+
+
+#define PATH_ARR_SIZE 5
+/*
+ * These are the number paths of length 1 to 5, that we are allowing to emanate
+ * from a single file of interest. For example, we allow 1000 paths of length
+ * 1, to emanate from each file of interest. This essentially represents the
+ * potential wakeup paths, which need to be limited in order to avoid massive
+ * uncontrolled wakeup storms. The common use case should be a single ep which
+ * is connected to n file sources. In this case each file source has 1 path
+ * of length 1. Thus, the numbers below should be more than sufficient. These
+ * path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
+ * and delete can't add additional paths. Protected by the epmutex.
+ */
+static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
+static int path_count[PATH_ARR_SIZE];
+
+static int path_count_inc(int nests)
+{
+ /* Allow an arbitrary number of depth 1 paths */
+ if (nests == 0)
+ return 0;
+
+ if (++path_count[nests] > path_limits[nests])
+ return -1;
+ return 0;
+}
+
+static void path_count_init(void)
+{
+ int i;
+
+ for (i = 0; i < PATH_ARR_SIZE; i++)
+ path_count[i] = 0;
+}
+
+static int reverse_path_check_proc(void *priv, void *cookie, int call_nests)
+{
+ int error = 0;
+ struct file *file = priv;
+ struct file *child_file;
+ struct epitem *epi;
+
+ list_for_each_entry(epi, &file->f_ep_links, fllink) {
+ child_file = epi->ep->file;
+ if (is_file_epoll(child_file)) {
+ if (list_empty(&child_file->f_ep_links)) {
+ if (path_count_inc(call_nests)) {
+ error = -1;
+ break;
+ }
+ } else {
+ error = ep_call_nested(&poll_loop_ncalls,
+ EP_MAX_NESTS,
+ reverse_path_check_proc,
+ child_file, child_file,
+ current);
+ }
+ if (error != 0)
+ break;
+ } else {
+ printk(KERN_ERR "reverse_path_check_proc: "
+ "file is not an ep!\n");
+ }
+ }
+ return error;
+}
+
+/**
+ * reverse_path_check - The tfile_check_list is list of file *, which have
+ * links that are proposed to be newly added. We need to
+ * make sure that those added links don't add too many
+ * paths such that we will spend all our time waking up
+ * eventpoll objects.
+ *
+ * Returns: Returns zero if the proposed links don't create too many paths,
+ * -1 otherwise.
+ */
+static int reverse_path_check(void)
+{
+ int length = 0;
+ int error = 0;
+ struct file *current_file;
+
+ /* let's call this for all tfiles */
+ list_for_each_entry(current_file, &tfile_check_list, f_tfile_llink) {
+ length++;
+ path_count_init();
+ error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+ reverse_path_check_proc, current_file,
+ current_file, current);
+ if (error)
+ break;
+ }
+ return error;
+}
+