[RFC][PATCH] fanotify: register event pid for pidfd reporting

[AnonymeMeow <anonymemeow@gmail.com>]

pidfd_prepare() may still fail if userspace reads the event after the
task has been reaped and the pid has not been registered with pidfs.
Register the event pid with pidfs when creating the fanotify event if
pidfd reporting was requested, so pidfd_prepare() can later create a
pidfd for the reaped task.

Signed-off-by: AnonymeMeow <anonymemeow@gmail.com>
---

Hi, Christian,

I tested this patch set and found that fanotify still usually can not
report pidfds for reaped tasks, because the event pid may not have
been registered before it exits, and registering it after the task
has been reaped is not allowed, so fanotify still fails to report a
pidfd for the event.

This patch registers the event pid when the fanotify event is created.
With this change, the current LTP fanotify21 test fails as expected.

But I do not know whether it is worth doing so to improve fanotify's
ability to report pidfds for reaped tasks. And, what should fanotify
do if pidfs_register_pid() fails? Should the error be reported to
userspace through the pidfd info record, using the raw error when
FAN_REPORT_FD_ERROR is set and FAN_EPIDFD otherwise? But the event
struct in fanotify does not have spare space to store this error code.
Or should it just ignore the failure here and let pidfd_prepare()
fail later in copy_event_to_user()? I would appreciate your feedback.

With Best Regards,
AnonymeMeow

Link: https://lore.kernel.org/lkml/20260528-schmuckvoll-heilen-garen-be77b4208671@brauner/

---
 fs/notify/fanotify/fanotify.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 38290b9c07f7..24b2af397126 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -14,6 +14,7 @@
 #include <linux/sched/mm.h>
 #include <linux/statfs.h>
 #include <linux/stringhash.h>
+#include <linux/pidfs.h>
 
 #include "fanotify.h"
 
@@ -868,6 +869,15 @@ static struct fanotify_event *fanotify_alloc_event(
 	else
 		pid = get_pid(task_tgid(current));
 
+	if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD)) {
+		int err = pidfs_register_pid(pid);
+		if (err) {
+			/*
+			 * What to do here? Pass this err to userspace via pidfd?
+			 */
+		}
+	}
+
 	/* Mix event info, FAN_ONDIR flag and pid into event merge key */
 	hash ^= hash_long((unsigned long)pid | ondir, FANOTIFY_EVENT_HASH_BITS);
 	fanotify_init_event(event, hash, mask);
-- 
2.54.0