Fallback to not using ECN if IP_TOS is not supported

On Linux <3.13 sendmsg/sendmmsg system calls
return EINVAL without sending anything if they encounter
an `IP_TOS` cmsg. To ensure we can still send
on such systems, remember if we got an EINVAL error
and switch to "fallback mode" in which we do not try
to transmit ECN at all and only use well-supported features.

quinn-udp/src/lib.rs

@@ -5,7 +5,7 @@ use std::os::unix::io::AsRawFd;
 use std::os::windows::io::AsRawSocket;
 use std::{
     net::{IpAddr, Ipv6Addr, SocketAddr},
-    sync::atomic::{AtomicUsize, Ordering},
+    sync::atomic::{AtomicBool, AtomicUsize, Ordering},
     time::{Duration, Instant},
 };
 
@@ -37,6 +37,13 @@ pub const BATCH_SIZE: usize = imp::BATCH_SIZE;
 pub struct UdpState {
     max_gso_segments: AtomicUsize,
     gro_segments: usize,
+
+    /// True if we have received EINVAL error from `sendmsg` or `sendmmsg` system call at least once.
+    ///
+    /// If enabled, we assume that old kernel is used and switch to fallback mode.
+    /// In particular, we do not use IP_TOS cmsg_type in this case,
+    /// which is not supported on Linux <3.13 and results in not sending the UDP packet at all.
+    sendmsg_einval: AtomicBool,
 }
 
 impl UdpState {
@@ -62,6 +69,18 @@ impl UdpState {
     pub fn gro_segments(&self) -> usize {
         self.gro_segments
     }
+
+    /// Returns true if we previously got an EINVAL error from `sendmsg` or `sendmmsg` syscall.
+    #[inline]
+    pub fn sendmsg_einval(&self) -> bool {
+        self.sendmsg_einval.load(Ordering::Relaxed)
+    }
+
+    /// Sets the flag indicating we got EINVAL error from `sendmsg` or `sendmmsg` syscall.
+    #[inline]
+    pub fn set_sendmsg_einval(&self) {
+        self.sendmsg_einval.store(true, Ordering::Relaxed)
+    }
 }
 
 impl Default for UdpState {

quinn-udp/src/unix.rs

@@ -6,7 +6,7 @@ use std::{
     mem::{self, MaybeUninit},
     net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6},
     os::unix::io::AsRawFd,
-    sync::atomic::AtomicUsize,
+    sync::atomic::{AtomicBool, AtomicUsize},
     time::Instant,
 };
 
@@ -189,6 +189,7 @@ fn send(
             &mut iovecs[i],
             &mut cmsgs[i],
             encode_src_ip,
+            state.sendmsg_einval(),
         );
     }
     let num_transmits = transmits.len().min(BATCH_SIZE);
@@ -221,6 +222,12 @@ fn send(
                         }
                     }
 
+                    if e.raw_os_error() == Some(libc::EINVAL) {
+                        // Some arguments to `sendmsg` are not supported.
+                        // Switch to fallback mode.
+                        state.set_sendmsg_einval();
+                    }
+
                     // Other errors are ignored, since they will ususally be handled
                     // by higher level retransmits and timeouts.
                     // - PermissionDenied errors have been observed due to iptable rules.
@@ -459,6 +466,7 @@ pub fn udp_state() -> UdpState {
     UdpState {
         max_gso_segments: AtomicUsize::new(gso::max_gso_segments()),
         gro_segments: gro::gro_segments(),
+        sendmsg_einval: AtomicBool::new(false),
     }
 }
 
@@ -472,6 +480,7 @@ fn prepare_msg(
     ctrl: &mut cmsg::Aligned<[u8; CMSG_LEN]>,
     #[allow(unused_variables)] // only used on FreeBSD & macOS
     encode_src_ip: bool,
+    sendmsg_einval: bool,
 ) {
     iov.iov_base = transmit.contents.as_ptr() as *const _ as *mut _;
     iov.iov_len = transmit.contents.len();
@@ -493,7 +502,9 @@ fn prepare_msg(
     let mut encoder = unsafe { cmsg::Encoder::new(hdr) };
     let ecn = transmit.ecn.map_or(0, |x| x as libc::c_int);
     if transmit.destination.is_ipv4() {
-        encoder.push(libc::IPPROTO_IP, libc::IP_TOS, ecn as IpTosTy);
+        if !sendmsg_einval {
+            encoder.push(libc::IPPROTO_IP, libc::IP_TOS, ecn as IpTosTy);
+        }
     } else {
         encoder.push(libc::IPPROTO_IPV6, libc::IPV6_TCLASS, ecn);
     }