[PATCH v3] fanotify.7, fanotify_mark.2: Document FAN_FS_ERROR

@@ -214,6 +214,35 @@ Create an event when a marked file or directory itself is deleted.
 An fanotify group that identifies filesystem objects by file handles
 is required.
 .TP
+.BR FAN_FS_ERROR " (since Linux 5.15)"
+.\" commit 9709bd548f11a092d124698118013f66e1740f9b
+Create an event when a filesystem error is detected.
+Support for this notification type
+is dependent on the underlying filesystem.
+.IP
+The filesystems that currently support
+.BR FAN_FS_ERROR
+are:
+.RS
+.IP "* ext4 "
+.RE
+.IP
+A user can also expect to receive additional information
+in the form of a
+.BR FAN_EVENT_INFO_TYPE_ERROR
+record
+alongside the generic metadata event.
+.IP
+A fanotify group initialized with
+.BR FAN_REPORT_FID
+is required, or the error
+.B EINVAL
+will occur.
+.IP
+See
+.BR fanotify (7)
+for additional details.
+.TP
 .BR FAN_MOVED_FROM " (since Linux 5.1)"
 .\" commit 235328d1fa4251c6dcb32351219bb553a58838d2
 Create an event when a file or directory has been moved from a marked

@@ -389,6 +418,13 @@ and mask contains a flag for permission events
 or
 .BR FAN_ACCESS_PERM ).
 .TP
+.B EINVAL
+The group was initialized without
+.B FAN_REPORT_FID
+but one or more events in
+.I mask
+require it.
+.TP
 .B ENODEV
 The filesystem object indicated by
 .I pathname

@@ -188,6 +188,24 @@ struct fanotify_event_info_fid {
 .EE
 .in
 .PP
+In case of a FAN_FS_ERROR event,
+besides the file handle record,
+an additional record describing the error that occurred
+is included in the read buffer.
+The structure described below follows the generic
+.I fanotify_event_metadata
+structure within the read buffer:
+.PP
+.in +4n
+.EX
+struct fanotify_event_info_error {
+    struct fanotify_event_info_header hdr;
+    __s32 error;
+    __u32 error_count;
+};
+.EE
+.in
+.PP
 For performance reasons, it is recommended to use a large
 buffer size (for example, 4096 bytes),
 so that multiple events can be retrieved by a single

@@ -311,6 +329,9 @@ A child file or directory was deleted in a watched parent.
 .B FAN_DELETE_SELF
 A watched file or directory was deleted.
 .TP
+.B FAN_FS_ERROR
+A filesystem error was detected.
+.TP
 .B FAN_MOVED_FROM
 A file or directory has been moved from a watched parent directory.
 .TP

@@ -510,6 +531,32 @@ and the file handle is followed by a null terminated string that identifies the
 name of a directory entry in that directory, or '.' to identify the directory
 object itself.
 .PP
+The fields of the
+.I fanotify_event_info_error
+structure are as follows:
+.TP
+.I hdr
+This is a structure of type
+.IR fanotify_event_info_header .
+and is a generic header that contains information used to
+describe an additional information record attached to the event.
+For
+.IR fanotify_event_info_error ,
+.I info_type
+will have the value
+.BR FAN_EVENT_INFO_TYPE_ERROR .
+.I len
+has the size of the additional information record including the
+.IR fanotify_event_info_header
+itself.
+.TP
+.I error
+Identifies the type of error that occurred
+.TP
+.I error_count
+This is a counter of the number of errors suppressed
+since the last error was read.
+.PP
 The following macros are provided to iterate over a buffer containing
 fanotify event metadata returned by a
 .BR read (2)

@@ -599,6 +646,30 @@ field.
 In that case, the audit subsystem will log information about the access
 decision to the audit logs.
 .\"
+.SS Monitoring filesystems for errors
+A single FAN_FS_ERROR event is stored per filesystem at once.
+Extra error messages are suppressed and accounted for
+inside the current FAN_FS_ERROR event record,
+but details about the errors are lost.
+.PP
+Error types reported by FAN_FS_ERROR are generic errno values
+but not all kinds of errors types are reported by all filesystems.
+Refer to the filesystem documentation
+for additional information on the type of errors that are reported,
+and the meaning of those errors.
+.PP
+Errors not directly related to a file (i.e. super block corruption)
+are reported with an invalid
+.IR file_handle .
+For these errors, the
+.I file_handle
+will have the field
+.I handle_type
+set to
+.BR FILEID_INVALID ,
+and the handle buffer size set to
+.BR 0 .
+.\"
 .SS Closing the fanotify file descriptor
 When all file descriptors referring to the fanotify notification group are
 closed, the fanotify group is released and its resources