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authorRemco Bloemen <remco@wicked.ventures>2018-06-24 00:03:49 +0800
committerAmir Bandeali <abandeali1@gmail.com>2018-06-26 07:19:07 +0800
commit562fec01d89cae43b813e0f22ffe924953d5fadc (patch)
treed6f058046f18b2c146f73a3718e088e4bd313c84 /packages/contracts
parentb8051c8fed0f42778692de135df922626da3fb4f (diff)
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Add Greg's documentation to MixinErc20Transfer
Diffstat (limited to 'packages/contracts')
-rw-r--r--packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol48
1 files changed, 35 insertions, 13 deletions
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
index 54fe7d726..32d5f394e 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
@@ -44,38 +44,60 @@ contract MixinERC20Transfer is
// Decode asset data.
address token = assetData.readAddress(16);
+ // Transfer tokens.
+ // We do a raw call so we can check the success separate
+ // from the return data.
+ // We construct calldata for the `token.transferFrom` ABI.
+ // The layout of this calldata is in the table below.
+ //
+ // | Area | Offset | Length | Contents |
+ // |----------|--------|---------|-------------------------------------|
+ // | Header | 0 | 4 | function selector |
+ // | Params | | 3 * 32 | function parameters: |
+ // | | 4 | | 1. from |
+ // | | 36 | | 2. to |
+ // | | 68 | | 3. amount |
+
bytes4 transferFromSelector = IERC20Token(token).transferFrom.selector;
bool success;
assembly {
/////// Setup State ///////
// `cdStart` is the start of the calldata for `token.transferFrom` (equal to free memory ptr).
let cdStart := mload(64)
- // `cdEnd` is the end of the calldata for `token.transferFrom`.
- let cdEnd := add(cdStart, 100)
/////// Setup Header Area ///////
// This area holds the 4-byte `transferFromSelector`.
+ // Any trailing data in transferFromSelector will be
+ // overwritten in the next `mstore` call.
mstore(cdStart, transferFromSelector)
/////// Setup Params Area ///////
- // Each parameter is padded to 32-bytes. The entire Params Area is 96 bytes.
- // A 20-byte mask is applied to addresses to zero-out the unused bytes.
+ // Each parameter is padded to 32-bytes.
+ // The entire Params Area is 96 bytes.
+ // A 20-byte mask is applied to addresses to
+ // zero-out the unused bytes.
mstore(add(cdStart, 4), and(from, 0xffffffffffffffffffffffffffffffffffffffff))
mstore(add(cdStart, 36), and(to, 0xffffffffffffffffffffffffffffffffffffffff))
mstore(add(cdStart, 68), amount)
- /////// Call `token.transferFrom` using the constructed calldata ///////
+ /////// Call `token.transferFrom` using the calldata ///////
success := call(
- gas,
- token,
- 0,
- cdStart,
- sub(cdEnd, cdStart),
- cdStart,
- 32
+ gas, // forward all gas
+ token, // call address of token contract
+ 0, // don't send any ETH
+ cdStart, // pointer to start of input
+ 100, // length of input
+ cdStart, // write output over input
+ 32 // output size should be 32 bytes
)
- // The transfer succeeded if the call succeeded and either
+ /////// Check return data. ///////
+ // If there is no return data, we assume the token incorrectly
+ // does not return a bool. In this case we expect it to revert
+ // on failure, which was handled above.
+ // If the token does return data, we require that it is a single
+ // nonzero 32 bytes value.
+ // So the transfer succeeded if the call succeeded and either
// returned nothing, or returned a non-zero 32 byte value.
success := and(success, or(
iszero(returndatasize),