[PATCH] fanotify: self describing event metadata

[Amir Goldstein <amir73il@gmail.com>]

Use the 'reserved' u8 field in event metadata to describe event
optional information.

When event->pid is thread id, set the flag FAN_EVENT_INFO_TID in
event->flags.

Rename the init flag that is used to request reporting thread id
in event to FAN_REPORT_TID.

This change is semantic, because in the only case that user requests
FAN_REPORT_TID and fanotify is not able to meet that request,
event->pid will be zero anyway.

However, for future event info requests, it is better to be explicit
about whether fanotify was able to meet the request, similar to how
statx() API behaves.

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

Jan,

While working on reporting file handles, I realized that the API would
be more solid if the event info flags are bi-directional similar to
statx().

Even if you disapprove of the way this patch modifies the event foramt,
or if you would like to take more time to consider that, I would still
like to rename the fanotify_init flag to FAN_REPORT_TID, becasue I think
the name is better describing. See example man page with new name [1].

I realize that the reserved/flags union is a bit ugly, but I think it
will be less painful than introducing event metadata format v4 at this
time.

What do you thing?

Amir.

[1] https://github.com/amir73il/man-pages/commits/fanotify_tid

 fs/notify/fanotify/fanotify.c      |  2 +-
 fs/notify/fanotify/fanotify_user.c |  4 +++-
 include/linux/fanotify.h           |  2 +-
 include/uapi/linux/fanotify.h      | 10 ++++++++--
 4 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 2c57186caa2e..5769cf3ff035 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -171,7 +171,7 @@ struct fanotify_event_info *fanotify_alloc_event(struct fsnotify_group *group,
 		goto out;
 init: __maybe_unused
 	fsnotify_init_event(&event->fse, inode, mask);
-	if (FAN_GROUP_FLAG(group, FAN_EVENT_INFO_TID))
+	if (FAN_GROUP_FLAG(group, FAN_REPORT_TID))
 		event->pid = get_pid(task_pid(current));
 	else
 		event->pid = get_pid(task_tgid(current));
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index e03be5071362..79c4ba349780 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -130,9 +130,11 @@ static int fill_event_metadata(struct fsnotify_group *group,
 	metadata->event_len = FAN_EVENT_METADATA_LEN;
 	metadata->metadata_len = FAN_EVENT_METADATA_LEN;
 	metadata->vers = FANOTIFY_METADATA_VERSION;
-	metadata->reserved = 0;
+	metadata->flags = 0;
 	metadata->mask = fsn_event->mask & FANOTIFY_OUTGOING_EVENTS;
 	metadata->pid = pid_vnr(event->pid);
+	if (metadata->pid && FAN_GROUP_FLAG(group, FAN_REPORT_TID))
+		metadata->flags |= FAN_EVENT_INFO_TID;
 	if (unlikely(fsn_event->mask & FAN_Q_OVERFLOW))
 		metadata->fd = FAN_NOFD;
 	else {
diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index 11c85bd3d82e..a5a60691e48b 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -19,7 +19,7 @@
 				 FAN_CLASS_PRE_CONTENT)
 
 #define FANOTIFY_INIT_FLAGS	(FANOTIFY_CLASS_BITS | \
-				 FAN_EVENT_INFO_TID | \
+				 FAN_REPORT_TID | \
 				 FAN_CLOEXEC | FAN_NONBLOCK | \
 				 FAN_UNLIMITED_QUEUE | FAN_UNLIMITED_MARKS)
 
diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
index 00b2304ed124..3e2ca5c97892 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h
@@ -41,7 +41,7 @@
 #define FAN_ENABLE_AUDIT	0x00000040
 
 /* Flags to determine fanotify event format */
-#define FAN_EVENT_INFO_TID	0x00000100	/* event->pid is thread id */
+#define FAN_REPORT_TID		0x00000100	/* Report thread id in event */
 
 /* Deprecated - do not use this in programs and do not add new flags here! */
 #define FAN_ALL_INIT_FLAGS	(FAN_CLOEXEC | FAN_NONBLOCK | \
@@ -94,10 +94,16 @@
 
 #define FANOTIFY_METADATA_VERSION	3
 
+/* Flags reported on event->flags */
+#define FAN_EVENT_INFO_TID	0x01	/* event->pid is thread id */
+
 struct fanotify_event_metadata {
 	__u32 event_len;
 	__u8 vers;
-	__u8 reserved;
+	union {
+		__u8 reserved;
+		__u8 flags;
+	};
 	__u16 metadata_len;
 	__aligned_u64 mask;
 	__s32 fd;
-- 
2.17.1