Thanks to: https://www.virtualbox.org/pipermail/vbox-dev/2019-July/015297.html (MIT License; adapted to 6.0.12.)

--- vboxdrv/r0drv/linux/mp-r0drv-linux.c	2019-09-16 17:43:37.905927323 -0300
+++ vboxdrv/r0drv/linux/mp-r0drv-linux.c	2019-09-16 17:46:34.168917574 -0300
@@ -284,11 +284,11 @@
     {
         /* Fire the function on all other CPUs without waiting for completion. */
 # if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 27)
-        int rc = smp_call_function(rtmpLinuxAllWrapper, &Args, 0 /* wait */);
+        smp_call_function(rtmpLinuxAllWrapper, &Args, 0 /* wait */);
 # else
         int rc = smp_call_function(rtmpLinuxAllWrapper, &Args, 0 /* retry */, 0 /* wait */);
-# endif
         Assert(!rc); NOREF(rc);
+# endif
     }
 #endif
 
@@ -326,7 +326,6 @@
 {
 #ifdef CONFIG_SMP
     IPRT_LINUX_SAVE_EFL_AC();
-    int rc;
     RTMPARGS Args;
 
     RTTHREADPREEMPTSTATE PreemptState = RTTHREADPREEMPTSTATE_INITIALIZER;
@@ -338,13 +337,14 @@
 
     RTThreadPreemptDisable(&PreemptState);
 # if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 27)
-    rc = smp_call_function(rtmpLinuxWrapper, &Args, 1 /* wait */);
+    smp_call_function(rtmpLinuxWrapper, &Args, 1 /* wait */);
 # else /* older kernels */
+    int rc;
     rc = smp_call_function(rtmpLinuxWrapper, &Args, 0 /* retry */, 1 /* wait */);
+    Assert(rc == 0); NOREF(rc);
 # endif /* older kernels */
     RTThreadPreemptRestore(&PreemptState);
 
-    Assert(rc == 0); NOREF(rc);
     IPRT_LINUX_RESTORE_EFL_AC();
 #else
     RT_NOREF(pfnWorker, pvUser1, pvUser2);

--- vboxnetflt/linux/VBoxNetFlt-linux.c	2019-09-16 18:09:53.150840192 -0300
+++ vboxnetflt/linux/VBoxNetFlt-linux.c	2019-09-16 18:11:50.424833705 -0300
@@ -2123,7 +2123,10 @@
 #endif
     if (in_dev != NULL)
     {
-        for_ifa(in_dev) {
+       /* macros for_ifa() and endfor_ifa() disappear for kernel 5.3
+        * Code them directly */
+        struct in_ifaddr *ifa;
+        for (ifa = in_dev->ifa_list; ifa; ifa = ifa->ifa_next) {
             if (VBOX_IPV4_IS_LOOPBACK(ifa->ifa_address))
                 return NOTIFY_OK;
 
@@ -2137,7 +2140,7 @@
 
             pThis->pSwitchPort->pfnNotifyHostAddress(pThis->pSwitchPort,
                 /* :fAdded */ true, kIntNetAddrType_IPv4, &ifa->ifa_address);
-        } endfor_ifa(in_dev);
+        }
     }
 
     /*