fanotify: copy event fid info to user

If group requested FAN_REPORT_FID and event has file identifier,
copy that information to user reading the event after event metadata.

fid information is formatted as struct fanotify_event_info_fid
that includes a generic header struct fanotify_event_info_header,
so that other info types could be defined in the future using the
same header.

metadata->event_len includes the length of the fid information.

The fid information includes the filesystem's fsid (see statfs(2))
followed by an NFS file handle of the file that could be passed as
an argument to open_by_handle_at(2).

Cc: <linux-api@vger.kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify.h

@@ -95,6 +95,11 @@ static inline bool fanotify_event_has_ext_fh(struct fanotify_event *event)
 		event->fh_len > FANOTIFY_INLINE_FH_LEN;
 }
 
+static inline void *fanotify_event_fh(struct fanotify_event *event)
+{
+	return fanotify_fid_fh(&event->fid, event->fh_len);
+}
+
 /*
  * Structure for permission fanotify events. It gets allocated and freed in
  * fanotify_handle_event() since we wait there for user response. When the

fs/notify/fanotify/fanotify_user.c

@@ -47,6 +47,18 @@ struct kmem_cache *fanotify_mark_cache __read_mostly;
 struct kmem_cache *fanotify_event_cachep __read_mostly;
 struct kmem_cache *fanotify_perm_event_cachep __read_mostly;
 
+#define FANOTIFY_EVENT_ALIGN 4
+
+static int fanotify_event_info_len(struct fanotify_event *event)
+{
+	if (!fanotify_event_has_fid(event))
+		return 0;
+
+	return roundup(sizeof(struct fanotify_event_info_fid) +
+		       sizeof(struct file_handle) + event->fh_len,
+		       FANOTIFY_EVENT_ALIGN);
+}
+
 /*
  * Get an fsnotify notification event if one exists and is small
  * enough to fit in "count". Return an error pointer if the count
@@ -57,6 +69,9 @@ struct kmem_cache *fanotify_perm_event_cachep __read_mostly;
 static struct fsnotify_event *get_one_event(struct fsnotify_group *group,
 					    size_t count)
 {
+	size_t event_size = FAN_EVENT_METADATA_LEN;
+	struct fanotify_event *event;
+
 	assert_spin_locked(&group->notification_lock);
 
 	pr_debug("%s: group=%p count=%zd\n", __func__, group, count);
@@ -64,11 +79,18 @@ static struct fsnotify_event *get_one_event(struct fsnotify_group *group,
 	if (fsnotify_notify_queue_is_empty(group))
 		return NULL;
 
-	if (FAN_EVENT_METADATA_LEN > count)
+	if (FAN_GROUP_FLAG(group, FAN_REPORT_FID)) {
+		event = FANOTIFY_E(fsnotify_peek_first_event(group));
+		event_size += fanotify_event_info_len(event);
+	}
+
+	if (event_size > count)
 		return ERR_PTR(-EINVAL);
 
-	/* held the notification_lock the whole time, so this is the
-	 * same event we peeked above */
+	/*
+	 * Held the notification_lock the whole time, so this is the
+	 * same event we peeked above
+	 */
 	return fsnotify_remove_first_event(group);
 }
 
@@ -174,6 +196,48 @@ static int process_access_response(struct fsnotify_group *group,
 	return 0;
 }
 
+static int copy_fid_to_user(struct fanotify_event *event, char __user *buf)
+{
+	struct fanotify_event_info_fid info = { };
+	struct file_handle handle = { };
+	size_t fh_len = event->fh_len;
+	size_t len = fanotify_event_info_len(event);
+
+	if (!len)
+		return 0;
+
+	if (WARN_ON_ONCE(len < sizeof(info) + sizeof(handle) + fh_len))
+		return -EFAULT;
+
+	/* Copy event info fid header followed by vaiable sized file handle */
+	info.hdr.info_type = FAN_EVENT_INFO_TYPE_FID;
+	info.hdr.len = len;
+	info.fsid = event->fid.fsid;
+	if (copy_to_user(buf, &info, sizeof(info)))
+		return -EFAULT;
+
+	buf += sizeof(info);
+	len -= sizeof(info);
+	handle.handle_type = event->fh_type;
+	handle.handle_bytes = fh_len;
+	if (copy_to_user(buf, &handle, sizeof(handle)))
+		return -EFAULT;
+
+	buf += sizeof(handle);
+	len -= sizeof(handle);
+	if (copy_to_user(buf, fanotify_event_fh(event), fh_len))
+		return -EFAULT;
+
+	/* Pad with 0's */
+	buf += fh_len;
+	len -= fh_len;
+	WARN_ON_ONCE(len < 0 || len >= FANOTIFY_EVENT_ALIGN);
+	if (len > 0 && clear_user(buf, len))
+		return -EFAULT;
+
+	return 0;
+}
+
 static ssize_t copy_event_to_user(struct fsnotify_group *group,
 				  struct fsnotify_event *fsn_event,
 				  char __user *buf, size_t count)
@@ -197,6 +261,8 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		fd = create_fd(group, event, &f);
 		if (fd < 0)
 			return fd;
+	} else if (fanotify_event_has_fid(event)) {
+		metadata.event_len += fanotify_event_info_len(event);
 	}
 	metadata.fd = fd;
 
@@ -208,14 +274,20 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	if (WARN_ON_ONCE(metadata.event_len > count))
 		goto out_close_fd;
 
-	if (copy_to_user(buf, &metadata, metadata.event_len))
+	if (copy_to_user(buf, &metadata, FAN_EVENT_METADATA_LEN))
 		goto out_close_fd;
 
 	if (fanotify_is_perm_event(event->mask))
 		FANOTIFY_PE(fsn_event)->fd = fd;
 
-	if (fd != FAN_NOFD)
+	if (fanotify_event_has_path(event)) {
 		fd_install(fd, f);
+	} else if (fanotify_event_has_fid(event)) {
+		ret = copy_fid_to_user(event, buf + FAN_EVENT_METADATA_LEN);
+		if (ret < 0)
+			return ret;
+	}
+
 	return metadata.event_len;
 
 out_close_fd:

include/uapi/linux/fanotify.h

@@ -107,6 +107,26 @@ struct fanotify_event_metadata {
 	__s32 pid;
 };
 
+#define FAN_EVENT_INFO_TYPE_FID		1
+
+/* Variable length info record following event metadata */
+struct fanotify_event_info_header {
+	__u8 info_type;
+	__u8 pad;
+	__u16 len;
+};
+
+/* Unique file identifier info record */
+struct fanotify_event_info_fid {
+	struct fanotify_event_info_header hdr;
+	__kernel_fsid_t fsid;
+	/*
+	 * Following is an opaque struct file_handle that can be passed as
+	 * an argument to open_by_handle_at(2).
+	 */
+	unsigned char handle[0];
+};
+
 struct fanotify_response {
 	__s32 fd;
 	__u32 response;