Do not generate extraneous IN_CLOSE inotify events.

IN_CLOSE should only be generated when a file description loses its last
reference; not when a file descriptor is closed.

See fs/file_table.c:__fput.

Updates #5348.

PiperOrigin-RevId: 353810697

pkg/sentry/kernel/fd_table.go

@@ -159,13 +159,6 @@ func (f *FDTable) dropVFS2(ctx context.Context, file *vfs.FileDescription) {
 		panic(fmt.Sprintf("UnlockPOSIX failed: %v", err))
 	}
 
-	// Generate inotify events.
-	ev := uint32(linux.IN_CLOSE_NOWRITE)
-	if file.IsWritable() {
-		ev = linux.IN_CLOSE_WRITE
-	}
-	file.Dentry().InotifyWithParent(ctx, ev, 0, vfs.PathEvent)
-
 	// Drop the table's reference.
 	file.DecRef(ctx)
 }

pkg/sentry/vfs/file_description.go

@@ -161,6 +161,13 @@ func (fd *FileDescription) Init(impl FileDescriptionImpl, flags uint32, mnt *Mou
 // DecRef decrements fd's reference count.
 func (fd *FileDescription) DecRef(ctx context.Context) {
 	fd.FileDescriptionRefs.DecRef(func() {
+		// Generate inotify events.
+		ev := uint32(linux.IN_CLOSE_NOWRITE)
+		if fd.IsWritable() {
+			ev = linux.IN_CLOSE_WRITE
+		}
+		fd.Dentry().InotifyWithParent(ctx, ev, 0, PathEvent)
+
 		// Unregister fd from all epoll instances.
 		fd.epollMu.Lock()
 		epolls := fd.epolls

test/syscalls/linux/inotify.cc

@@ -782,6 +782,38 @@ TEST(Inotify, MoveWatchedTargetGeneratesEvents) {
   EXPECT_EQ(events[0].cookie, events[1].cookie);
 }
 
+// Tests that close events are only emitted when a file description drops its
+// last reference.
+TEST(Inotify, DupFD) {
+  SKIP_IF(IsRunningWithVFS1());
+
+  const TempPath file = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFile());
+  const FileDescriptor inotify_fd =
+      ASSERT_NO_ERRNO_AND_VALUE(InotifyInit1(IN_NONBLOCK));
+
+  const int wd = ASSERT_NO_ERRNO_AND_VALUE(
+      InotifyAddWatch(inotify_fd.get(), file.path(), IN_ALL_EVENTS));
+
+  FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(Open(file.path(), O_RDONLY));
+  FileDescriptor fd2 = ASSERT_NO_ERRNO_AND_VALUE(fd.Dup());
+
+  std::vector<Event> events =
+      ASSERT_NO_ERRNO_AND_VALUE(DrainEvents(inotify_fd.get()));
+  EXPECT_THAT(events, Are({
+                          Event(IN_OPEN, wd),
+                      }));
+
+  fd.reset();
+  events = ASSERT_NO_ERRNO_AND_VALUE(DrainEvents(inotify_fd.get()));
+  EXPECT_THAT(events, Are({}));
+
+  fd2.reset();
+  events = ASSERT_NO_ERRNO_AND_VALUE(DrainEvents(inotify_fd.get()));
+  EXPECT_THAT(events, Are({
+                          Event(IN_CLOSE_NOWRITE, wd),
+                      }));
+}
+
 TEST(Inotify, CoalesceEvents) {
   const TempPath root = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
   const FileDescriptor fd =