net_util: Allow retrieving the MAC address from the TAP device

This can be used to preserve the host MAC address as part of the
configuration when the TAP device is precreated.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

net_util/src/tap.rs

@@ -6,6 +6,7 @@
 // found in the THIRD-PARTY file.
 
 use super::{create_sockaddr, create_socket, Error as NetUtilError, MacAddr};
+use mac::MAC_ADDR_LEN;
 use net_gen;
 use std::fs::File;
 use std::io::{Error as IoError, Read, Result as IoResult, Write};
@@ -29,6 +30,8 @@ pub enum Error {
     /// Failed to create a socket.
     NetUtil(NetUtilError),
     InvalidIfname,
+    /// Error parsing MAC data
+    MacParsing(()),
 }
 
 pub type Result<T> = ::std::result::Result<T, Error>;
@@ -176,6 +179,15 @@ impl Tap {
 
     /// Set mac addr for tap interface.
     pub fn set_mac_addr(&self, addr: MacAddr) -> Result<()> {
+        // Checking if the mac address already matches the desired one
+        // is useful to avoid making the "set ioctl" in the case where
+        // the VMM is running without the privilege to do that.
+        // In practice this comes from a reboot after the configuration
+        // has been update with the kernel generated address.
+        if self.get_mac_addr()? == addr {
+            return Ok(());
+        }
+
         let sock = create_socket().map_err(Error::NetUtil)?;
 
         let mut ifreq = self.get_ifreq();
@@ -206,6 +218,31 @@ impl Tap {
         Ok(())
     }
 
+    /// Get mac addr for tap interface.
+    pub fn get_mac_addr(&self) -> Result<MacAddr> {
+        let sock = create_socket().map_err(Error::NetUtil)?;
+
+        let ifreq = self.get_ifreq();
+
+        // ioctl is safe. Called with a valid sock fd, and we check the return.
+        #[allow(clippy::cast_lossless)]
+        let ret =
+            unsafe { ioctl_with_ref(&sock, net_gen::sockios::SIOCGIFHWADDR as c_ulong, &ifreq) };
+        if ret < 0 {
+            return Err(Error::IoctlError(IoError::last_os_error()));
+        }
+
+        // We only access one field of the ifru union, hence this is safe.
+        let addr = unsafe {
+            let ifru_hwaddr = ifreq.ifr_ifru.ifru_hwaddr.as_ref();
+            MacAddr::from_bytes(
+                &*(&ifru_hwaddr.sa_data[0..MAC_ADDR_LEN] as *const _ as *const [u8]),
+            )
+            .map_err(Error::MacParsing)?
+        };
+        Ok(addr)
+    }
+
     /// Set the netmask for the subnet that the tap interface will exist on.
     pub fn set_netmask(&self, netmask: net::Ipv4Addr) -> Result<()> {
         let sock = create_socket().map_err(Error::NetUtil)?;