fix: use repr packed to avoid padding in the syscall interface

I was hoping to _not_ do this as it might have a perf implication, but:

- Padding is "undefined" so we could end up leaking rust memory (and
  causing non-determinism at runtime).
- Requiring that everything be "packed" simplifies the spec.
- In practice, there should be no performance issues (with existing
  syscalls). Specifically, we can re-order fields to avoid unaligned
  loads.

part of https://github.com/filecoin-project/fvm-specs/issues/63

Author: Stebalien

shared/src/sys/mod.rs

@@ -11,8 +11,8 @@ pub type Codec = u64;
 ///
 /// Internally, this type is a tuple of `u64`s storing the "low" and "high" bits of a little-endian
 /// u128.
-#[repr(C)]
 #[derive(Debug, Copy, Clone)]
+#[repr(packed, C)]
 pub struct TokenAmount {
     pub lo: u64,
     pub hi: u64,

shared/src/sys/out.rs

@@ -8,26 +8,25 @@
 //! Read more at https://github.com/rust-lang/rust/issues/73755.
 
 pub mod actor {
-    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
+    #[repr(packed, C)]
     pub struct ResolveAddress {
         pub resolved: i32,
         pub value: u64,
     }
 }
 
 pub mod ipld {
-    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
+    #[repr(packed, C)]
     pub struct IpldOpen {
-        /// TODO could be more efficient to align id, size, codec, depending on padding.
-        pub id: u32,
         pub codec: u64,
+        pub id: u32,
         pub size: u32,
     }
 
-    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
+    #[repr(packed, C)]
     pub struct IpldStat {
         pub codec: u64,
         pub size: u32,
@@ -37,8 +36,8 @@ pub mod ipld {
 pub mod send {
     use crate::sys::BlockId;
 
-    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
+    #[repr(packed, C)]
     pub struct Send {
         pub exit_code: u32,
         pub return_id: BlockId,
@@ -48,11 +47,11 @@ pub mod send {
 pub mod crypto {
     use crate::{ActorID, ChainEpoch};
 
-    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
+    #[repr(packed, C)]
     pub struct VerifyConsensusFault {
-        pub fault: u32,
         pub epoch: ChainEpoch,
         pub target: ActorID,
+        pub fault: u32,
     }
 }