Drop one dirent reference after referenced by file

When pipe is created, a dirent of pipe will be
created and its initial reference is set as 0.
Cause all dirent will only be destroyed when
the reference decreased to -1, so there is already
a 'initial reference' of dirent after it created.
For destroying dirent after all reference released,
the correct way is to drop the 'initial reference'
once someone hold a reference to the dirent, such
as fs.NewFile, otherwise the reference of dirent
will stay 0 all the time, and will cause memory
leak of dirent.
Except pipe, timerfd/eventfd/epoll has the same
problem

Here is a simple case to create memory leak of dirent
for pipe/timerfd/eventfd/epoll in C langange, after
run the case, pprof the runsc process, you will
find lots dirents of pipe/timerfd/eventfd/epoll not
freed:

int main(int argc, char *argv[])
{
	int i;
	int n;
	int pipefd[2];

	if (argc != 3) {
		printf("Usage: %s epoll|timerfd|eventfd|pipe <iterations>\n", argv[0]);
	}

	n = strtol(argv[2], NULL, 10);

	if (strcmp(argv[1], "epoll") == 0) {
		for (i = 0; i < n; ++i)
			close(epoll_create(1));
	} else if (strcmp(argv[1], "timerfd") == 0) {
		for (i = 0; i < n; ++i)
			close(timerfd_create(CLOCK_REALTIME, 0));
	} else if (strcmp(argv[1], "eventfd") == 0) {
		for (i = 0; i < n; ++i)
			close(eventfd(0, 0));
	} else if (strcmp(argv[1], "pipe") == 0) {
		for (i = 0; i < n; ++i)
			if (pipe(pipefd) == 0) {
				close(pipefd[0]);
				close(pipefd[1]);
			}
	}

	printf("%s %s test finished\r\n",argv[1],argv[2]);
	return 0;
}

Change-Id: Ia1b8a1fb9142edb00c040e44ec644d007f81f5d2
PiperOrigin-RevId: 251531096

pkg/sentry/fs/timerfd/timerfd.go

@@ -54,6 +54,8 @@ type TimerOperations struct {
 // NewFile returns a timerfd File that receives time from c.
 func NewFile(ctx context.Context, c ktime.Clock) *fs.File {
 	dirent := fs.NewDirent(anon.NewInode(ctx), "anon_inode:[timerfd]")
+	// Release the initial dirent reference after NewFile takes a reference.
+	defer dirent.DecRef()
 	tops := &TimerOperations{}
 	tops.timer = ktime.NewTimer(c, tops)
 	// Timerfds reject writes, but the Write flag must be set in order to

pkg/sentry/kernel/epoll/epoll.go

@@ -156,6 +156,8 @@ var cycleMu sync.Mutex
 func NewEventPoll(ctx context.Context) *fs.File {
 	// name matches fs/eventpoll.c:epoll_create1.
 	dirent := fs.NewDirent(anon.NewInode(ctx), fmt.Sprintf("anon_inode:[eventpoll]"))
+	// Release the initial dirent reference after NewFile takes a reference.
+	defer dirent.DecRef()
 	return fs.NewFile(ctx, dirent, fs.FileFlags{}, &EventPoll{
 		files: make(map[FileIdentifier]*pollEntry),
 	})

pkg/sentry/kernel/eventfd/eventfd.go

@@ -69,6 +69,8 @@ type EventOperations struct {
 func New(ctx context.Context, initVal uint64, semMode bool) *fs.File {
 	// name matches fs/eventfd.c:eventfd_file_create.
 	dirent := fs.NewDirent(anon.NewInode(ctx), "anon_inode:[eventfd]")
+	// Release the initial dirent reference after NewFile takes a reference.
+	defer dirent.DecRef()
 	return fs.NewFile(ctx, dirent, fs.FileFlags{Read: true, Write: true}, &EventOperations{
 		val:     initVal,
 		semMode: semMode,