From 4a11c6adc783099386a3c8ac82da34cf82915224 Mon Sep 17 00:00:00 2001
From: WGH <wgh@torlan.ru>
Date: Sat, 6 Sep 2025 05:41:20 +0300
Subject: [PATCH] Calculate cmsg buffer size statically

Although generic const fn max hasn't been stabilized yet[1], it's possible
to write a concrete impl for usize.

Follow-up to #178 and #225.

[1] https://github.com/rust-lang/rust/issues/92391
---
 phantun/src/utils.rs | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/phantun/src/utils.rs b/phantun/src/utils.rs
index d0617c4..1b68c1f 100644
--- a/phantun/src/utils.rs
+++ b/phantun/src/utils.rs
@@ -62,13 +62,11 @@ pub async fn udp_recv_pktinfo(
     buf: &mut [u8],
 ) -> std::io::Result<(usize, SocketAddr, IpAddr)> {
     sock.async_io(Interest::READABLE, || {
-        // according to documented struct definition in RFC 3542,
-        // sizeof(in6_pktinfo) should always be larger than sizeof(in_pktinfo),
-        // this assert just double checks that. The goal is to avoid
-        // a heap allocation with Vec at runtime.
-        assert!(cmsg_space::<nix::libc::in6_pktinfo>() >= cmsg_space::<nix::libc::in_pktinfo>());
-
-        let mut control_message_buffer = [0u8; cmsg_space::<nix::libc::in6_pktinfo>()];
+        const CONTROL_MESSAGE_BUFFER_SIZE: usize = max_usize(
+            cmsg_space::<nix::libc::in_pktinfo>(),
+            cmsg_space::<nix::libc::in6_pktinfo>(),
+        );
+        let mut control_message_buffer = [0u8; CONTROL_MESSAGE_BUFFER_SIZE];
         let iov = &mut [std::io::IoSliceMut::new(buf)];
         let res = nix::sys::socket::recvmsg::<SockaddrStorage>(
             sock.as_raw_fd(),
@@ -151,3 +149,7 @@ pub fn assign_ipv6_address(device_name: &str, local: Ipv6Addr, peer: Ipv6Addr) {
         .unwrap();
     rtnl.send(&nl_header).unwrap();
 }
+
+const fn max_usize(a: usize, b: usize) -> usize {
+    if a > b { a } else { b }
+}