vhost_user: ignore unknown features from backend

virtiofsd 1.11.0 added support for VHOST_USER_PROTOCOL_F_DEVICE_STATE.
Upgrading virtiofsd meant that the latest released version of Cloud
Hypervisor (39.0) was no longer able to communicate with it, because
rather than just ignoring the unsupported feature, it got an
unrecoverable "invalid message" error.  This demonstrates that it's
better for frontends to just ignore offers of unsupported features.
If the backend requires some feature, it'll get a chance to know that
when we send VHOST_USER_SET_PROTOCOL_FEATURES anyway.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost/CHANGELOG.md

@@ -5,6 +5,7 @@
 - [[#241]](/~https://github.com/rust-vmm/vhost/pull/241) Add shared objects support
 
 ### Changed
+- [[#243]](/~https://github.com/rust-vmm/vhost/pull/243) Ignore unknown bits in `VHOST_USER_GET_PROTOCOL_FEATURES` response.
 
 ### Remove
 - [[#246]](/~https://github.com/rust-vmm/vhost/pull/246) Remove support for FS_* requests

vhost/src/vhost_user/frontend.rs

@@ -362,12 +362,9 @@ impl VhostUserFrontend for Frontend {
         let hdr = node.send_request_header(FrontendReq::GET_PROTOCOL_FEATURES, None)?;
         let val = node.recv_reply::<VhostUserU64>(&hdr)?;
         node.protocol_features = val.value;
-        // Should we support forward compatibility?
-        // If so just mask out unrecognized flags instead of return errors.
-        match VhostUserProtocolFeatures::from_bits(node.protocol_features) {
-            Some(val) => Ok(val),
-            None => error_code(VhostUserError::InvalidMessage),
-        }
+        Ok(VhostUserProtocolFeatures::from_bits_truncate(
+            node.protocol_features,
+        ))
     }
 
     fn set_protocol_features(&mut self, features: VhostUserProtocolFeatures) -> Result<()> {
@@ -811,6 +808,8 @@ mod tests {
 
     use std::path::PathBuf;
 
+    const INVALID_PROTOCOL_FEATURE: u64 = 1 << 63;
+
     fn temp_path() -> PathBuf {
         PathBuf::from(format!(
             "/tmp/vhost_test_{}",
@@ -935,7 +934,8 @@ mod tests {
 
         let pfeatures = VhostUserProtocolFeatures::all();
         let hdr = VhostUserMsgHeader::new(FrontendReq::GET_PROTOCOL_FEATURES, 0x4, 8);
-        let msg = VhostUserU64::new(pfeatures.bits());
+        // Unknown feature bits should be ignored.
+        let msg = VhostUserU64::new(pfeatures.bits() | INVALID_PROTOCOL_FEATURE);
         peer.send_message(&hdr, &msg, None).unwrap();
         let features = frontend.get_protocol_features().unwrap();
         assert_eq!(features, pfeatures);