vmm: Allow IP configuration on named TAP interfaces

This commit changes existing behavior of named TAP interfaces.
When booting a VM with configuration for a named TAP interface,
cloud-hypervisor will create the interface and apply a given
IP configuration to that interface. If the named interface
already exists on the system, the configuration is NOT overwritten.

Setting the ip and netmask fields in a tap interface configuration
for a named tap interface now works by handing this configuration
to the virtio_devices::Net object when it is created with a name.

This commit also touches net_util to make sure that the ip configuration
of existing TAP interfaces is not modified with ip or netmask handed to
open_tap.

Signed-off-by: Markus Sütter <markus.suetter@secunet.com>

net_util/src/open_tap.rs

@@ -72,6 +72,10 @@ pub fn open_tap(
     let mut taps: Vec<Tap> = Vec::new();
     let mut ifname: String = String::new();
     let vnet_hdr_size = vnet_hdr_len() as i32;
+    // Check if the given interface exists before we create it.
+    let tap_existed = if_name.map_or(false, |n| {
+        Path::new(&format!("/sys/class/net/{n}")).exists()
+    });
 
     // In case the tap interface already exists, check if the number of
     // queues is appropriate. The tap might not support multiqueue while
@@ -87,12 +91,20 @@ pub fn open_tap(
                 Some(name) => Tap::open_named(name, num_rx_q, flags).map_err(Error::TapOpen)?,
                 None => Tap::new(num_rx_q).map_err(Error::TapOpen)?,
             };
+            // Don't overwrite ip configuration of existing interfaces:
+            if !tap_existed {
                 if let Some(ip) = ip_addr {
                     tap.set_ip_addr(ip).map_err(Error::TapSetIp)?;
                 }
                 if let Some(mask) = netmask {
                     tap.set_netmask(mask).map_err(Error::TapSetNetmask)?;
                 }
+            } else {
+                warn!(
+                    "Tap {} already exists. IP configuration will not be overwritten.",
+                    if_name.unwrap_or_default()
+                );
+            }
             if let Some(mac) = host_mac {
                 tap.set_mac_addr(*mac).map_err(Error::TapSetMac)?
             } else {

vmm/src/device_manager.rs

@@ -2464,8 +2464,8 @@ impl DeviceManager {
                     virtio_devices::Net::new(
                         id.clone(),
                         Some(tap_if_name),
-                        None,
-                        None,
+                        Some(net_cfg.ip),
+                        Some(net_cfg.mask),
                         Some(net_cfg.mac),
                         &mut net_cfg.host_mac,
                         net_cfg.mtu,