io_uring/rw: forbid multishot async reads

At the moment we can't sanely handle queuing an async request from a
multishot context, so disable them. It shouldn't matter as pollable
files / socekts don't normally do async.

Patching it in __io_read() is not the cleanest way, but it's simpler
than other options, so let's fix it there and clean up on top.

Cc: [email protected]
Reported-by: chase xd <[email protected]>
Fixes: fc68fcd ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/7d51732c125159d17db4fe16f51ec41b936973f8.1739919038.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

io_uring/rw.c

@@ -880,7 +880,15 @@ static int __io_read(struct io_kiocb *req, unsigned int issue_flags)
 	if (unlikely(ret))
 		return ret;
 
-	ret = io_iter_do_read(rw, &io->iter);
+	if (unlikely(req->opcode == IORING_OP_READ_MULTISHOT)) {
+		void *cb_copy = rw->kiocb.ki_complete;
+
+		rw->kiocb.ki_complete = NULL;
+		ret = io_iter_do_read(rw, &io->iter);
+		rw->kiocb.ki_complete = cb_copy;
+	} else {
+		ret = io_iter_do_read(rw, &io->iter);
+	}
 
 	/*
 	 * Some file systems like to return -EOPNOTSUPP for an IOCB_NOWAIT
@@ -904,7 +912,8 @@ static int __io_read(struct io_kiocb *req, unsigned int issue_flags)
 	} else if (ret == -EIOCBQUEUED) {
 		return IOU_ISSUE_SKIP_COMPLETE;
 	} else if (ret == req->cqe.res || ret <= 0 || !force_nonblock ||
-		   (req->flags & REQ_F_NOWAIT) || !need_complete_io(req)) {
+		   (req->flags & REQ_F_NOWAIT) || !need_complete_io(req) ||
+		   (issue_flags & IO_URING_F_MULTISHOT)) {
 		/* read all, failed, already did sync or don't want to retry */
 		goto done;
 	}