[PATCH] fanotify: report thread pidfds for FAN_REPORT_TID

[AnonymeMeow <anonymemeow@gmail.com>]

The FAN_REPORT_PIDFD and FAN_REPORT_TID flags used to be mutually
exclusive because by the time the pidfd support was introduced to
fanotify, pidfds could only be created for thread group leaders. Now
that the pidfd API supports thread-specific pidfds via PIDFD_THREAD,
this restriction can be lifted.

This patch allows listeners using FAN_REPORT_PIDFD | FAN_REPORT_TID
to receive the pidfd referring to the thread that triggered the
event.

Signed-off-by: AnonymeMeow <anonymemeow@gmail.com>
---
 fs/notify/fanotify/fanotify_user.c | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index ae904451dfc0..ebdd48942029 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -19,6 +19,7 @@
 #include <linux/memcontrol.h>
 #include <linux/statfs.h>
 #include <linux/exportfs.h>
+#include <linux/pidfd.h>
 
 #include <asm/ioctls.h>
 
@@ -903,25 +904,21 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 		metadata.fd = fd >= 0 ? fd : FAN_NOFD;
 
 	if (pidfd_mode) {
-		/*
-		 * Complain if the FAN_REPORT_PIDFD and FAN_REPORT_TID mutual
-		 * exclusion is ever lifted. At the time of incoporating pidfd
-		 * support within fanotify, the pidfd API only supported the
-		 * creation of pidfds for thread-group leaders.
-		 */
-		WARN_ON_ONCE(FAN_GROUP_FLAG(group, FAN_REPORT_TID));
+		unsigned int tid_mode = FAN_GROUP_FLAG(group, FAN_REPORT_TID);
+		enum pid_type pidtype = tid_mode ? PIDTYPE_PID : PIDTYPE_TGID;
+		unsigned int pidfd_flags = tid_mode ? PIDFD_THREAD : 0;
 
 		/*
-		 * The PIDTYPE_TGID check for an event->pid is performed
+		 * The pid_has_task() check for an event->pid is performed
 		 * preemptively in an attempt to catch out cases where the event
-		 * listener reads events after the event generating process has
+		 * listener reads events after the event generating task has
 		 * already terminated.  Depending on flag FAN_REPORT_FD_ERROR,
 		 * report either -ESRCH or FAN_NOPIDFD to the event listener in
 		 * those cases with all other pidfd creation errors reported as
 		 * the error code itself or as FAN_EPIDFD.
 		 */
-		if (metadata.pid && pid_has_task(event->pid, PIDTYPE_TGID))
-			pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
+		if (metadata.pid && pid_has_task(event->pid, pidtype))
+			pidfd = pidfd_prepare(event->pid, pidfd_flags, &pidfd_file);
 
 		if (!FAN_GROUP_FLAG(group, FAN_REPORT_FD_ERROR) && pidfd < 0)
 			pidfd = pidfd == -ESRCH ? FAN_NOPIDFD : FAN_EPIDFD;
@@ -1628,14 +1625,6 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 #endif
 		return -EINVAL;
 
-	/*
-	 * A pidfd can only be returned for a thread-group leader; thus
-	 * FAN_REPORT_PIDFD and FAN_REPORT_TID need to remain mutually
-	 * exclusive.
-	 */
-	if ((flags & FAN_REPORT_PIDFD) && (flags & FAN_REPORT_TID))
-		return -EINVAL;
-
 	/* Don't allow mixing mnt events with inode events for now */
 	if (flags & FAN_REPORT_MNT) {
 		if (class != FAN_CLASS_NOTIF)
-- 
2.54.0