Clarify webauthn decoding from pr comments

account-integrations/safe/src/SafeWebAuthnPlugin.sol

@@ -67,7 +67,9 @@ contract SafeWebAuthnPlugin is BaseAccount {
  return _publicKey;
  }
 
- struct LocalVarStruct {
+ // Struct declaration to hold multiple local vars.
+ // Prevents stack from getting too deep for evm.
+ struct LocalVarWrapper {
  bytes1 authenticatorDataFlagMask;
  bytes32 clientChallenge;
  uint256 clientChallengeDataOffset;
@@ -81,9 +83,10 @@ contract SafeWebAuthnPlugin is BaseAccount {
  bytes calldata clientData;
  uint256[2] calldata signature;
  uint256[2] calldata pubKey;
- LocalVarStruct memory s;
+ LocalVarWrapper memory wrapper;
 
- { 
+ // scope to contain local variables that can be popped from the stack after use
+ {
  // parse length of all fixed-length params (including length)
  uint i = 0;
  uint dataLen = 32;
@@ -92,12 +95,13 @@ contract SafeWebAuthnPlugin is BaseAccount {
  i += dataLen; // advance index
 
  // decode fixed length params (values to memory)
- dataLen = 4 * 32; //lenFixedParams - 32; // -32 already read length
+ dataLen = paramLen - 32; // length already read
+ dataLen -= 2 * 2 * 32; // exclude fixed length arrays
  (
- s.authenticatorDataFlagMask,
+ wrapper.authenticatorDataFlagMask,
  , // some number
- s.clientChallenge,
- s.clientChallengeDataOffset
+ wrapper.clientChallenge,
+ wrapper.clientChallengeDataOffset
  ) = abi.decode(
  userOp.signature[i:i+dataLen],
  (
@@ -130,8 +134,7 @@ contract SafeWebAuthnPlugin is BaseAccount {
  paramLen = abi.decode(userOp.signature[i:i+dataLen], (uint256));
  i += dataLen; // advance index
  // assign authenticatorData to sig splice
- dataLen = paramLen;//((paramLen >> 5) + 1) << 5; // (round up to next slot)
-
+ dataLen = paramLen;
  authenticatorData = userOp.signature[i:i+dataLen];
  i += ((dataLen >> 5) + 1) << 5; // advance index (round up to next slot)
 
@@ -140,17 +143,17 @@ contract SafeWebAuthnPlugin is BaseAccount {
  paramLen = abi.decode(userOp.signature[i:i+dataLen], (uint256));
  i += dataLen; // advance index
  // assign clientData to sig splice
- dataLen = paramLen;// ((paramLen >> 5) + 1) << 5; // (round up to next slot)
+ dataLen = paramLen;
  clientData = userOp.signature[i:i+dataLen];
- i += ((dataLen >> 5) + 1) << 5; // advance index (round up to next slot)
- }
+ // i += ((dataLen >> 5) + 1) << 5; // advance index (round up to next slot)
+ } // end scope to free vars from stack
 
  bool verified = FCL_WebAuthn.checkSignature(
  authenticatorData,
- s.authenticatorDataFlagMask,
+ wrapper.authenticatorDataFlagMask,
  clientData,
- s.clientChallenge,
- s.clientChallengeDataOffset,
+ wrapper.clientChallenge,
+ wrapper.clientChallengeDataOffset,
  signature,
  pubKey
  );