net_util: modify or provide safety comments

Signed-off-by: Wei Liu <liuwe@microsoft.com>

net_util/src/lib.rs

@@ -66,32 +66,34 @@ fn create_sockaddr(ip_addr: net::Ipv4Addr) -> net_gen::sockaddr {
     let addr_in = net_gen::sockaddr_in {
         sin_family: net_gen::AF_INET as u16,
         sin_port: 0,
+        // SAFETY: ip_addr can be safely transmute to in_addr
         sin_addr: unsafe { mem::transmute(ip_addr.octets()) },
         __pad: [0; 8usize],
     };
 
+    // SAFETY: addr_in can be safely transmute to sockaddr
     unsafe { mem::transmute(addr_in) }
 }
 
 fn create_inet_socket() -> Result<net::UdpSocket> {
-    // This is safe since we check the return value.
+    // SAFETY: we check the return value.
     let sock = unsafe { libc::socket(libc::AF_INET, libc::SOCK_DGRAM, 0) };
     if sock < 0 {
         return Err(Error::CreateSocket(IoError::last_os_error()));
     }
 
-    // This is safe; nothing else will use or hold onto the raw sock fd.
+    // SAFETY: nothing else will use or hold onto the raw sock fd.
     Ok(unsafe { net::UdpSocket::from_raw_fd(sock) })
 }
 
 fn create_unix_socket() -> Result<net::UdpSocket> {
-    // This is safe since we check the return value.
+    // SAFETY: we check the return value.
     let sock = unsafe { libc::socket(libc::AF_UNIX, libc::SOCK_DGRAM, 0) };
     if sock < 0 {
         return Err(Error::CreateSocket(IoError::last_os_error()));
     }
 
-    // This is safe; nothing else will use or hold onto the raw sock fd.
+    // SAFETY: nothing else will use or hold onto the raw sock fd.
     Ok(unsafe { net::UdpSocket::from_raw_fd(sock) })
 }