epoll: limit paths

The current epoll code can be tickled to run basically indefinitely in
both loop detection path check (on ep_insert()), and in the wakeup paths.
The programs that tickle this behavior set up deeply linked networks of
epoll file descriptors that cause the epoll algorithms to traverse them
indefinitely. A couple of these sample programs have been previously
posted in this thread: https://lkml.org/lkml/2011/2/25/297.

To fix the loop detection path check algorithms, I simply keep track of
the epoll nodes that have been already visited. Thus, the loop detection
becomes proportional to the number of epoll file descriptor and links.
This dramatically decreases the run-time of the loop check algorithm. In
one diabolical case I tried it reduced the run-time from 15 mintues (all
in kernel time) to .3 seconds.

Fixing the wakeup paths could be done at wakeup time in a similar manner
by keeping track of nodes that have already been visited, but the
complexity is harder, since there can be multiple wakeups on different
cpus...Thus, I've opted to limit the number of possible wakeup paths when
the paths are created.

This is accomplished, by noting that the end file descriptor points that
are found during the loop detection pass (from the newly added link), are
actually the sources for wakeup events. I keep a list of these file
descriptors and limit the number and length of these paths that emanate
from these 'source file descriptors'. In the current implemetation I
allow 1000 paths of length 1, 500 of length 2, 100 of length 3, 50 of
length 4 and 10 of length 5. Note that it is sufficient to check the
'source file descriptors' reachable from the newly added link, since no
other 'source file descriptors' will have newly added links. This allows
us to check only the wakeup paths that may have gotten too long, and not
re-check all possible wakeup paths on the system.

In terms of the path limit selection, I think its first worth noting that
the most common case for epoll, is probably the model where you have 1
epoll file descriptor that is monitoring n number of 'source file
descriptors'. In this case, each 'source file descriptor' has a 1 path of
length 1. Thus, I believe that the limits I'm proposing are quite
reasonable and in fact may be too generous. Thus, I'm hoping that the
proposed limits will not prevent any workloads that currently work to
fail.

In terms of locking, I have extended the use of the 'epmutex' to all
epoll_ctl add and remove operations. Currently its only used in a subset
of the add paths. I need to hold the epmutex, so that we can correctly
traverse a coherent graph, to check the number of paths. I believe that
this additional locking is probably ok, since its in the setup/teardown
paths, and doesn't affect the running paths, but it certainly is going to
add some extra overhead. Also, worth noting is that the epmuex was
recently added to the ep_ctl add operations in the initial path loop
detection code using the argument that it was not on a critical path.

Another thing to note here, is the length of epoll chains that is allowed.
Currently, eventpoll.c defines:

/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4

This basically means that I am limited to a graph depth of 5 (EP_MAX_NESTS
+ 1). However, this limit is currently only enforced during the loop
check detection code, and only when the epoll file descriptors are added
in a certain order. Thus, this limit is currently easily bypassed. The
newly added check for wakeup paths, stricly limits the wakeup paths to a
length of 5, regardless of the order in which ep's are linked together.
Thus, a side-effect of the new code is a more consistent enforcement of
the graph depth.

Thus far, I've tested this, using the sample programs previously
mentioned, which now either return quickly or return -EINVAL. I've also
testing using the piptest.c epoll tester, which showed no difference in
performance. I've also created a number of different epoll networks and
tested that they behave as expectded.

I believe this solves the original diabolical test cases, while still
preserving the sane epoll nesting.

Signed-off-by: Jason Baron <[e-mail]>
Cc: Nelson Elhage <[e-mail]>
Cc: Davide Libenzi <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/eventpoll.c

···
 
 	/* The user that created the eventpoll descriptor */
 	struct user_struct *user;
+
+	struct file *file;
+
+	/* used to optimize loop detection check */
+	int visited;
+	struct list_head visited_list_link;
 };
 
 /* Wait structure used by the poll hooks */
···
 /* Slab cache used to allocate "struct eppoll_entry" */
 static struct kmem_cache *pwq_cache __read_mostly;
 
+/* Visited nodes during ep_loop_check(), so we can unset them when we finish */
+static LIST_HEAD(visited_list);
+
+/*
+ * List of files with newly added links, where we may need to limit the number
+ * of emanating paths. Protected by the epmutex.
+ */
+static LIST_HEAD(tfile_check_list);
+
 #ifdef CONFIG_SYSCTL
 
 #include <linux/sysctl.h>
···
 };
 #endif /* CONFIG_SYSCTL */
 
+static const struct file_operations eventpoll_fops;
+
+static inline int is_file_epoll(struct file *f)
+{
+	return f->f_op == &eventpoll_fops;
+}
 
 /* Setup the structure that is used as key for the RB tree */
 static inline void ep_set_ffd(struct epoll_filefd *ffd,
···
 	.llseek		= noop_llseek,
 };
 
-/* Fast test to see if the file is an eventpoll file */
-static inline int is_file_epoll(struct file *f)
-{
-	return f->f_op == &eventpoll_fops;
-}
-
 /*
  * This is called from eventpoll_release() to unlink files from the eventpoll
  * interface. We need to have this facility to cleanup correctly files that are
···
 	rb_insert_color(&epi->rbn, &ep->rbr);
 }
 
+
+
+#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)
+{
+	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;
+}
+
 /*
  * Must be called with "mtx" held.
  */
···
 	 */
 	ep_rbtree_insert(ep, epi);
 
+	/* now check if we've created too many backpaths */
+	error = -EINVAL;
+	if (reverse_path_check())
+		goto error_remove_epi;
+
 	/* We have to drop the new item inside our item list to keep track of it */
 	spin_lock_irqsave(&ep->lock, flags);
 
···
 		ep_poll_safewake(&ep->poll_wait);
 
 	return 0;
+
+error_remove_epi:
+	spin_lock(&tfile->f_lock);
+	if (ep_is_linked(&epi->fllink))
+		list_del_init(&epi->fllink);
+	spin_unlock(&tfile->f_lock);
+
+	rb_erase(&epi->rbn, &ep->rbr);
 
 error_unregister:
 	ep_unregister_pollwait(ep, epi);
···
 	int error = 0;
 	struct file *file = priv;
 	struct eventpoll *ep = file->private_data;
+	struct eventpoll *ep_tovisit;
 	struct rb_node *rbp;
 	struct epitem *epi;
 
 	mutex_lock_nested(&ep->mtx, call_nests + 1);
+	ep->visited = 1;
+	list_add(&ep->visited_list_link, &visited_list);
 	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		if (unlikely(is_file_epoll(epi->ffd.file))) {
+			ep_tovisit = epi->ffd.file->private_data;
+			if (ep_tovisit->visited)
+				continue;
 			error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
-					       ep_loop_check_proc, epi->ffd.file,
-					       epi->ffd.file->private_data, current);
+					ep_loop_check_proc, epi->ffd.file,
+					ep_tovisit, current);
 			if (error != 0)
 				break;
+		} else {
+			/*
+			 * If we've reached a file that is not associated with
+			 * an ep, then we need to check if the newly added
+			 * links are going to add too many wakeup paths. We do
+			 * this by adding it to the tfile_check_list, if it's
+			 * not already there, and calling reverse_path_check()
+			 * during ep_insert().
+			 */
+			if (list_empty(&epi->ffd.file->f_tfile_llink))
+				list_add(&epi->ffd.file->f_tfile_llink,
+					 &tfile_check_list);
 		}
 	}
 	mutex_unlock(&ep->mtx);
···
  */
 static int ep_loop_check(struct eventpoll *ep, struct file *file)
 {
-	return ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+	int ret;
+	struct eventpoll *ep_cur, *ep_next;
+
+	ret = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
 			      ep_loop_check_proc, file, ep, current);
+	/* clear visited list */
+	list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
+							visited_list_link) {
+		ep_cur->visited = 0;
+		list_del(&ep_cur->visited_list_link);
+	}
+	return ret;
+}
+
+static void clear_tfile_check_list(void)
+{
+	struct file *file;
+
+	/* first clear the tfile_check_list */
+	while (!list_empty(&tfile_check_list)) {
+		file = list_first_entry(&tfile_check_list, struct file,
+					f_tfile_llink);
+		list_del_init(&file->f_tfile_llink);
+	}
+	INIT_LIST_HEAD(&tfile_check_list);
 }
 
 /*
···
  */
 SYSCALL_DEFINE1(epoll_create1, int, flags)
 {
-	int error;
+	int error, fd;
 	struct eventpoll *ep = NULL;
+	struct file *file;
 
 	/* Check the EPOLL_* constant for consistency.  */
 	BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
···
 	 * Creates all the items needed to setup an eventpoll file. That is,
 	 * a file structure and a free file descriptor.
 	 */
-	error = anon_inode_getfd("[eventpoll]", &eventpoll_fops, ep,
+	fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
+	if (fd < 0) {
+		error = fd;
+		goto out_free_ep;
+	}
+	file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
 				 O_RDWR | (flags & O_CLOEXEC));
-	if (error < 0)
-		ep_free(ep);
+	if (IS_ERR(file)) {
+		error = PTR_ERR(file);
+		goto out_free_fd;
+	}
+	fd_install(fd, file);
+	ep->file = file;
+	return fd;
 
+out_free_fd:
+	put_unused_fd(fd);
+out_free_ep:
+	ep_free(ep);
 	return error;
 }
 
···
 	/*
 	 * When we insert an epoll file descriptor, inside another epoll file
 	 * descriptor, there is the change of creating closed loops, which are
-	 * better be handled here, than in more critical paths.
+	 * better be handled here, than in more critical paths. While we are
+	 * checking for loops we also determine the list of files reachable
+	 * and hang them on the tfile_check_list, so we can check that we
+	 * haven't created too many possible wakeup paths.
 	 *
-	 * We hold epmutex across the loop check and the insert in this case, in
-	 * order to prevent two separate inserts from racing and each doing the
-	 * insert "at the same time" such that ep_loop_check passes on both
-	 * before either one does the insert, thereby creating a cycle.
+	 * We need to hold the epmutex across both ep_insert and ep_remove
+	 * b/c we want to make sure we are looking at a coherent view of
+	 * epoll network.
 	 */
-	if (unlikely(is_file_epoll(tfile) && op == EPOLL_CTL_ADD)) {
+	if (op == EPOLL_CTL_ADD || op == EPOLL_CTL_DEL) {
 		mutex_lock(&epmutex);
 		did_lock_epmutex = 1;
-		error = -ELOOP;
-		if (ep_loop_check(ep, tfile) != 0)
-			goto error_tgt_fput;
 	}
-
+	if (op == EPOLL_CTL_ADD) {
+		if (is_file_epoll(tfile)) {
+			error = -ELOOP;
+			if (ep_loop_check(ep, tfile) != 0)
+				goto error_tgt_fput;
+		} else
+			list_add(&tfile->f_tfile_llink, &tfile_check_list);
+	}
 
 	mutex_lock_nested(&ep->mtx, 0);
 
···
 			error = ep_insert(ep, &epds, tfile, fd);
 		} else
 			error = -EEXIST;
+		clear_tfile_check_list();
 		break;
 	case EPOLL_CTL_DEL:
 		if (epi)
···
 	mutex_unlock(&ep->mtx);
 
 error_tgt_fput:
-	if (unlikely(did_lock_epmutex))
+	if (did_lock_epmutex)
 		mutex_unlock(&epmutex);
 
 	fput(tfile);

include/linux/eventpoll.h

···
 static inline void eventpoll_init_file(struct file *file)
 {
 	INIT_LIST_HEAD(&file->f_ep_links);
+	INIT_LIST_HEAD(&file->f_tfile_llink);
 }
 
 

include/linux/fs.h

···
 #ifdef CONFIG_EPOLL
 	/* Used by fs/eventpoll.c to link all the hooks to this file */
 	struct list_head	f_ep_links;
+	struct list_head	f_tfile_llink;
 #endif /* #ifdef CONFIG_EPOLL */
 	struct address_space	*f_mapping;
 #ifdef CONFIG_DEBUG_WRITECOUNT