quinn-udp: sanitise segment_size

In order to perform GSO, an application needs to batch datagrams
together into a big buffer. As part of such a batch, the last datagram
is allowed to be less than the `segment_size` as that still allows the
kernel to unambigously segment the content into individual packets.

When GSO gets disabled at runtime due to lack of driver support,
applications need to segment such a prepared batch themselves into
individual `Transmit`s. Doing that may lead to situations in which a
`Transmit` is handed to `quinn-udp` which still has the original
`segment_size` set, yet its content is shorter than that.

Not all kernels like that. Specifically, it has been observed that
Android is particularly picky about these parameters.

We already sanitise the `segment_size` such that it is not set when it
is equal to the content length. To further increase the robustness of
`quinn-udp`, we extend this check to only set it when it is strictly
less than the content length.

Related: #2201
Related: #2145
Related: firezone/firezone#8932

Author: thomaseizinger

quinn-udp/src/unix.rs

@@ -612,11 +612,13 @@ fn prepare_msg(
         encoder.push(libc::IPPROTO_IPV6, libc::IPV6_TCLASS, ecn);
     }
 
-    // Only set the segment size if it is different from the size of the contents.
-    // Some network drivers don't like being told to do GSO even if there is effectively only a single segment.
+    // Only set the segment size if it is less than the size of the contents.
+    // Some network drivers don't like being told to do GSO even if there is effectively only a single segment (i.e. `segment_size == transmit.contents.len()`)
+    // Additionally, a `segment_size` that is greater than the content also means there is effectively only a single segment.
+    // This case is actually quite common when splitting up a prepared GSO batch again after GSO has been disabled because the last datagram in a GSO batch is allowed to be smaller than the segment size.
     if let Some(segment_size) = transmit
         .segment_size
-        .filter(|segment_size| *segment_size != transmit.contents.len())
+        .filter(|segment_size| *segment_size < transmit.contents.len())
     {
         gso::set_segment_size(&mut encoder, segment_size as u16);
     }