On Sun, 6 Feb 2011, Nelson Elhage wrote:
> Suppose thread A does epoll_ctl(e1, EPOLL_CTL_ADD, e2, evt) concurrently with
> thread B doing epoll_ctl(e2, EPOLL_CTL_ADD, e1, evt).
>
> Since you do the recursive scan before grabbing ep->mtx, I think there is
> nothing to prevent the two scans from both succeeding, followed by the two
> inserts, so you end up with a cycle anyways.
Good point!
> One possibility is grabbing epmutex, or another global mutex, for both the scan
> and the duration of inserting one epoll fd onto another. That's really
> heavyweight, but maybe inserting an epoll fd onto another epoll fd is rare
> enough that it's Ok.
Yes, that'd be fine, since that is not a fast path.
Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
- Davide
---
fs/eventpoll.c | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 94 insertions(+), 2 deletions(-)
Index: linux-2.6.mod/fs/eventpoll.c
===================================================================
--- linux-2.6.mod.orig/fs/eventpoll.c 2011-02-06 11:42:38.000000000 -0800
+++ linux-2.6.mod/fs/eventpoll.c 2011-02-06 15:17:25.000000000 -0800
@@ -224,6 +224,9 @@
*/
static DEFINE_MUTEX(epmutex);
+/* Used to check for epoll file descriptor inclusion loops */
+static struct nested_calls poll_loop_ncalls;
+
/* Used for safe wake up implementation */
static struct nested_calls poll_safewake_ncalls;
@@ -592,7 +595,6 @@
*/
for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
epi = rb_entry(rbp, struct epitem, rbn);
-
ep_unregister_pollwait(ep, epi);
}
@@ -711,7 +713,6 @@
while (!list_empty(lsthead)) {
epi = list_first_entry(lsthead, struct epitem, fllink);
-
ep = epi->ep;
list_del_init(&epi->fllink);
mutex_lock(&ep->mtx);
@@ -1198,6 +1199,82 @@
return res;
}
+/**
+ * ep_loop_check_proc - Callback function to be passed to the @ep_call_nested()
+ * API, to verify that adding an epoll file inside another
+ * epoll structure, does not violate the constraints, in
+ * terms of closed loops, or too deep chains (which can
+ * result in excessive stack usage).
+ *
+ * @priv: Pointer to the epoll file to be currently checked.
+ * @cookie: Original cookie for this call. This is the top-of-the-chain epoll
+ * data structure pointer.
+ * @call_nests: Current dept of the @ep_call_nested() call stack.
+ *
+ * Returns: Returns zero if adding the epoll @file inside current epoll
+ * structure @ep does not violate the constraints, or -1 otherwise.
+ */
+static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
+{
+ int error = 0;
+ struct file *file = priv;
+ struct eventpoll *ep = file->private_data;
+ struct rb_node *rbp;
+ struct epitem *epi;
+
+ mutex_lock(&ep->mtx);
+ 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))) {
+ error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+ ep_loop_check_proc, epi->ffd.file,
+ ep, current);
+ if (error != 0)
+ break;
+ }
+ }
+ mutex_unlock(&ep->mtx);
+
+ return error;
+}
+
+/**
+ * ep_loop_check - Performs a check to verify that adding an epoll file (@file)
+ * another epoll file (represented by @ep) does not create
+ * closed loops or too deep chains.
+ *
+ * @ep: Pointer to the epoll private data structure.
+ * @file: Pointer to the epoll file to be checked.
+ *
+ * Returns: Returns zero if adding the epoll @file inside current epoll
+ * structure @ep does not violate the constraints, or -1 otherwise.
+ */
+static int ep_loop_check(struct eventpoll *ep, struct file *file)
+{
+ int error;
+
+ /*
+ * It needs to grab the @epmutex lock, before doing this.
+ * This because two concurrent @ep_loop_check() might be going on,
+ * resulting in a closed loop due to both @ep_loop_check() functions
+ * succeeding:
+ *
+ * e1 = epoll_create();
+ * e2 = epoll_create();
+ *
+ * THREAD-0 THREAD-1
+ *
+ * epoll_ctl(e1, EPOLL_CTL_ADD, e2) epoll_ctl(e2, EPOLL_CTL_ADD, e1)
+ *
+ */
+ mutex_lock(&epmutex);
+ error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+ ep_loop_check_proc, file, ep, current);
+ mutex_unlock(&epmutex);
+
+ return error;
+}
+
/*
* Open an eventpoll file descriptor.
*/
@@ -1287,6 +1364,15 @@
*/
ep = file->private_data;
+ /*
+ * 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.
+ */
+ if (unlikely(is_file_epoll(tfile) && op == EPOLL_CTL_ADD &&
+ ep_loop_check(ep, tfile) != 0))
+ goto error_tgt_fput;
+
mutex_lock(&ep->mtx);
/*
@@ -1441,6 +1527,12 @@
EP_ITEM_COST;
BUG_ON(max_user_watches < 0);
+ /*
+ * Initialize the structure used to perform epoll file descriptor
+ * inclusion loops checks.
+ */
+ ep_nested_calls_init(&poll_loop_ncalls);
+
/* Initialize the structure used to perform safe poll wait head wake ups */
ep_nested_calls_init(&poll_safewake_ncalls);
--
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