[PATCH 5.10 61/63] eventpoll: add EPOLL_URING_WAKE poll wakeup flag

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Jens Axboe <axboe@kernel.dk>

[ Upstream commit caf1aeaffc3b09649a56769e559333ae2c4f1802 ]

We can have dependencies between epoll and io_uring. Consider an epoll
context, identified by the epfd file descriptor, and an io_uring file
descriptor identified by iofd. If we add iofd to the epfd context, and
arm a multishot poll request for epfd with iofd, then the multishot
poll request will repeatedly trigger and generate events until terminated
by CQ ring overflow. This isn't a desired behavior.

Add EPOLL_URING so that io_uring can pass it in as part of the poll wakeup
key, and io_uring can check for that to detect a potential recursive
invocation.

Cc: stable@vger.kernel.org # 6.0
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/eventpoll.c                 |   18 ++++++++++--------
 include/uapi/linux/eventpoll.h |    6 ++++++
 2 files changed, 16 insertions(+), 8 deletions(-)

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -548,7 +548,8 @@ out_unlock:
  */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
-static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
+static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
+			     unsigned pollflags)
 {
 	struct eventpoll *ep_src;
 	unsigned long flags;
@@ -579,16 +580,17 @@ static void ep_poll_safewake(struct even
 	}
 	spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
 	ep->nests = nests + 1;
-	wake_up_locked_poll(&ep->poll_wait, EPOLLIN);
+	wake_up_locked_poll(&ep->poll_wait, EPOLLIN | pollflags);
 	ep->nests = 0;
 	spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
 }
 
 #else
 
-static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
+static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
+			     unsigned pollflags)
 {
-	wake_up_poll(&ep->poll_wait, EPOLLIN);
+	wake_up_poll(&ep->poll_wait, EPOLLIN | pollflags);
 }
 
 #endif
@@ -815,7 +817,7 @@ static void ep_free(struct eventpoll *ep
 
 	/* We need to release all tasks waiting for these file */
 	if (waitqueue_active(&ep->poll_wait))
-		ep_poll_safewake(ep, NULL);
+		ep_poll_safewake(ep, NULL, 0);
 
 	/*
 	 * We need to lock this because we could be hit by
@@ -1284,7 +1286,7 @@ out_unlock:
 
 	/* We have to call this outside the lock */
 	if (pwake)
-		ep_poll_safewake(ep, epi);
+		ep_poll_safewake(ep, epi, pollflags & EPOLL_URING_WAKE);
 
 	if (!(epi->event.events & EPOLLEXCLUSIVE))
 		ewake = 1;
@@ -1589,7 +1591,7 @@ static int ep_insert(struct eventpoll *e
 
 	/* We have to call this outside the lock */
 	if (pwake)
-		ep_poll_safewake(ep, NULL);
+		ep_poll_safewake(ep, NULL, 0);
 
 	return 0;
 
@@ -1692,7 +1694,7 @@ static int ep_modify(struct eventpoll *e
 
 	/* We have to call this outside the lock */
 	if (pwake)
-		ep_poll_safewake(ep, NULL);
+		ep_poll_safewake(ep, NULL, 0);
 
 	return 0;
 }
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -41,6 +41,12 @@
 #define EPOLLMSG	(__force __poll_t)0x00000400
 #define EPOLLRDHUP	(__force __poll_t)0x00002000
 
+/*
+ * Internal flag - wakeup generated by io_uring, used to detect recursion back
+ * into the io_uring poll handler.
+ */
+#define EPOLL_URING_WAKE	((__force __poll_t)(1U << 27))
+
 /* Set exclusive wakeup mode for the target file descriptor */
 #define EPOLLEXCLUSIVE	((__force __poll_t)(1U << 28))