osmo_io: Guard osmo_iofd_register() with invalid file descriptor

Let's return an error if both osmo_iofd_setup() and osmo_iofd_register()
are called with an invalid file descriptor like -1.  Either one of them
must have been called with a valid file descriptor.

Change-Id: Ie4561cefad82e1bf5d37dd1a4815f4bc805343e6
---
M src/core/osmo_io.c
1 file changed, 18 insertions(+), 0 deletions(-)

diff --git a/src/core/osmo_io.c b/src/core/osmo_io.c
index 5a5e05c..f0d213c 100644
--- a/src/core/osmo_io.c
+++ b/src/core/osmo_io.c
@@ -672,6 +672,11 @@
 
 	if (fd >= 0)
 		iofd->fd = fd;
+	else if (iofd->fd < 0) {
+		/* this might happen if both osmo_iofd_setup() and osmo_iofd_register() are called with -1 */
+		LOGPIO(iofd, LOGL_ERROR, "Cannot register io_fd using invalid fd == %d\n", iofd->fd);
+		return -EBADF;
+	}
 
 	if (osmo_iofd_ops.register_fd)
 		rc = osmo_iofd_ops.register_fd(iofd);

