* This mutex is acquired by ep_free() during the epoll file
* cleanup path and it is also acquired by eventpoll_release_file()
* if a file has been pushed inside an epoll set and it is then
- * close()d without a previous call toepoll_ctl(EPOLL_CTL_DEL).
+ * close()d without a previous call to epoll_ctl(EPOLL_CTL_DEL).
+ * It is also acquired when inserting an epoll fd onto another epoll
+ * fd. We do this so that we walk the epoll tree and ensure that this
+ * insertion does not create a cycle of epoll file descriptors, which
+ * could lead to deadlock. We need a global mutex to prevent two
+ * simultaneous inserts (A into B and B into A) from racing and
+ * constructing a cycle without either insert observing that it is
+ * going to.
* It is possible to drop the "ep->mtx" and to use the global
* mutex "epmutex" (together with "ep->lock") to have it working,
* but having "ep->mtx" will make the interface more scalable.
/*
* This structure is stored inside the "private_data" member of the file
- * structure and rapresent the main data sructure for the eventpoll
+ * structure and represents the main data structure for the eventpoll
* interface.
*/
struct eventpoll {
- /* Protect the this structure access */
+ /* Protect the access to this structure */
spinlock_t lock;
/*
*/
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;
spin_lock_init(&ncalls->lock);
}
+/**
+ * ep_events_available - Checks if ready events might be available.
+ *
+ * @ep: Pointer to the eventpoll context.
+ *
+ * Returns: Returns a value different than zero if ready events are available,
+ * or zero otherwise.
+ */
+static inline int ep_events_available(struct eventpoll *ep)
+{
+ return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
+}
+
/**
* ep_call_nested - Perform a bound (possibly) nested call, by checking
* that the recursion limit is not exceeded, and that
/*
* This is the callback that is passed to the wait queue wakeup
- * machanism. It is called by the stored file descriptors when they
+ * mechanism. It is called by the stored file descriptors when they
* have events to report.
*/
static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
goto out_unlock;
/*
- * If we are trasfering events to userspace, we can hold no locks
+ * If we are transferring events to userspace, we can hold no locks
* (because we're accessing user memory, and because of linux f_op->poll()
- * semantics). All the events that happens during that period of time are
+ * semantics). All the events that happen during that period of time are
* chained in ep->ovflist and requeued later on.
*/
if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
return ep_scan_ready_list(ep, ep_send_events_proc, &esed);
}
+static inline struct timespec ep_set_mstimeout(long ms)
+{
+ struct timespec now, ts = {
+ .tv_sec = ms / MSEC_PER_SEC,
+ .tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
+ };
+
+ ktime_get_ts(&now);
+ return timespec_add_safe(now, ts);
+}
+
+/**
+ * ep_poll - Retrieves ready events, and delivers them to the caller supplied
+ * event buffer.
+ *
+ * @ep: Pointer to the eventpoll context.
+ * @events: Pointer to the userspace buffer where the ready events should be
+ * stored.
+ * @maxevents: Size (in terms of number of events) of the caller event buffer.
+ * @timeout: Maximum timeout for the ready events fetch operation, in
+ * milliseconds. If the @timeout is zero, the function will not block,
+ * while if the @timeout is less than zero, the function will block
+ * until at least one event has been retrieved (or an error
+ * occurred).
+ *
+ * Returns: Returns the number of ready events which have been fetched, or an
+ * error code, in case of error.
+ */
static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
int maxevents, long timeout)
{
- int res, eavail, timed_out = 0;
+ int res = 0, eavail, timed_out = 0;
unsigned long flags;
- long slack;
+ long slack = 0;
wait_queue_t wait;
- struct timespec end_time;
ktime_t expires, *to = NULL;
if (timeout > 0) {
- ktime_get_ts(&end_time);
- timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
+ struct timespec end_time = ep_set_mstimeout(timeout);
+
slack = select_estimate_accuracy(&end_time);
to = &expires;
*to = timespec_to_ktime(end_time);
} else if (timeout == 0) {
+ /*
+ * Avoid the unnecessary trip to the wait queue loop, if the
+ * caller specified a non blocking operation.
+ */
timed_out = 1;
+ spin_lock_irqsave(&ep->lock, flags);
+ goto check_events;
}
-retry:
+fetch_events:
spin_lock_irqsave(&ep->lock, flags);
- res = 0;
- if (list_empty(&ep->rdllist)) {
+ if (!ep_events_available(ep)) {
/*
* We don't have any available event to return to the caller.
* We need to sleep here, and we will be wake up by
* to TASK_INTERRUPTIBLE before doing the checks.
*/
set_current_state(TASK_INTERRUPTIBLE);
- if (!list_empty(&ep->rdllist) || timed_out)
+ if (ep_events_available(ep) || timed_out)
break;
if (signal_pending(current)) {
res = -EINTR;
set_current_state(TASK_RUNNING);
}
+check_events:
/* Is it worth to try to dig for events ? */
- eavail = !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
+ eavail = ep_events_available(ep);
spin_unlock_irqrestore(&ep->lock, flags);
*/
if (!res && eavail &&
!(res = ep_send_events(ep, events, maxevents)) && !timed_out)
- goto retry;
+ goto fetch_events;
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,
+ epi->ffd.file->private_data, 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)
+{
+ return ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
+ ep_loop_check_proc, file, ep, current);
+}
+
/*
* Open an eventpoll file descriptor.
*/
struct epoll_event __user *, event)
{
int error;
+ int did_lock_epmutex = 0;
struct file *file, *tfile;
struct eventpoll *ep;
struct epitem *epi;
*/
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.
+ *
+ * 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.
+ */
+ if (unlikely(is_file_epoll(tfile) && op == EPOLL_CTL_ADD)) {
+ mutex_lock(&epmutex);
+ did_lock_epmutex = 1;
+ error = -ELOOP;
+ if (ep_loop_check(ep, tfile) != 0)
+ goto error_tgt_fput;
+ }
+
+
mutex_lock(&ep->mtx);
/*
mutex_unlock(&ep->mtx);
error_tgt_fput:
+ if (unlikely(did_lock_epmutex))
+ mutex_unlock(&epmutex);
+
fput(tfile);
error_fput:
fput(file);
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);
git.openpandora.org / pandora-kernel.git / blobdiff