path: root/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 = codeBitmap(code)
        d[codehash] = m
    }
    return OpCode(code[udest]) == JUMPDEST && m.codeSegment(udest)
}

// bitvec is a bit vector which maps bytes in a program.
// An unset bit means the byte is an opcode, a set bit means
// it's data (i.e. argument of PUSHxx).
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
}

// codeBitmap collects data locations in code.
func codeBitmap(code []byte) bitvec {
    // The bitmap 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.
    bits := make(bitvec, len(code)/8+1+4)
    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
}