From 0e9513f2b7c72556ff499789420815aacc2070a9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Markus=20S=C3=BCtter?= <markus.suetter@secunet.com>
Date: Mon, 4 Dec 2023 11:21:07 +0100
Subject: [PATCH] vmm: Allow IP configuration on named TAP interfaces
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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  | 22 +++++++++++++++++-----
 vmm/src/device_manager.rs |  4 ++--
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/net_util/src/open_tap.rs b/net_util/src/open_tap.rs
index dd5d49868..4276012d6 100644
--- a/net_util/src/open_tap.rs
+++ b/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
@@ -87,11 +91,19 @@ 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)?,
             };
-            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)?;
+            // 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)?
diff --git a/vmm/src/device_manager.rs b/vmm/src/device_manager.rs
index ed0f7d116..883f0c040 100644
--- a/vmm/src/device_manager.rs
+++ b/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,