}
+	return 0;
+}
+
+void aggr_post_handler(struct kprobe *p, struct pt_regs *regs,
+		unsigned long flags)
+{
+	struct kprobe *kp;
+
+	list_for_each_entry(kp, &p->list, list) {
+		if (kp->post_handler) {
+			curr_kprobe = kp;
+			kp->post_handler(kp, regs, flags);
+			curr_kprobe = NULL;
+		}
+	}
+	return;
+}
+
+int aggr_fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr)
+{
+	/*
+	 * if we faulted "during" the execution of a user specified
+	 * probe handler, invoke just that probe's fault handler
+	 */
+	if (curr_kprobe && curr_kprobe->fault_handler) {
+		if (curr_kprobe->fault_handler(curr_kprobe, regs, trapnr))
+			return 1;
+	}
+	return 0;
+}
+
+/*
+ * Fill in the required fields of the "manager kprobe". Replace the
+ * earlier kprobe in the hlist with the manager kprobe
+ */
+static inline void add_aggr_kprobe(struct kprobe *ap, struct kprobe *p)
+{
+	ap->addr = p->addr;
+	ap->opcode = p->opcode;
+	memcpy(&ap->ainsn, &p->ainsn, sizeof(struct arch_specific_insn));
+
+	ap->pre_handler = aggr_pre_handler;
+	ap->post_handler = aggr_post_handler;
+	ap->fault_handler = aggr_fault_handler;
+
+	INIT_LIST_HEAD(&ap->list);
+	list_add(&p->list, &ap->list);
+
+	INIT_HLIST_NODE(&ap->hlist);
+	hlist_del(&p->hlist);
+	hlist_add_head(&ap->hlist,
+		&kprobe_table[hash_ptr(ap->addr, KPROBE_HASH_BITS)]);
+}
+
+/*
+ * This is the second or subsequent kprobe at the address - handle
+ * the intricacies
+ * TODO: Move kcalloc outside the spinlock
+ */
+static int register_aggr_kprobe(struct kprobe *old_p, struct kprobe *p)
+{
+	int ret = 0;
+	struct kprobe *ap;
+
+	if (old_p->break_handler || p->break_handler) {
+		ret = -EEXIST;	/* kprobe and jprobe can't (yet) coexist */
+	} else if (old_p->pre_handler == aggr_pre_handler) {
+		list_add(&p->list, &old_p->list);
+	} else {
+		ap = kcalloc(1, sizeof(struct kprobe), GFP_ATOMIC);
+		if (!ap)
+			return -ENOMEM;
+		add_aggr_kprobe(ap, old_p);
+		list_add(&p->list, &ap->list);
+	}
+	return ret;
+}
+
+/* kprobe removal house-keeping routines */
+static inline void cleanup_kprobe(struct kprobe *p, unsigned long flags)
+{
+	*p->addr = p->opcode;
+	hlist_del(&p->hlist);
+	flush_icache_range((unsigned long) p->addr,
+		   (unsigned long) p->addr + sizeof(kprobe_opcode_t));
+	spin_unlock_irqrestore(&kprobe_lock, flags);
+	arch_remove_kprobe(p);
+}
+
+static inline void cleanup_aggr_kprobe(struct kprobe *old_p,
+		struct kprobe *p, unsigned long flags)
+{
+	list_del(&p->list);
+	if (list_empty(&old_p->list)) {
+		cleanup_kprobe(old_p, flags);
+		kfree(old_p);
+	} else
+		spin_unlock_irqrestore(&kprobe_lock, flags);
+}
+
 int register_kprobe(struct kprobe *p)
 {
 	int ret = 0;
 	unsigned long flags = 0;
+	struct kprobe *old_p;
 
 	if ((ret = arch_prepare_kprobe(p)) != 0) {
 		goto rm_kprobe;
 	}
 	spin_lock_irqsave(&kprobe_lock, flags);
-	INIT_HLIST_NODE(&p->hlist);
-	if (get_kprobe(p->addr)) {
-		ret = -EEXIST;
+	old_p = get_kprobe(p->addr);
+	if (old_p) {
+		ret = register_aggr_kprobe(old_p, p);
 		goto out;
 	}
-	arch_copy_kprobe(p);
 
+	arch_copy_kprobe(p);
+	INIT_HLIST_NODE(&p->hlist);
 	hlist_add_head(&p->hlist,
 		       &kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);
 
···
 void unregister_kprobe(struct kprobe *p)
 {
 	unsigned long flags;
+	struct kprobe *old_p;
+
 	spin_lock_irqsave(&kprobe_lock, flags);
-	if (!get_kprobe(p->addr)) {
+	old_p = get_kprobe(p->addr);
+	if (old_p) {
+		if (old_p->pre_handler == aggr_pre_handler)
+			cleanup_aggr_kprobe(old_p, p, flags);
+		else
+			cleanup_kprobe(p, flags);
+	} else
 		spin_unlock_irqrestore(&kprobe_lock, flags);
-		return;
-	}
-	*p->addr = p->opcode;
-	hlist_del(&p->hlist);
-	flush_icache_range((unsigned long) p->addr,
-			   (unsigned long) p->addr + sizeof(kprobe_opcode_t));
-	spin_unlock_irqrestore(&kprobe_lock, flags);
-	arch_remove_kprobe(p);
 }
 
 static struct notifier_block kprobe_exceptions_nb = {