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-rw-r--r--fs/eventpoll.c38
1 files changed, 37 insertions, 1 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 3534d36a14..c5a9a483fb 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -876,6 +876,34 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep
}
/*
+ * The ffd.file pointer may be in the process of being torn down due to
+ * being closed, but we may not have finished eventpoll_release() yet.
+ *
+ * Normally, even with the atomic_long_inc_not_zero, the file may have
+ * been free'd and then gotten re-allocated to something else (since
+ * files are not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).
+ *
+ * But for epoll, users hold the ep->mtx mutex, and as such any file in
+ * the process of being free'd will block in eventpoll_release_file()
+ * and thus the underlying file allocation will not be free'd, and the
+ * file re-use cannot happen.
+ *
+ * For the same reason we can avoid a rcu_read_lock() around the
+ * operation - 'ffd.file' cannot go away even if the refcount has
+ * reached zero (but we must still not call out to ->poll() functions
+ * etc).
+ */
+static struct file *epi_fget(const struct epitem *epi)
+{
+ struct file *file;
+
+ file = epi->ffd.file;
+ if (!atomic_long_inc_not_zero(&file->f_count))
+ file = NULL;
+ return file;
+}
+
+/*
* Differs from ep_eventpoll_poll() in that internal callers already have
* the ep->mtx so we need to start from depth=1, such that mutex_lock_nested()
* is correctly annotated.
@@ -883,14 +911,22 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep
static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
int depth)
{
- struct file *file = epi->ffd.file;
+ struct file *file = epi_fget(epi);
__poll_t res;
+ /*
+ * We could return EPOLLERR | EPOLLHUP or something, but let's
+ * treat this more as "file doesn't exist, poll didn't happen".
+ */
+ if (!file)
+ return 0;
+
pt->_key = epi->event.events;
if (!is_file_epoll(file))
res = vfs_poll(file, pt);
else
res = __ep_eventpoll_poll(file, pt, depth);
+ fput(file);
return res & epi->event.events;
}