source: bootcd/isolinux/syslinux-6.03/com32/gpllib/disk/msdos.c

/* ----------------------------------------------------------------------- *
 *
 *   Copyright 2009 Pierre-Alexandre Meyer
 *
 *   Some parts borrowed from chain.c32:
 *
 *   Copyright 2003-2009 H. Peter Anvin - All Rights Reserved
 *   Copyright 2009 Intel Corporation; author: H. Peter Anvin
 *
 *   This file is part of Syslinux, and is made available under
 *   the terms of the GNU General Public License version 2.
 *
 * ----------------------------------------------------------------------- */

#include <stdlib.h>

#include <disk/common.h>
#include <disk/geom.h>
#include <disk/msdos.h>
#include <disk/partition.h>
#include <disk/read.h>

static int is_extended_partition(struct part_entry *ptab)
{
    return (ptab->ostype == 0x05 ||
            ptab->ostype == 0x0f || ptab->ostype == 0x85);
}

static int msdos_magic_present(const char *ptab)
{
    return (*(uint16_t *) (ptab + 0x1fe) == 0xaa55);
}

/**
 * process_extended_partition - execute a callback for each partition contained listed in an ebr
 * @drive_info:         driveinfo struct describing the drive
 * @partition_offset:   Absolute start (lba) of the extended partition
 * @ebr_offset:         Relative start (lba) of the current ebr processed within
 *                      the extended partition
 * @callback:           Callback to execute
 * @error:              Buffer for I/O errors
 * @nb_part_seen:       Number of partitions found on the disk so far
 **/
static int process_extended_partition(struct driveinfo *drive_info,
                                      const int partition_offset,
                                      const int ebr_offset,
                                      p_callback callback, int nb_part_seen)
{
    int status = 0;
    /* The ebr is located at the first sector of the extended partition */
    char *ebr = malloc(SECTOR * sizeof(char));

    if (read_sectors(drive_info, ebr, partition_offset + ebr_offset, 1) == -1)
        goto abort;

    /* Check msdos magic signature */
    if (!msdos_magic_present(ebr))
        goto abort;

    struct part_entry *ptab =
        (struct part_entry *)(ebr + PARTITION_TABLES_OFFSET);

    for (int i = 0; i < 4; i++) {
        if (status == -1)
            goto abort;

        if (!is_extended_partition(&ptab[i])) {
            /*
             * This EBR partition table entry points to the
             * logical partition associated to that EBR
             */
            int logical_partition_start = ebr_offset + ptab[i].start_lba;

            /* Last EBR in the extended partition? */
            if (!logical_partition_start)
                continue;

            /*
             * Check for garbage:
             * 3rd and 4th entries in an EBR should be zero
             * Some (malformed) partitioning software still add some
             * data partitions there.
             */
            if (ptab[i].start_lba <= 0 || ptab[i].length <= 0)
                continue;

            nb_part_seen++;
            callback(drive_info,
                     &ptab[i],
                     partition_offset + logical_partition_start, nb_part_seen);
        } else
            status = process_extended_partition(drive_info,
                                                partition_offset,
                                                ptab[i].start_lba,
                                                callback, nb_part_seen);
    }

    free(ebr);
    return 0;

abort:
    free(ebr);
    return -1;
}

/**
 * process_mbr - execute a callback for each partition contained in an {m,e}br
 * @drive_info: driveinfo struct describing the drive
 * @ptab:       Pointer to the partition table
 * @callback:   Callback to execute
 **/
static int process_mbr(struct driveinfo *drive_info, struct part_entry *ptab,
                       p_callback callback)
{
    int status = 0;

    for (int i = 0; i < 4; i++) {
        if (status == -1)
            return -1;

        if (ptab[i].start_sect > 0) {
            if (is_extended_partition(&ptab[i])) {
                callback(drive_info, &ptab[i], ptab[i].start_lba, i + 1);
                status =
                    process_extended_partition(drive_info, ptab[i].start_lba, 0,
                                               callback, 4);
            } else
                callback(drive_info, &ptab[i], ptab[i].start_lba, i + 1);
        }
    }

    return 0;
}

/**
 * parse_partition_table - execute a callback for each partition entry
 * @d:          driveinfo struct describing the drive
 * @callback:   Callback to execute
 *
 * The signature of the callback should be the following:
 *
 * void callback(struct driveinfo *drive_info,
 *               struct part_entry *ptab,
 *               int offset_root,
 *               int nb_part_seen)
 **/
int parse_partition_table(struct driveinfo *d, p_callback callback)
{
    char *mbr = malloc(SECTOR * sizeof(char));

    if (read_mbr(d->disk, mbr) == -1)
        return -1;
    else {
        /* Check msdos magic signature */
        if (!msdos_magic_present(mbr))
            return -1;

        struct part_entry *ptab =
            (struct part_entry *)(mbr + PARTITION_TABLES_OFFSET);
        return process_mbr(d, ptab, callback);
    }
}