fanotify: create overflow event type

The special overflow event is allocated as struct fanotify_path_event,
but with a null path.

Use a special event type to identify the overflow event, so the helper
fanotify_has_event_path() will always indicate a non null path.

Allocating the overflow event doesn't need any of the fancy stuff in
fanotify_alloc_event(), so create a simplified helper for allocating the
overflow event.

There is also no need to store and report the pid with an overflow event.

Link: https://lore.kernel.org/r/20200708111156.24659-7-amir73il@gmail.com
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify.c

@@ -344,11 +344,11 @@ static struct inode *fanotify_fid_inode(struct inode *to_tell, u32 event_mask,
 	return fsnotify_data_inode(data, data_type);
 }
 
-struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
-					    struct inode *inode, u32 mask,
-					    const void *data, int data_type,
-					    const struct qstr *file_name,
-					    __kernel_fsid_t *fsid)
+static struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
+						struct inode *inode, u32 mask,
+						const void *data, int data_type,
+						const struct qstr *file_name,
+						__kernel_fsid_t *fsid)
 {
 	struct fanotify_event *event = NULL;
 	struct fanotify_fid_event *ffe = NULL;
@@ -426,8 +426,7 @@ struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
 	 * event queue, so event reported on parent is merged with event
 	 * reported on child when both directory and child watches exist.
 	 */
-	fsnotify_init_event(&event->fse, (unsigned long)id);
-	event->mask = mask;
+	fanotify_init_event(event, (unsigned long)id, mask);
 	if (FAN_GROUP_FLAG(group, FAN_REPORT_TID))
 		event->pid = get_pid(task_pid(current));
 	else
@@ -443,15 +442,8 @@ struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
 		fanotify_encode_fh(fanotify_event_dir_fh(event), id, gfp);
 
 	if (fanotify_event_has_path(event)) {
-		struct path *p = fanotify_event_path(event);
-
-		if (path) {
-			*p = *path;
-			path_get(path);
-		} else {
-			p->mnt = NULL;
-			p->dentry = NULL;
-		}
+		*fanotify_event_path(event) = *path;
+		path_get(path);
 	}
 out:
 	memalloc_unuse_memcg();
@@ -640,6 +632,9 @@ static void fanotify_free_event(struct fsnotify_event *fsn_event)
 	case FANOTIFY_EVENT_TYPE_FID_NAME:
 		fanotify_free_name_event(event);
 		break;
+	case FANOTIFY_EVENT_TYPE_OVERFLOW:
+		kfree(event);
+		break;
 	default:
 		WARN_ON_ONCE(1);
 	}

fs/notify/fanotify/fanotify.h

@@ -63,6 +63,7 @@ enum fanotify_event_type {
 	FANOTIFY_EVENT_TYPE_FID_NAME, /* variable length */
 	FANOTIFY_EVENT_TYPE_PATH,
 	FANOTIFY_EVENT_TYPE_PATH_PERM,
+	FANOTIFY_EVENT_TYPE_OVERFLOW, /* struct fanotify_event */
 };
 
 struct fanotify_event {
@@ -72,6 +73,14 @@ struct fanotify_event {
 	struct pid *pid;
 };
 
+static inline void fanotify_init_event(struct fanotify_event *event,
+				       unsigned long id, u32 mask)
+{
+	fsnotify_init_event(&event->fse, id);
+	event->mask = mask;
+	event->pid = NULL;
+}
+
 struct fanotify_fid_event {
 	struct fanotify_event fae;
 	__kernel_fsid_t fsid;
@@ -202,9 +211,3 @@ static inline struct path *fanotify_event_path(struct fanotify_event *event)
 	else
 		return NULL;
 }
-
-struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
-					    struct inode *inode, u32 mask,
-					    const void *data, int data_type,
-					    const struct qstr *file_name,
-					    __kernel_fsid_t *fsid);

fs/notify/fanotify/fanotify_user.c

@@ -836,13 +836,26 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
 				 FSNOTIFY_OBJ_TYPE_INODE, mask, flags, fsid);
 }
 
+static struct fsnotify_event *fanotify_alloc_overflow_event(void)
+{
+	struct fanotify_event *oevent;
+
+	oevent = kmalloc(sizeof(*oevent), GFP_KERNEL_ACCOUNT);
+	if (!oevent)
+		return NULL;
+
+	fanotify_init_event(oevent, 0, FS_Q_OVERFLOW);
+	oevent->type = FANOTIFY_EVENT_TYPE_OVERFLOW;
+
+	return &oevent->fse;
+}
+
 /* fanotify syscalls */
 SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 {
 	struct fsnotify_group *group;
 	int f_flags, fd;
 	struct user_struct *user;
-	struct fanotify_event *oevent;
 
 	pr_debug("%s: flags=%x event_f_flags=%x\n",
 		 __func__, flags, event_f_flags);
@@ -897,13 +910,11 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	atomic_inc(&user->fanotify_listeners);
 	group->memcg = get_mem_cgroup_from_mm(current->mm);
 
-	oevent = fanotify_alloc_event(group, NULL, FS_Q_OVERFLOW, NULL,
-				      FSNOTIFY_EVENT_NONE, NULL, NULL);
-	if (unlikely(!oevent)) {
+	group->overflow_event = fanotify_alloc_overflow_event();
+	if (unlikely(!group->overflow_event)) {
 		fd = -ENOMEM;
 		goto out_destroy_group;
 	}
-	group->overflow_event = &oevent->fse;
 
 	if (force_o_largefile())
 		event_f_flags |= O_LARGEFILE;