add stricter ABI-compatibility checks for punned types (#2434)

* add stricter ABI-compatibility checks for punned types

* enforce type isomorphism for each pair of cast types to eliminate potential safety hole

beacon_chain/beacon_chain_db.nim

@@ -375,7 +375,7 @@ proc putState*(db: BeaconChainDB, key: Eth2Digest, value: var BeaconState) =
   updateImmutableValidators(db, db.immutableValidatorsMem, value.validators)
   db.put(
     subkey(BeaconStateNoImmutableValidators, key),
-    cast[ref BeaconStateNoImmutableValidators](addr value)[])
+    isomorphicCast[BeaconStateNoImmutableValidators](value))
 
 proc putState*(db: BeaconChainDB, value: var BeaconState) =
   db.putState(hash_tree_root(value), value)
@@ -461,7 +461,7 @@ proc getStateOnlyMutableValidators(
   case db.get(
     subkey(
       BeaconStateNoImmutableValidators, key),
-      cast[ref BeaconStateNoImmutableValidators](addr output)[])
+      isomorphicCast[BeaconStateNoImmutableValidators](output))
   of GetResult.found:
     let numValidators = output.validators.len
     doAssert db.immutableValidatorsMem.len >= numValidators

beacon_chain/beacon_chain_db_immutable.nim

@@ -71,13 +71,21 @@ type
     current_justified_checkpoint*: Checkpoint
     finalized_checkpoint*: Checkpoint
 
-static:
+func getSizeofSig(x: auto, n: int = 0): seq[(string, int, int)] =
+  for name, value in x.fieldPairs:
+    when value is tuple|object:
+      result.add getSizeofSig(value, n + 1)
+    result.add((name, sizeof(value), n))
+
+template isomorphicCast*[T, U](x: var U): T =
   # Each of these pairs of types has ABI-compatible memory representations, so
   # that the SSZ serialization can read and write directly from an object with
   # only mutable portions of BeaconState into a full BeaconState without using
-  # any extra copies.
-  doAssert sizeof(Validator) == sizeof(ValidatorStatus)
-  doAssert sizeof(BeaconState) == sizeof(BeaconStateNoImmutableValidators)
+  # extra copies.
+  static:
+    doAssert sizeof(T) == sizeof(U)
+    doAssert getSizeofSig(T()) == getSizeofSig(U())
+  cast[ref T](addr x)[]
 
 proc loadImmutableValidators*(dbSeq: var auto): seq[ImmutableValidatorData] =
   for i in 0 ..< dbSeq.len: