[PATCH 1/2] net/tap: fix FreeBSD portability problem receiving TAP FD

["Daniel P. Berrangé" <berrange@redhat.com>]

The CMSG_LEN and CMSG_SPACE macros must not be assumed to return the
same value. The msg_controllen field must be initialized using
CMSG_SPACE when using SCM_RIGHTS.

This ought to fix any FD receive issues users might be hitting on
64-bit FeeBSD / NetBSD platforms. The flaw was noticed first in
GNULIB

   https://lists.gnu.org/archive/html/bug-gnulib/2021-02/msg00066.html

and QEMU's code has the same logic bug.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 net/tap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/tap.c b/net/tap.c
index bae895e287..276a9077fc 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -467,7 +467,7 @@ static int recv_fd(int c)
     cmsg->cmsg_level = SOL_SOCKET;
     cmsg->cmsg_type = SCM_RIGHTS;
     cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
-    msg.msg_controllen = cmsg->cmsg_len;
+    msg.msg_controllen = CMSG_SPACE(sizeof(fd));
 
     iov.iov_base = req;
     iov.iov_len = sizeof(req);
-- 
2.31.1