muvm-guest: pwbridge: actually read fd indexes in the Transport msg

The message payload contains indexes for the attached file descriptors.
We must use them instead of relying on the order, which was never guaranteed,
and *actually* differs in practice between architectures in compilers
because current PipeWire code calls the fd-attaching function inside of
function call arguments, which *don't* have a defined evaluation order
in the C language standard.

Signed-off-by: Val Packett <[email protected]>

Author: valpackett

crates/muvm/src/guest/bridge/pipewire.rs

@@ -21,6 +21,7 @@ const CROSS_DOMAIN_CMD_READ: u8 = 6;
 const CROSS_DOMAIN_CMD_WRITE: u8 = 7;
 
 const SPA_TYPE_STRUCT: u32 = 14;
+const SPA_TYPE_FD: u32 = 18;
 
 const PW_OPC_CORE_CREATE_OBJECT: u8 = 6;
 const PW_OPC_CORE_ADD_MEM: u8 = 6;
@@ -53,6 +54,10 @@ fn read_u32(data: &[u8], at: usize) -> u32 {
     u32::from_ne_bytes(data[at..(at + 4)].try_into().unwrap())
 }
 
+fn read_u64(data: &[u8], at: usize) -> u64 {
+    u64::from_ne_bytes(data[at..(at + 8)].try_into().unwrap())
+}
+
 #[derive(Debug)]
 struct CoreCreateObject<'a> {
     obj_type: &'a CStr,
@@ -115,6 +120,27 @@ impl<'a> ClientUpdateProperties<'a> {
     }
 }
 
+#[derive(Debug)]
+struct ClientNodeTransport {
+    readfd: u64,
+    writefd: u64,
+    // .. don't care about the remaining ones
+}
+
+impl ClientNodeTransport {
+    fn new(data: &[u8]) -> Self {
+        let ty = read_u32(data, 4);
+        assert_eq!(ty, SPA_TYPE_STRUCT);
+        let readfd_ty = read_u32(data, 12);
+        assert_eq!(readfd_ty, SPA_TYPE_FD);
+        let readfd = read_u64(data, 16);
+        let writefd_ty = read_u32(data, 28);
+        assert_eq!(writefd_ty, SPA_TYPE_FD);
+        let writefd = read_u64(data, 32);
+        ClientNodeTransport { readfd, writefd }
+    }
+}
+
 struct PipeWireHeader {
     id: u32,
     opcode: u8,
@@ -267,10 +293,21 @@ impl ProtocolHandler for PipeWireProtocolHandler {
                     let rsc = resources.pop_front().ok_or(Errno::EIO)?;
                     fds.push(Self::create_guest_to_host_eventfd(this, hdr.id, rsc)?);
                 } else if hdr.opcode == PW_OPC_CLIENT_NODE_TRANSPORT {
-                    let rsc1 = resources.pop_front().ok_or(Errno::EIO)?;
-                    fds.push(Self::create_host_to_guest_eventfd(this, hdr.id, rsc1)?);
-                    let rsc2 = resources.pop_front().ok_or(Errno::EIO)?;
-                    fds.push(Self::create_guest_to_host_eventfd(this, hdr.id, rsc2)?);
+                    let msg = ClientNodeTransport::new(&data[PipeWireHeader::SIZE..]);
+                    // We need to take elements out by index without shifting the indices.
+                    let mut resources: Vec<_> = resources.drain(..hdr.num_fd).map(Some).collect();
+                    let writefd_rsc = resources.get_mut(msg.writefd as usize).ok_or(Errno::EIO)?;
+                    fds.push(Self::create_guest_to_host_eventfd(
+                        this,
+                        hdr.id,
+                        writefd_rsc.take().ok_or(Errno::EIO)?,
+                    )?);
+                    let readfd_rsc = resources.get_mut(msg.readfd as usize).ok_or(Errno::EIO)?;
+                    fds.push(Self::create_host_to_guest_eventfd(
+                        this,
+                        hdr.id,
+                        readfd_rsc.take().ok_or(Errno::EIO)?,
+                    )?);
                 } else {
                     unimplemented!()
                 }