fix: Ensure ConfigSpace has stable layout

Rust doesn't guarantee struct layout by default, so `repr(C)` or `repr(transparent)` are necessary for safety.

Signed-off-by: Kornel <kornel@cloudflare.com>

src/utils/src/net/mac.rs

@@ -22,6 +22,7 @@ pub const MAC_ADDR_LEN: u8 = 6;
 
 /// Represents a MAC address
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+#[repr(transparent)]
 /// Representation of a MAC address.
 pub struct MacAddr {
     bytes: [u8; MAC_ADDR_LEN as usize],

src/vmm/src/devices/virtio/net/device.rs

@@ -96,11 +96,12 @@ fn init_vnet_hdr(buf: &mut [u8]) {
 }
 
 #[derive(Debug, Default, Clone, Copy)]
+#[repr(C)]
 pub struct ConfigSpace {
     pub guest_mac: MacAddr,
 }
 
-// SAFETY: `ConfigSpace` contains only PODs.
+// SAFETY: `ConfigSpace` contains only PODs in `repr(C)` or `repr(transparent)`, without padding.
 unsafe impl ByteValued for ConfigSpace {}
 
 /// VirtIO network device.