io_uring: revert consumed iov_iter bytes on error

Some consumers of the iov_iter will return an error, but still have
bytes consumed in the iterator. This is an issue for -EAGAIN, since we
rely on a sane iov_iter state across retries.

Fix this by ensuring that we revert consumed bytes, if any, if the file
operations have consumed any bytes from iterator. This is similar to what
generic_file_read_iter() does, and is always safe as we have the previous
bytes count handy already.

Fixes: ff6165b2 ("io_uring: retain iov_iter state over io_read/io_write calls")
Reported-by: Dmitry Shulyak <yashulyak@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3153,6 +3153,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 	} else if (ret == -EAGAIN) {
 		if (!force_nonblock)
 			goto done;
+		/* some cases will consume bytes even on error returns */
+		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
 		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		if (ret)
 			goto out_free;
@@ -3294,6 +3296,8 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 	if (!force_nonblock || ret2 != -EAGAIN) {
 		kiocb_done(kiocb, ret2, cs);
 	} else {
+		/* some cases will consume bytes even on error returns */
+		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
 copy_iov:
 		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		if (!ret)