hfs_block_is_alloc(HFS_INFO * hfs, TSK_DADDR_T a_addr)
{
    TSK_FS_INFO *fs = &(hfs->fs_info);
    TSK_OFF_T b;
    size_t b2;

    if (hfs->blockmap_file == NULL) {
        if ((hfs->blockmap_file =
                tsk_fs_file_open_meta(fs, NULL,
                    HFS_ALLOCATION_FILE_ID)) == NULL) {
            tsk_error_errstr2_concat(" - Loading blockmap file");
            return -1;
        }

        /* cache the data attribute */
        hfs->blockmap_attr =
            tsk_fs_attrlist_get(hfs->blockmap_file->meta->attr,
            TSK_FS_ATTR_TYPE_DEFAULT);
        if (!hfs->blockmap_attr) {
            tsk_error_errstr2_concat
                (" - Data Attribute not found in Blockmap File");
            return -1;
        }
        hfs->blockmap_cache_start = -1;
        hfs->blockmap_cache_len = 0;
    }

    b = (TSK_OFF_T) a_addr / 8;
    if (b > hfs->blockmap_file->meta->size) {
        tsk_error_set_errno(TSK_ERR_FS_CORRUPT);
        tsk_error_set_errstr("hfs_block_is_alloc: block %" PRIuDADDR
            " is too large for bitmap (%" PRIuOFF ")", a_addr,
            hfs->blockmap_file->meta->size);
        return -1;
    }

    if ((hfs->blockmap_cache_start == -1)
        || (hfs->blockmap_cache_start > b)
        || (hfs->blockmap_cache_start + hfs->blockmap_cache_len <= b)) {
        size_t cnt = tsk_fs_attr_read(hfs->blockmap_attr, b,
            hfs->blockmap_cache,
            sizeof(hfs->blockmap_cache), 0);
        if (cnt < 1) {
            tsk_error_set_errstr2
                ("hfs_block_is_alloc: Error reading block bitmap at offset %"
                PRIuOFF, b);
            return -1;
        }
        hfs->blockmap_cache_start = b;
        hfs->blockmap_cache_len = cnt;
    }
    b2 = (size_t) (b - hfs->blockmap_cache_start);
    return (hfs->blockmap_cache[b2] & (1 << (7 - (a_addr % 8)))) != 0;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.