common/math: optimized modexp (+ fuzzer) (#25525)

This adds a * core/vm, tests: optimized modexp + fuzzer * common/math: modexp optimizations * core/vm: special case base 1 in big modexp * core/vm: disable fastexp
2022-10-12 10:34:52 +02:00
parent a007ab786c
commit bed3b10086
4 changed files with 181 additions and 2 deletions
@@ -0,0 +1,84 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package modexp
+
+import (
+	"fmt"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
+	"github.com/ethereum/go-ethereum/core/vm"
+)
+
+// The function must return
+// 1 if the fuzzer should increase priority of the
+//    given input during subsequent fuzzing (for example, the input is lexically
+//    correct and was parsed successfully);
+// -1 if the input must not be added to corpus even if gives new coverage; and
+// 0  otherwise
+// other values are reserved for future use.
+func Fuzz(input []byte) int {
+	if len(input) <= 96 {
+		return -1
+	}
+	// Abort on too expensive inputs
+	precomp := vm.PrecompiledContractsBerlin[common.BytesToAddress([]byte{5})]
+	if gas := precomp.RequiredGas(input); gas > 40_000_000 {
+		return 0
+	}
+	var (
+		baseLen = new(big.Int).SetBytes(getData(input, 0, 32)).Uint64()
+		expLen  = new(big.Int).SetBytes(getData(input, 32, 32)).Uint64()
+		modLen  = new(big.Int).SetBytes(getData(input, 64, 32)).Uint64()
+	)
+	// Handle a special case when both the base and mod length is zero
+	if baseLen == 0 && modLen == 0 {
+		return -1
+	}
+	input = input[96:]
+	// Retrieve the operands and execute the exponentiation
+	var (
+		base = new(big.Int).SetBytes(getData(input, 0, baseLen))
+		exp  = new(big.Int).SetBytes(getData(input, baseLen, expLen))
+		mod  = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
+	)
+	if mod.BitLen() == 0 {
+		// Modulo 0 is undefined, return zero
+		return -1
+	}
+	var a = math.FastExp(new(big.Int).Set(base), new(big.Int).Set(exp), new(big.Int).Set(mod))
+	var b = base.Exp(base, exp, mod)
+	if a.Cmp(b) != 0 {
+		panic(fmt.Sprintf("Inequality %x != %x", a, b))
+	}
+	return 1
+}
+
+// getData returns a slice from the data based on the start and size and pads
+// up to size with zero's. This function is overflow safe.
+func getData(data []byte, start uint64, size uint64) []byte {
+	length := uint64(len(data))
+	if start > length {
+		start = length
+	}
+	end := start + size
+	if end > length {
+		end = length
+	}
+	return common.RightPadBytes(data[start:end], int(size))
+}