go-ethereum/core/vm/analysis.go

// Copyright 2014 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 vm

import (
	"math/big"

	"github.com/ethereum/go-ethereum/common"
)

// destinations stores one map per contract (keyed by hash of code).
// The maps contain an entry for each location of a JUMPDEST
// instruction.
type destinations map[common.Hash]bitvec

// has checks whether code has a JUMPDEST at dest.
func (d destinations) has(codehash common.Hash, code []byte, dest *big.Int) bool {
	// PC cannot go beyond len(code) and certainly can't be bigger than 63bits.
	// Don't bother checking for JUMPDEST in that case.
	udest := dest.Uint64()
	if dest.BitLen() >= 63 || udest >= uint64(len(code)) {
		return false
	}

	m, analysed := d[codehash]
	if !analysed {
		m = jumpdests(code)
		d[codehash] = m
	}
	return OpCode(code[udest]) == JUMPDEST && m.codeSegment(udest)
	//	return (m[udest/8] & (1 << (udest % 8))) != 0
}

// bitvec is a bit vector which maps bytes in a program
// An unset bit means the byte is a code-segemnt, a set bit means it's data-segment
type bitvec []byte

func (bits *bitvec) set(pos uint64) {
	(*bits)[pos/8] |= 0x80 >> (pos % 8)
}
func (bits *bitvec) set8(pos uint64) {
	(*bits)[pos/8] |= 0xFF >> (pos % 8)
	(*bits)[pos/8+1] |= ^(0xFF >> (pos % 8))
}

// codeSegment checks if the position is in a code segment
func (bits *bitvec) codeSegment(pos uint64) bool {
	return ((*bits)[pos/8] & (0x80 >> (pos % 8))) == 0
}

// jumpdests creates a map that contains an entry for each
// PC location that is a JUMPDEST instruction.
func jumpdests(code []byte) []byte {
	//The map is 4 bytes longer than necessary, in case the code
	// ends with a PUSH32, the algorithm will push zeroes onto the
	// bitvector outside the bounds of the actual code.
	m := make([]byte, len(code)/8+1+4)
	bits := bitvec(m)
	for pc := uint64(0); pc < uint64(len(code)); {
		op := OpCode(code[pc])

		if op >= PUSH1 && op <= PUSH32 {
			numbits := op - PUSH1 + 1
			pc++
			for ; numbits >= 8; numbits -= 8 {
				bits.set8(pc) // 8
				pc += 8
			}
			for ; numbits > 0; numbits-- {
				bits.set(pc)
				pc++
			}
		} else {
			pc++
		}
	}
	return bits
}