Ensure all packed structs are also repr(C)

Using #[repr(packed)] alone does not guarantee that the struct fields
will stay in the specified order, and as of a change in Rust 1.80, the
compiler will actually reorder such structs in practice in some cases:
<rust-lang/rust#125360>

Add "C" to all structs that were previously #[repr(packed)] alone, since
these are all trying to match an externally-defined layout where order
matters. None of these would get reordered in practice today, even with
the Rust 1.80 change, but this ensures they will always stay consistent.

Change-Id: I397fd0bd531a34e0f1726afb830bcd7fcc6a2f05
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/5758933
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-by: Frederick Mayle <fmayle@google.com>

devices/src/pci/pci_configuration.rs

@@ -1100,7 +1100,7 @@ mod tests {
 
     use super::*;
 
-    #[repr(packed)]
+    #[repr(C, packed)]
     #[derive(Clone, Copy, AsBytes)]
     #[allow(dead_code)]
     struct TestCap {

devices/src/virtio/vsock/sys/windows/protocol.rs

@@ -20,7 +20,7 @@ pub struct virtio_vsock_config {
 
 /// The message header for data packets sent on the tx/rx queues
 #[derive(Copy, Clone, Debug, Default, AsBytes, FromZeroes, FromBytes)]
-#[repr(packed)]
+#[repr(C, packed)]
 #[allow(non_camel_case_types)]
 pub struct virtio_vsock_hdr {
     pub src_cid: Le64,

third_party/vmm_vhost/src/message.rs

@@ -447,7 +447,7 @@ bitflags! {
 }
 
 /// A generic message to encapsulate a 64-bit value.
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Clone, Copy, AsBytes, FromZeroes, FromBytes)]
 pub struct VhostUserU64 {
     /// The encapsulated 64-bit common value.
@@ -623,7 +623,7 @@ impl VhostUserMsgValidator for VhostUserSingleMemoryRegion {
 }
 
 /// Vring state descriptor.
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Clone, Copy, AsBytes, FromZeroes, FromBytes)]
 pub struct VhostUserVringState {
     /// Vring index.
@@ -735,7 +735,7 @@ bitflags! {
 }
 
 /// Message to read/write device configuration space.
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Clone, Copy, AsBytes, FromZeroes, FromBytes)]
 pub struct VhostUserConfig {
     /// Offset of virtio device's configuration space.
@@ -820,21 +820,21 @@ impl VhostUserMsgValidator for VhostUserInflight {
 
 /*
  * TODO: support dirty log, live migration and IOTLB operations.
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct VhostUserVringArea {
     pub index: u32,
     pub flags: u32,
     pub size: u64,
     pub offset: u64,
 }
 
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct VhostUserLog {
     pub size: u64,
     pub offset: u64,
 }
 
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct VhostUserIotlb {
     pub iova: u64,
     pub size: u64,
@@ -1057,7 +1057,7 @@ impl VhostUserShmemUnmapMsg {
 }
 
 /// Inflight I/O descriptor state for split virtqueues
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Clone, Copy, Default)]
 pub struct DescStateSplit {
     /// Indicate whether this descriptor (only head) is inflight or not.
@@ -1078,7 +1078,7 @@ impl DescStateSplit {
 }
 
 /// Inflight I/O queue region for split virtqueues
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct QueueRegionSplit {
     /// Features flags of this region
     pub features: u64,
@@ -1109,7 +1109,7 @@ impl QueueRegionSplit {
 }
 
 /// Inflight I/O descriptor state for packed virtqueues
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Clone, Copy, Default)]
 pub struct DescStatePacked {
     /// Indicate whether this descriptor (only head) is inflight or not.
@@ -1142,7 +1142,7 @@ impl DescStatePacked {
 }
 
 /// Inflight I/O queue region for packed virtqueues
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct QueueRegionPacked {
     /// Features flags of this region
     pub features: u64,
@@ -1188,7 +1188,7 @@ impl QueueRegionPacked {
 }
 
 /// Virtio shared memory descriptor.
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Copy, Clone, FromZeroes, FromBytes, AsBytes)]
 pub struct VhostSharedMemoryRegion {
     /// The shared memory region's shmid.