parse_dex.kt

import java.io.IOException
import java.io.RandomAccessFile
import java.nio.charset.StandardCharsets

/**
 * https://android.googlesource.com/platform/dalvik.git/+/master/tools/dexdeps/src/com/android/dexdeps
 *
 * Extracts information about methods and fields from .dex file
 */
internal class DexParser(
    private val dexFile: RandomAccessFile
) {

    private val tmpBuf = ByteArray(INT_SIZE_IN_BYTES)
    private var isBigEndian = false

    @Throws(IOException::class)
    fun load(): DexInfo {
        val headerItem = parseHeaderItem()
        return DexInfo(
            methods = headerItem.methodIdsSize,
            fields = headerItem.fieldIdsCount,
        )
    }

    @Throws(IOException::class)
    @Suppress("MagicNumber")
    private fun parseHeaderItem(): HeaderItem {
        dexFile.seek(0)
        verifyMagic()

        val headerItem = HeaderItem()

        /*
         * Read the endian tag, so we properly swap things as we read
         * them from here on.
         */
        dexFile.seek((DEX_FILE_MAGIC_LENGTH + 4 + 20 + 4 + 4).toLong())
        headerItem.endianTag = readInt()
        isBigEndian = if (headerItem.endianTag == ENDIAN_CONSTANT) {
            false
        } else if (headerItem.endianTag == REVERSE_ENDIAN_CONSTANT) {
            /* file is big-endian (!), reverse future reads */
            true
        } else {
            throw IOException("Endian constant has unexpected value " + Integer.toHexString(headerItem.endianTag))
        }
        dexFile.seek((DEX_FILE_MAGIC_LENGTH + 4 + 20).toLong()) // magic, checksum, signature
        /*headerItem.fileSize =*/readInt()
        /*headerItem.headerSize =*/readInt()
        /*headerItem.endianTag =*/readInt()
        /*headerItem.linkSize =*/readInt()
        /*headerItem.linkOff =*/readInt()
        /*headerItem.mapOff =*/readInt()
        /*headerItem.stringIdsSize =*/ readInt()
        /*headerItem.stringIdsOff =*/readInt()
        /*headerItem.typeIdsSize =*/ readInt()
        /*headerItem.typeIdsOff =*/readInt()
        /*headerItem.protoIdsSize =*/readInt()
        /*headerItem.protoIdsOff =*/readInt()
        headerItem.fieldIdsCount = readInt()
        /*headerItem.fieldIdsOff =*/readInt()
        headerItem.methodIdsSize = readInt()
        /*headerItem.methodIdsOff =*/readInt()
        /*headerItem.classDefsSize =*/ readInt()
        /*headerItem.classDefsOff =*/readInt()
        /*headerItem.dataSize =*/readInt()
        /*headerItem.dataOff =*/readInt()
        return headerItem
    }

    @Throws(IOException::class)
    private fun verifyMagic() {
        val magic = ByteArray(DEX_FILE_MAGIC_LENGTH)
        dexFile.readFully(magic)
        if (!verifyMagic(magic)) {
            throw IOException("Magic number is wrong -- are you sure this is a DEX file?")
        }
    }

    /**
     * Reads a signed 32-bit integer, byte-swapping if necessary.
     */
    @Throws(IOException::class)
    @Suppress("MagicNumber")
    private fun readInt(): Int {
        dexFile.readFully(tmpBuf, 0, INT_SIZE_IN_BYTES)
        return if (isBigEndian) {
            tmpBuf[3].toInt()
                .and(0xff)
                .or(tmpBuf[2].toInt().and(0xff).shl(8))
                .or(tmpBuf[1].toInt().and(0xff).shl(16))
                .or(tmpBuf[0].toInt().and(0xff).shl(24))
        } else {
            tmpBuf[0].toInt()
                .and(0xff)
                .or(tmpBuf[1].toInt() and 0xff shl 8)
                .or(tmpBuf[2].toInt() and 0xff shl 16)
                .or(tmpBuf[3].toInt() and 0xff shl 24)
        }
    }

    private fun verifyMagic(magic: ByteArray): Boolean {
        return magic.contentEquals(DEX_FILE_MAGIC_v035) ||
            magic.contentEquals(DEX_FILE_MAGIC_v037) ||
            magic.contentEquals(DEX_FILE_MAGIC_v038) ||
            magic.contentEquals(DEX_FILE_MAGIC_v039)
    }

    /**
     * Holds the contents of a header_item.
     */
    internal class HeaderItem {

        var endianTag = 0
        var fieldIdsCount = 0
        var methodIdsSize = 0
    }

    companion object {
        private const val DEX_FILE_MAGIC_LENGTH = 8

        /* expected magic values */
        private val DEX_FILE_MAGIC_v035 = "dex\n035\u0000".toByteArray(StandardCharsets.US_ASCII)

        // Dex version 036 skipped because of an old dalvik bug on some versions
        // of android where dex files with that version number would erroneously
        // be accepted and run. See: art/runtime/dex_file.cc
        // V037 was introduced in API LEVEL 24
        private val DEX_FILE_MAGIC_v037 = "dex\n037\u0000".toByteArray(StandardCharsets.US_ASCII)

        // V038 was introduced in API LEVEL 26
        private val DEX_FILE_MAGIC_v038 = "dex\n038\u0000".toByteArray(StandardCharsets.US_ASCII)

        // V039 was introduced in API LEVEL 28
        private val DEX_FILE_MAGIC_v039 = "dex\n039\u0000".toByteArray(StandardCharsets.US_ASCII)
        private const val ENDIAN_CONSTANT = 0x12345678
        private const val REVERSE_ENDIAN_CONSTANT = 0x78563412
        private const val INT_SIZE_IN_BYTES = 4
    }
}