Hive-test fixes (ethereum#14419)

* core: Fix for consensus test gasLimit > 2^63-1 https://github.com/ethereum/tests/blob/develop/BlockchainTests/bcInvalidHeaderTest.json#L238

* core: fix testcase for uncle gasUsage > gasLimit : https://github.com/ethereum/tests/blob/develop/BlockchainTests/EIP150/bcUncleHeaderValiditiy.json#L986

* math/big: rename TTM63m1 -> MaxBig63, + go fmt

* core: documentation

common/math/big.go

@@ -27,6 +27,8 @@ var (
 	tt256     = BigPow(2, 256)
 	tt256m1   = new(big.Int).Sub(tt256, big.NewInt(1))
 	MaxBig256 = new(big.Int).Set(tt256m1)
+	tt63      = BigPow(2, 63)
+	MaxBig63  = new(big.Int).Sub(tt63, big.NewInt(1))
 )
 
 const (

consensus/ethash/consensus.go

@@ -243,6 +243,15 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 	if expected.Cmp(header.Difficulty) != 0 {
 		return fmt.Errorf("invalid difficulty: have %v, want %v", header.Difficulty, expected)
 	}
+	// Verify that the gas limit is <= 2^63-1
+	if header.GasLimit.Cmp(math.MaxBig63) > 0 {
+		return fmt.Errorf("invalid gasLimit: have %v, max %v", header.GasLimit, math.MaxBig63)
+	}
+	// Verify that the gasUsed is <= gasLimit
+	if header.GasUsed.Cmp(header.GasLimit) > 0 {
+		return fmt.Errorf("invalid gasUsed: have %v, gasLimit %v", header.GasUsed, header.GasLimit)
+	}
+
 	// Verify that the gas limit remains within allowed bounds
 	diff := new(big.Int).Set(parent.GasLimit)
 	diff = diff.Sub(diff, header.GasLimit)