uretprobes: Introduce uprobe_consumer->ret_handler()

Enclose return probes implementation, introduce ->ret_handler() and update
existing code to rely on ->handler() *and* ->ret_handler() for uprobe and
uretprobe respectively.
Signed-off-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

include/linux/uprobes.h

@@ -46,6 +46,9 @@ enum uprobe_filter_ctx {
 
 struct uprobe_consumer {
 	int (*handler)(struct uprobe_consumer *self, struct pt_regs *regs);
+	int (*ret_handler)(struct uprobe_consumer *self,
+				unsigned long func,
+				struct pt_regs *regs);
 	bool (*filter)(struct uprobe_consumer *self,
 				enum uprobe_filter_ctx ctx,
 				struct mm_struct *mm);

kernel/events/uprobes.c

@@ -838,6 +838,14 @@ int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *
 	struct uprobe *uprobe;
 	int ret;
 
+	/* Uprobe must have at least one set consumer */
+	if (!uc->handler && !uc->ret_handler)
+		return -EINVAL;
+
+	/* TODO: Implement return probes */
+	if (uc->ret_handler)
+		return -ENOSYS;
+
 	/* Racy, just to catch the obvious mistakes */
 	if (offset > i_size_read(inode))
 		return -EINVAL;
@@ -1497,10 +1505,13 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
 
 	down_read(&uprobe->register_rwsem);
 	for (uc = uprobe->consumers; uc; uc = uc->next) {
-		int rc = uc->handler(uc, regs);
+		int rc = 0;
 
-		WARN(rc & ~UPROBE_HANDLER_MASK,
-			"bad rc=0x%x from %pf()\n", rc, uc->handler);
+		if (uc->handler) {
+			rc = uc->handler(uc, regs);
+			WARN(rc & ~UPROBE_HANDLER_MASK,
+				"bad rc=0x%x from %pf()\n", rc, uc->handler);
+		}
 		remove &= rc;
 	}