MSCompoundFileReader.php

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<?php
/*
 * Copyright 2019 Wikimedia Foundation
 *
 * 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.
 */

class MSCompoundFileReader {
    private $file;
    private $header;
    private $mime;
    private $mimeFromClsid;
    private $error;
    private $errorCode;
    private $valid = false;

    private $sectorLength;
    private $difat;
    private $fat = [];
    private $fileLength;

    const TYPE_UNALLOCATED = 0;
    const TYPE_STORAGE = 1;
    const TYPE_STREAM = 2;
    const TYPE_ROOT = 5;

    const ERROR_FILE_OPEN = 1;
    const ERROR_SEEK = 2;
    const ERROR_READ = 3;
    const ERROR_INVALID_SIGNATURE = 4;
    const ERROR_READ_PAST_END = 5;
    const ERROR_INVALID_FORMAT = 6;

    private static $mimesByClsid = [
        // From http://justsolve.archiveteam.org/wiki/Microsoft_Compound_File
        '00020810-0000-0000-C000-000000000046' => 'application/vnd.ms-excel',
        '00020820-0000-0000-C000-000000000046' => 'application/vnd.ms-excel',
        '00020906-0000-0000-C000-000000000046' => 'application/msword',
        '64818D10-4F9B-11CF-86EA-00AA00B929E8' => 'application/vnd.ms-powerpoint',
    ];

    public static function readFile( $fileName ) {
        $handle = fopen( $fileName, 'r' );
        if ( $handle === false ) {
            return [
                'valid' => false,
                'error' => 'file does not exist',
                'errorCode' => self::ERROR_FILE_OPEN
            ];
        }
        return self::readHandle( $handle );
    }

    public static function readHandle( $fileHandle ) {
        $reader = new self( $fileHandle );
        $info = [
            'valid' => $reader->valid,
            'mime' => $reader->mime,
            'mimeFromClsid' => $reader->mimeFromClsid
        ];
        if ( $reader->error ) {
            $info['error'] = $reader->error;
            $info['errorCode'] = $reader->errorCode;
        }
        return $info;
    }

    private function __construct( $fileHandle ) {
        $this->file = $fileHandle;
        try {
            $this->init();
        } catch ( RuntimeException $e ) {
            $this->valid = false;
            $this->error = $e->getMessage();
            $this->errorCode = $e->getCode();
        }
    }

    private function init() {
        $this->header = $this->unpackOffset( 0, [
            'header_signature' => 8,
            'header_clsid' => 16,
            'minor_version' => 2,
            'major_version' => 2,
            'byte_order' => 2,
            'sector_shift' => 2,
            'mini_sector_shift' => 2,
            'reserved' => 6,
            'num_dir_sectors' => 4,
            'num_fat_sectors' => 4,
            'first_dir_sector' => 4,
            'transaction_signature_number' => 4,
            'mini_stream_cutoff_size' => 4,
            'first_mini_fat_sector' => 4,
            'num_mini_fat_sectors' => 4,
            'first_difat_sector' => 4,
            'num_difat_sectors' => 4,
            'difat' => 436,
        ] );
        if ( $this->header['header_signature'] !== "\xd0\xcf\x11\xe0\xa1\xb1\x1a\xe1" ) {
            $this->error( 'invalid signature: ' . bin2hex( $this->header['header_signature'] ),
                self::ERROR_INVALID_SIGNATURE );
        }
        $this->sectorLength = 1 << $this->header['sector_shift'];
        $this->readDifat();
        $this->readDirectory();

        $this->valid = true;
    }

    private function sectorOffset( $sectorId ) {
        return $this->sectorLength * ( $sectorId + 1 );
    }

    private function decodeClsid( $binaryClsid ) {
        $parts = unpack( 'Va/vb/vc/C8d', $binaryClsid );
        return sprintf( "%08X-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X",
            $parts['a'],
            $parts['b'],
            $parts['c'],
            $parts['d1'],
            $parts['d2'],
            $parts['d3'],
            $parts['d4'],
            $parts['d5'],
            $parts['d6'],
            $parts['d7'],
            $parts['d8']
        );
    }

    private function unpackOffset( $offset, $struct ) {
        $block = $this->readOffset( $offset, array_sum( $struct ) );
        return $this->unpack( $block, 0, $struct );
    }

    private function unpackSector( $sectorNumber, $struct ) {
        $offset = $this->sectorOffset( $sectorNumber );
        return $this->unpackOffset( $offset, array_sum( $struct ) );
    }

    private function unpack( $block, $offset, $struct ) {
        $data = [];
        foreach ( $struct as $key => $length ) {
            if ( $length > 4 ) {
                $data[$key] = substr( $block, $offset, $length );
            } else {
                $data[$key] = $this->bin2dec( $block, $offset, $length );
            }
            $offset += $length;
        }
        return $data;
    }

    private function bin2dec( $str, $offset, $length ) {
        $value = 0;
        for ( $i = $length - 1; $i >= 0; $i-- ) {
            $value *= 256;
            $value += ord( $str[$offset + $i] );
        }
        return $value;
    }

    private function readOffset( $offset, $length ) {
        $this->fseek( $offset );
        Wikimedia\suppressWarnings();
        $block = fread( $this->file, $length );
        Wikimedia\restoreWarnings();
        if ( $block === false ) {
            $this->error( 'error reading from file', self::ERROR_READ );
        }
        if ( strlen( $block ) !== $length ) {
            $this->error( 'unable to read the required number of bytes from the file',
                self::ERROR_READ_PAST_END );
        }
        return $block;
    }

    private function readSector( $sectorId ) {
        return $this->readOffset( $this->sectorOffset( $sectorId ), 1 << $this->header['sector_shift'] );
    }

    private function error( $message, $code ) {
        throw new RuntimeException( $message, $code );
    }

    private function fseek( $offset ) {
        Wikimedia\suppressWarnings();
        $result = fseek( $this->file, $offset );
        Wikimedia\restoreWarnings();
        if ( $result !== 0 ) {
            $this->error( "unable to seek to offset $offset", self::ERROR_SEEK );
        }
    }

    private function readDifat() {
        $binaryDifat = $this->header['difat'];
        $nextDifatSector = $this->header['first_difat_sector'];
        for ( $i = 0; $i < $this->header['num_difat_sectors']; $i++ ) {
            $block = $this->readSector( $nextDifatSector );
            $binaryDifat .= substr( $block, 0, $this->sectorLength - 4 );
            $nextDifatSector = $this->bin2dec( $block, $this->sectorLength - 4, 4 );
            if ( $nextDifatSector == 0xFFFFFFFE ) {
                break;
            }
        }

        $this->difat = [];
        for ( $pos = 0; $pos < strlen( $binaryDifat ); $pos += 4 ) {
            $fatSector = $this->bin2dec( $binaryDifat, $pos, 4 );
            if ( $fatSector < 0xFFFFFFFC ) {
                $this->difat[] = $fatSector;
            } else {
                break;
            }
        }
    }

    private function getNextSectorIdFromFat( $sectorId ) {
        $entriesPerSector = intdiv( $this->sectorLength, 4 );
        $fatSectorId = intdiv( $sectorId, $entriesPerSector );
        $fatSectorArray = $this->getFatSector( $fatSectorId );
        return $fatSectorArray[$sectorId % $entriesPerSector];
    }

    private function getFatSector( $fatSectorId ) {
        if ( !isset( $this->fat[$fatSectorId] ) ) {
            $fat = [];
            if ( !isset( $this->difat[$fatSectorId] ) ) {
                $this->error( 'FAT sector requested beyond the end of the DIFAT', self::ERROR_INVALID_FORMAT );
            }
            $absoluteSectorId = $this->difat[$fatSectorId];
            $block = $this->readSector( $absoluteSectorId );
            for ( $pos = 0; $pos < strlen( $block ); $pos += 4 ) {
                $fat[] = $this->bin2dec( $block, $pos, 4 );
            }
            $this->fat[$fatSectorId] = $fat;
        }
        return $this->fat[$fatSectorId];
    }

    private function readDirectory() {
        $dirSectorId = $this->header['first_dir_sector'];
        $binaryDir = '';
        $seenSectorIds = [];
        while ( $dirSectorId !== 0xFFFFFFFE ) {
            if ( isset( $seenSectorIds[$dirSectorId] ) ) {
                $this->error( 'FAT loop detected', self::ERROR_INVALID_FORMAT );
            }
            $seenSectorIds[$dirSectorId] = true;

            $binaryDir .= $this->readSector( $dirSectorId );
            $dirSectorId = $this->getNextSectorIdFromFat( $dirSectorId );
        }

        $struct = [
            'name_raw' => 64,
            'name_length' => 2,
            'object_type' => 1,
            'color' => 1,
            'sid_left' => 4,
            'sid_right' => 4,
            'sid_child' => 4,
            'clsid' => 16,
            'state_bits' => 4,
            'create_time_low' => 4,
            'create_time_high' => 4,
            'modify_time_low' => 4,
            'modify_time_high' => 4,
            'first_sector' => 4,
            'size_low' => 4,
            'size_high' => 4,
        ];
        $entryLength = array_sum( $struct );

        for ( $pos = 0; $pos < strlen( $binaryDir ); $pos += $entryLength ) {
            $entry = $this->unpack( $binaryDir, $pos, $struct );

            // According to [MS-CFB] size_high may contain garbage due to a
            // bug in a writer, it's best to pretend it is zero
            $entry['size_high'] = 0;

            $type = $entry['object_type'];
            if ( $type == self::TYPE_UNALLOCATED ) {
                continue;
            }

            $name = iconv( 'UTF-16LE', 'UTF-8', substr( $entry['name_raw'], 0, $entry['name_length'] - 2 ) );

            $clsid = $this->decodeClsid( $entry['clsid'] );
            if ( $type == self::TYPE_ROOT && isset( self::$mimesByClsid[$clsid] ) ) {
                $this->mimeFromClsid = self::$mimesByClsid[$clsid];
            }

            if ( $name === 'Workbook' ) {
                $this->mime = 'application/vnd.ms-excel';
            } elseif ( $name === 'WordDocument' ) {
                $this->mime = 'application/msword';
            } elseif ( $name === 'PowerPoint Document' ) {
                $this->mime = 'application/vnd.ms-powerpoint';
            }
        }
    }
}