sd-netlink: make sure the root-level type is nested

In sd-netlink-message, we always guarantee that the currently selected
type-system is non-NULL. Otherwise, we would be unable to parse any types
in the current container level. Hence, this assertion must be true:
    message->container_type_system[m->n_containers] != NULL

During message_new() we currently do not verify that this assertion is
true. Instead, we blindly access nl_type->type_system and use it (which
might be NULL for basic types and unions). Fix this, by explicitly
checking that the root-level type is nested.

Note that this is *not* a strict requirement of netlink, but it's a strict
requirement for all message types we currently support. Furthermore, all
the callers of message_new() already verify that only supported types are
passed, therefore, this is a pure cosmetic check. However, it might be
needed on the future, so make sure we don't trap into this once we change
the type-system.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libsystemd/sd-netlink/netlink-message.c b/src/libsystemd/sd-netlink/netlink-message.c
index bfbc0e6c95..e39e4c646c 100644
--- a/src/libsystemd/sd-netlink/netlink-message.c
+++ b/src/libsystemd/sd-netlink/netlink-message.c
@@ -72,6 +72,9 @@ int message_new(sd_netlink *rtnl, sd_netlink_message **ret, uint16_t type) {
         if (r < 0)
                 return r;
 
+        if (type_get_type(nl_type) != NETLINK_TYPE_NESTED)
+                return -EINVAL;
+
         r = message_new_empty(rtnl, &m);
         if (r < 0)
                 return r;
@@ -85,8 +88,7 @@ int message_new(sd_netlink *rtnl, sd_netlink_message **ret, uint16_t type) {
 
         m->hdr->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
 
-        if (type_get_type(nl_type) == NETLINK_TYPE_NESTED)
-                type_get_type_system(nl_type, &m->container_type_system[0]);
+        type_get_type_system(nl_type, &m->container_type_system[0]);
         m->hdr->nlmsg_len = size;
         m->hdr->nlmsg_type = type;