android/util/apk/ZipUtils.java

/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.util.apk;

import android.util.Pair;

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;

/**
 * Assorted ZIP format helpers.
 *
 * <p>NOTE: Most helper methods operating on {@code ByteBuffer} instances expect that the byte
 * order of these buffers is little-endian.
 */
abstract class ZipUtils {
    private ZipUtils() {}

    private static final int ZIP_EOCD_REC_MIN_SIZE = 22;
    private static final int ZIP_EOCD_REC_SIG = 0x06054b50;
    private static final int ZIP_EOCD_CENTRAL_DIR_SIZE_FIELD_OFFSET = 12;
    private static final int ZIP_EOCD_CENTRAL_DIR_OFFSET_FIELD_OFFSET = 16;
    private static final int ZIP_EOCD_COMMENT_LENGTH_FIELD_OFFSET = 20;

    private static final int ZIP64_EOCD_LOCATOR_SIZE = 20;
    private static final int ZIP64_EOCD_LOCATOR_SIG_REVERSE_BYTE_ORDER = 0x504b0607;

    private static final int UINT16_MAX_VALUE = 0xffff;

    /**
     * Returns the ZIP End of Central Directory record of the provided ZIP file.
     *
     * @return contents of the ZIP End of Central Directory record and the record's offset in the
     *         file or {@code null} if the file does not contain the record.
     *
     * @throws IOException if an I/O error occurs while reading the file.
     */
    static Pair<ByteBuffer, Long> findZipEndOfCentralDirectoryRecord(RandomAccessFile zip)
            throws IOException {
        // ZIP End of Central Directory (EOCD) record is located at the very end of the ZIP archive.
        // The record can be identified by its 4-byte signature/magic which is located at the very
        // beginning of the record. A complication is that the record is variable-length because of
        // the comment field.
        // The algorithm for locating the ZIP EOCD record is as follows. We search backwards from
        // end of the buffer for the EOCD record signature. Whenever we find a signature, we check
        // the candidate record's comment length is such that the remainder of the record takes up
        // exactly the remaining bytes in the buffer. The search is bounded because the maximum
        // size of the comment field is 65535 bytes because the field is an unsigned 16-bit number.

        long fileSize = zip.length();
        if (fileSize < ZIP_EOCD_REC_MIN_SIZE) {
            return null;
        }

        // Optimization: 99.99% of APKs have a zero-length comment field in the EoCD record and thus
        // the EoCD record offset is known in advance. Try that offset first to avoid unnecessarily
        // reading more data.
        Pair<ByteBuffer, Long> result = findZipEndOfCentralDirectoryRecord(zip, 0);
        if (result != null) {
            return result;
        }

        // EoCD does not start where we expected it to. Perhaps it contains a non-empty comment
        // field. Expand the search. The maximum size of the comment field in EoCD is 65535 because
        // the comment length field is an unsigned 16-bit number.
        return findZipEndOfCentralDirectoryRecord(zip, UINT16_MAX_VALUE);
    }

    /**
     * Returns the ZIP End of Central Directory record of the provided ZIP file.
     *
     * @param maxCommentSize maximum accepted size (in bytes) of EoCD comment field. The permitted
     *        value is from 0 to 65535 inclusive. The smaller the value, the faster this method
     *        locates the record, provided its comment field is no longer than this value.
     *
     * @return contents of the ZIP End of Central Directory record and the record's offset in the
     *         file or {@code null} if the file does not contain the record.
     *
     * @throws IOException if an I/O error occurs while reading the file.
     */
    private static Pair<ByteBuffer, Long> findZipEndOfCentralDirectoryRecord(
            RandomAccessFile zip, int maxCommentSize) throws IOException {
        // ZIP End of Central Directory (EOCD) record is located at the very end of the ZIP archive.
        // The record can be identified by its 4-byte signature/magic which is located at the very
        // beginning of the record. A complication is that the record is variable-length because of
        // the comment field.
        // The algorithm for locating the ZIP EOCD record is as follows. We search backwards from
        // end of the buffer for the EOCD record signature. Whenever we find a signature, we check
        // the candidate record's comment length is such that the remainder of the record takes up
        // exactly the remaining bytes in the buffer. The search is bounded because the maximum
        // size of the comment field is 65535 bytes because the field is an unsigned 16-bit number.

        if ((maxCommentSize < 0) || (maxCommentSize > UINT16_MAX_VALUE)) {
            throw new IllegalArgumentException("maxCommentSize: " + maxCommentSize);
        }

        long fileSize = zip.length();
        if (fileSize < ZIP_EOCD_REC_MIN_SIZE) {
            // No space for EoCD record in the file.
            return null;
        }
        // Lower maxCommentSize if the file is too small.
        maxCommentSize = (int) Math.min(maxCommentSize, fileSize - ZIP_EOCD_REC_MIN_SIZE);

        ByteBuffer buf = ByteBuffer.allocate(ZIP_EOCD_REC_MIN_SIZE + maxCommentSize);
        buf.order(ByteOrder.LITTLE_ENDIAN);
        long bufOffsetInFile = fileSize - buf.capacity();
        zip.seek(bufOffsetInFile);
        zip.readFully(buf.array(), buf.arrayOffset(), buf.capacity());
        int eocdOffsetInBuf = findZipEndOfCentralDirectoryRecord(buf);
        if (eocdOffsetInBuf == -1) {
            // No EoCD record found in the buffer
            return null;
        }
        // EoCD found
        buf.position(eocdOffsetInBuf);
        ByteBuffer eocd = buf.slice();
        eocd.order(ByteOrder.LITTLE_ENDIAN);
        return Pair.create(eocd, bufOffsetInFile + eocdOffsetInBuf);
    }

    /**
     * Returns the position at which ZIP End of Central Directory record starts in the provided
     * buffer or {@code -1} if the record is not present.
     *
     * <p>NOTE: Byte order of {@code zipContents} must be little-endian.
     */
    private static int findZipEndOfCentralDirectoryRecord(ByteBuffer zipContents) {
        assertByteOrderLittleEndian(zipContents);

        // ZIP End of Central Directory (EOCD) record is located at the very end of the ZIP archive.
        // The record can be identified by its 4-byte signature/magic which is located at the very
        // beginning of the record. A complication is that the record is variable-length because of
        // the comment field.
        // The algorithm for locating the ZIP EOCD record is as follows. We search backwards from
        // end of the buffer for the EOCD record signature. Whenever we find a signature, we check
        // the candidate record's comment length is such that the remainder of the record takes up
        // exactly the remaining bytes in the buffer. The search is bounded because the maximum
        // size of the comment field is 65535 bytes because the field is an unsigned 16-bit number.

        int archiveSize = zipContents.capacity();
        if (archiveSize < ZIP_EOCD_REC_MIN_SIZE) {
            return -1;
        }
        int maxCommentLength = Math.min(archiveSize - ZIP_EOCD_REC_MIN_SIZE, UINT16_MAX_VALUE);
        int eocdWithEmptyCommentStartPosition = archiveSize - ZIP_EOCD_REC_MIN_SIZE;
        for (int expectedCommentLength = 0; expectedCommentLength <= maxCommentLength;
                expectedCommentLength++) {
            int eocdStartPos = eocdWithEmptyCommentStartPosition - expectedCommentLength;
            if (zipContents.getInt(eocdStartPos) == ZIP_EOCD_REC_SIG) {
                int actualCommentLength =
                        getUnsignedInt16(
                                zipContents, eocdStartPos + ZIP_EOCD_COMMENT_LENGTH_FIELD_OFFSET);
                if (actualCommentLength == expectedCommentLength) {
                    return eocdStartPos;
                }
            }
        }

        return -1;
    }

    /**
     * Returns {@code true} if the provided file contains a ZIP64 End of Central Directory
     * Locator.
     *
     * @param zipEndOfCentralDirectoryPosition offset of the ZIP End of Central Directory record
     *        in the file.
     *
     * @throws IOException if an I/O error occurs while reading the file.
     */
    public static final boolean isZip64EndOfCentralDirectoryLocatorPresent(
            RandomAccessFile zip, long zipEndOfCentralDirectoryPosition) throws IOException {

        // ZIP64 End of Central Directory Locator immediately precedes the ZIP End of Central
        // Directory Record.
        long locatorPosition = zipEndOfCentralDirectoryPosition - ZIP64_EOCD_LOCATOR_SIZE;
        if (locatorPosition < 0) {
            return false;
        }

        zip.seek(locatorPosition);
        // RandomAccessFile.readInt assumes big-endian byte order, but ZIP format uses
        // little-endian.
        return zip.readInt() == ZIP64_EOCD_LOCATOR_SIG_REVERSE_BYTE_ORDER;
    }

    /**
     * Returns the offset of the start of the ZIP Central Directory in the archive.
     *
     * <p>NOTE: Byte order of {@code zipEndOfCentralDirectory} must be little-endian.
     */
    public static long getZipEocdCentralDirectoryOffset(ByteBuffer zipEndOfCentralDirectory) {
        assertByteOrderLittleEndian(zipEndOfCentralDirectory);
        return getUnsignedInt32(
                zipEndOfCentralDirectory,
                zipEndOfCentralDirectory.position() + ZIP_EOCD_CENTRAL_DIR_OFFSET_FIELD_OFFSET);
    }

    /**
     * Sets the offset of the start of the ZIP Central Directory in the archive.
     *
     * <p>NOTE: Byte order of {@code zipEndOfCentralDirectory} must be little-endian.
     */
    public static void setZipEocdCentralDirectoryOffset(
            ByteBuffer zipEndOfCentralDirectory, long offset) {
        assertByteOrderLittleEndian(zipEndOfCentralDirectory);
        setUnsignedInt32(
                zipEndOfCentralDirectory,
                zipEndOfCentralDirectory.position() + ZIP_EOCD_CENTRAL_DIR_OFFSET_FIELD_OFFSET,
                offset);
    }

    /**
     * Returns the size (in bytes) of the ZIP Central Directory.
     *
     * <p>NOTE: Byte order of {@code zipEndOfCentralDirectory} must be little-endian.
     */
    public static long getZipEocdCentralDirectorySizeBytes(ByteBuffer zipEndOfCentralDirectory) {
        assertByteOrderLittleEndian(zipEndOfCentralDirectory);
        return getUnsignedInt32(
                zipEndOfCentralDirectory,
                zipEndOfCentralDirectory.position() + ZIP_EOCD_CENTRAL_DIR_SIZE_FIELD_OFFSET);
    }

    private static void assertByteOrderLittleEndian(ByteBuffer buffer) {
        if (buffer.order() != ByteOrder.LITTLE_ENDIAN) {
            throw new IllegalArgumentException("ByteBuffer byte order must be little endian");
        }
    }

    private static int getUnsignedInt16(ByteBuffer buffer, int offset) {
        return buffer.getShort(offset) & 0xffff;
    }

    private static long getUnsignedInt32(ByteBuffer buffer, int offset) {
        return buffer.getInt(offset) & 0xffffffffL;
    }

    private static void setUnsignedInt32(ByteBuffer buffer, int offset, long value) {
        if ((value < 0) || (value > 0xffffffffL)) {
            throw new IllegalArgumentException("uint32 value of out range: " + value);
        }
        buffer.putInt(buffer.position() + offset, (int) value);
    }
}