libisofs/libisofs/make_isohybrid_mbr.c

#include <ctype.h>
#include <stdint.h>
#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

/* for gettimeofday() */
#include <sys/time.h>

/* This code stems from syslinux-3.72/utils/isohybrid, a perl script
 under GPL which is Copyright 2002-2008 H. Peter Anvin.
 Line numbers in comments refer to the lines of that script.
 It has been analyzed and re-written in C language 2008 by Thomas Schmitt.
 As soon as it is part of a libisofs test version, H. Peter Anvin will
 be asked about his own copyright claims and/or license tolerance in
 respect to this C code.
 */

/* A helper function. One could replace it by one or two macros. */
static int lsb_to_buf(char **wpt, int value, int bits, int flag)
{
    int b;

    for (b = 0; b < bits; b += 8)
        *((unsigned char *) ((*wpt)++)) = (value >> b) & 0xff;
    return (1);
}

/*
 * Create a MBR for an isohybrid enabled ISOLINUX boot image.
 *
 * It is assumed that the caller has verified the readiness of the boot image
 * by checking for 0xfb 0xc0 0x78 0x70 at bytes 0x40 to 0x43 of isolinux.bin.
 *
 * @param bin_lba    The predicted LBA of isolinux.bin within the emerging ISO.
 * @param img_blocks The predicted number of 2048 byte blocks in the ISO.
 *                   It will get rounded up to full MBs and that many blocks
 *                   must really be written as ISO 9660 image.
 * @param mbr        A buffer of at least 512 bytes to take the result which is
 *                   to be written as the very beginning of the ISO.
 * @param flag    unused yet, submit 0
 * @return  <0 = fatal, 0 = failed , 1 = ok , 2 = ok with size warning
 */
int make_isohybrid_mbr(int bin_lba, int *img_blocks, char *mbr, int flag)
{
    /* According to H. Peter Anvin this binary code stems from
     syslinux-3.72/mbr/isohdpfx.S
     */
    static int mbr_code_size = 271;
    static unsigned char mbr_code[271] = { 0xfa, 0x31, 0xc0, 0x8e, 0xd8, 0x8e,
            0xd0, 0xbc, 0x00, 0x7c, 0x89, 0xe6, 0x06, 0x57, 0x52, 0x8e, 0xc0,
            0xfb, 0xfc, 0xbf, 0x00, 0x06, 0xb9, 0x00, 0x01, 0xf3, 0xa5, 0xea,
            0x20, 0x06, 0x00, 0x00, 0x52, 0xb4, 0x41, 0xbb, 0xaa, 0x55, 0x31,
            0xc9, 0x30, 0xf6, 0xf9, 0xcd, 0x13, 0x72, 0x14, 0x81, 0xfb, 0x55,
            0xaa, 0x75, 0x0e, 0x83, 0xe1, 0x01, 0x74, 0x09, 0x66, 0xc7, 0x06,
            0xb4, 0x06, 0xb4, 0x42, 0xeb, 0x15, 0x5a, 0x51, 0xb4, 0x08, 0xcd,
            0x13, 0x83, 0xe1, 0x3f, 0x51, 0x0f, 0xb6, 0xc6,

            0x40, 0x50, 0xf7, 0xe1, 0x52, 0x50, 0xbb, 0x00, 0x7c, 0xb9, 0x04,
            0x00, 0x66, 0xa1, 0xb0, 0x07, 0xe8, 0x40, 0x00, 0x72, 0x74, 0x66,
            0x40, 0x80, 0xc7, 0x02, 0xe2, 0xf4, 0x66, 0x81, 0x3e, 0x40, 0x7c,
            0xfb, 0xc0, 0x78, 0x70, 0x75, 0x07, 0xfa, 0xbc, 0xf4, 0x7b, 0xe9,
            0xc6, 0x75, 0xe8, 0x79, 0x00, 0x69, 0x73, 0x6f, 0x6c, 0x69, 0x6e,
            0x75, 0x78, 0x2e, 0x62, 0x69, 0x6e, 0x20, 0x6d, 0x69, 0x73, 0x73,
            0x69, 0x6e, 0x67, 0x20, 0x6f, 0x72, 0x20, 0x63, 0x6f, 0x72, 0x72,
            0x75, 0x70, 0x74,

            0x2e, 0x0d, 0x0a, 0x66, 0x60, 0x66, 0x31, 0xd2, 0x66, 0x52, 0x66,
            0x50, 0x06, 0x53, 0x6a, 0x01, 0x6a, 0x10, 0x89, 0xe6, 0x66, 0xf7,
            0x36, 0xf0, 0x7b, 0xc0, 0xe4, 0x06, 0x88, 0xe1, 0x88, 0xc5, 0x92,
            0xf6, 0x36, 0xf6, 0x7b, 0x88, 0xc6, 0x08, 0xe1, 0x41, 0xb8, 0x01,
            0x02, 0x8a, 0x16, 0xfa, 0x7b, 0xcd, 0x13, 0x8d, 0x64, 0x10, 0x66,
            0x61, 0xc3, 0xe8, 0x1e, 0x00, 0x4f, 0x70, 0x65, 0x72, 0x61, 0x74,
            0x69, 0x6e, 0x67, 0x20, 0x73, 0x79, 0x73, 0x74, 0x65, 0x6d, 0x20,
            0x6c, 0x6f, 0x61,

            0x64, 0x20, 0x65, 0x72, 0x72, 0x6f, 0x72, 0x2e, 0x0d, 0x0a, 0x5e,
            0xac, 0xb4, 0x0e, 0x8a, 0x3e, 0x62, 0x04, 0xb3, 0x07, 0xcd, 0x10,
            0x3c, 0x0a, 0x75, 0xf1, 0xcd, 0x18, 0xf4, 0xeb, 0xfd };

    static int h = 64, s = 32;

    int i, warn_size = 0, id;
    char *wpt;
    off_t imgsize, cylsize, frac, padding, c, cc;

    /* For generating a weak random number */
    struct timeval tv;
    struct timezone tz;

    if (bin_lba < 0 || bin_lba >= (1 << 29))
        return (0); /* 1 TB limit of signed 32 bit addressing of 512 byte blocks */

    /*
     84: Gets size of image in bytes.
     89:
     */
    imgsize = ((off_t) img_blocks) * (off_t) 2048;

    /*
     90: Computes $padding, size of padded image and number $c of
     pseudo-cylinders ($h and $s are constants set in line 24).
     102: $cc is $c curbed to 1024.
     */
    cylsize = h * s * 512;
    frac = imgsize % cylsize;
    padding = (frac > 0 ? cylsize - frac : 0);
    imgsize += padding;
    *img_blocks = imgsize / (off_t) 2048;
    c = imgsize / cylsize;
    if (c > 1024) {
        warn_size = 1;
        cc = 1024;
    } else
        cc = c;

    /*
     107: Brings the hex-encoded bytes from the file __END__ into
     113: the MBR buffer. (This function holds them in mbr_code[].)
     */
    for (i = 0; i < mbr_code_size; i++)
        mbr[i] = mbr_code[i];

    /*
     118: Pads up to 432
     */
    for (i = mbr_code_size; i < 432; i++)
        mbr[i] = 0;

    /* From here on use write cursor wpt */
    wpt = mbr + 432;

    /*
     120: Converts LBA of isolinux.bin to blocksize 512 and writes
     to buffer. Appends 4 zero bytes.
     */
    lsb_to_buf(&wpt, bin_lba * 4, 32, 0);
    lsb_to_buf(&wpt, 0, 32, 0);

    /*
     121: I do not understand where $id should eventually come
     from. An environment variable ?
     125: Whatever, i use some 32-bit random value with no crypto strength.
     */
    gettimeofday(&tv, &tz);
    id = 0xffffffff & (tv.tv_sec ^ (tv.tv_usec * 2000));

    /*
     126: Adds 4 id bytes and 2 zero bytes.
     */
    lsb_to_buf(&wpt, id, 32, 0);
    lsb_to_buf(&wpt, 0, 16, 0);

    /*
     129: Composes 16 byte record from the parameters determined
     147: so far. Appends it to buffer and add 48 zero bytes.
     */
    /* There are 4 "partition slots". Only the first is non-zero here.
     In the perl script, the fields are set in variables and then
     written to buffer. I omit the variables.
     */
    /* 0x80 */
    lsb_to_buf(&wpt, 0x80, 8, 0);
    /* bhead */
    lsb_to_buf(&wpt, 0, 8, 0);
    /* bsect */
    lsb_to_buf(&wpt, 1, 8, 0);
    /* bcyl */
    lsb_to_buf(&wpt, 0, 8, 0);
    /* fstype */
    lsb_to_buf(&wpt, 0x83, 8, 0);
    /* ehead */
    lsb_to_buf(&wpt, h - 1, 8, 0);
    /* esect */
    lsb_to_buf(&wpt, s + (((cc - 1) & 0x300) >> 2), 8, 0);
    /* ecyl */
    lsb_to_buf(&wpt, (cc - 1) & 0xff, 8, 0);
    /* 0 */
    lsb_to_buf(&wpt, 0, 32, 0);
    /* psize */
    lsb_to_buf(&wpt, c * h * s, 32, 0);

    /* Fill the other three slots with zeros */
    for (i = 0; i < 3 * 4; i++)
        lsb_to_buf(&wpt, 0, 32, 0);

    /*
     148: Appends bytes 0x55 , 0xAA.
     */
    lsb_to_buf(&wpt, 0x55, 8, 0);
    lsb_to_buf(&wpt, 0xaa, 8, 0);

    return (1);
}