200 lines
6.8 KiB
Plaintext
200 lines
6.8 KiB
Plaintext
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The partition offset feature of libisofs can produce ISO 9660 images which bear
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a quite conventional partition table if copied onto a USB stick. The first
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partition marks the size of the ISO image but starts at a non-zero address.
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Thus it marks a small part of the device as unclaimed by partitions and
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available for storing boot loader code.
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Nevertheless the USB stick is mountable via its overall device file as well as
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via the partition device file. E.g. on GNU/Linux: /dev/sdb and /dev/sdb1.
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This is achieved by two distinct sets of meta-data which refer to the same
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file content.
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The dual-mount feature supports Rock Ridge and eventually Joliet too.
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It is capable of multi-session.
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Currently only offset 32 kB seems to make sense. Smaller offsets are prohibited
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by fundamental assumptions of libisofs and libisoburn. Larger offsets would
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extend the unclaimed area into vital blocks of the ISO image.
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--------------------------------------------------------------------------
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Meanwhile Debian
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[http://cdimage.debian.org/cdimage/daily-builds/daily/current/ daily]
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and [http://cdimage.debian.org/cdimage/weekly-builds/ weekly] builds make
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use of this feature with their bootable ISO images for i386 and amd64. E.g.
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[http://cdimage.debian.org/cdimage/daily-builds/daily/current/i386/iso-cd/debian-testing-i386-businesscard.iso debian-testing-i386-businesscard.iso].
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--------------------------------------------------------------------------
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Example:
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Testing mountability and ISOLINUX bootability from USB stick and CD.
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Overview:
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The test image was derived from one year old RIPLinux-9.3-non-X.iso which
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has an isohybrid MBR. Syslinux version seems to be 3.82. That MBR and the file
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tree from the mounted RIPLinux image was used to build a new ISO image
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with 16 * 2kB partition offset. Isohybrid MBR patching was done by xorriso.
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Details:
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The first 32 kB of an ISO 9660 image are called System Area and may host any
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byte pattern. In the case of RIPLinux-9.3-non-X.iso only the first 512 bytes
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are non-zero. But to avoid any assumptions, all 32 kB get copied here.
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{{{
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dd if=RIPLinux-9.3-non-X.iso bs=1K count=32 of=RIPLinux-9.3-non-X.sysarea
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}}}
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The files of the image are made accessible for reading
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{{{
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mount -o loop RIPLinux-9.3-non-X.iso /mnt
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}}}
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A new ISO image gets composed. The first three lines of arguments are taken
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from the prescriptions of ISOLINUX wiki and adapted to the names used in
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RIPLinux-9.3-non-X.iso.
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Option -isohybrid-mbr imports the copied System Area and patches the MBR
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according to rules published by hpa on Syslinux mailing list.
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Option -partition_offset 16 causes the first partition to start at 2 kB block
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number 16. It also prepares the image to be mountable by this partition, too.
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{{{
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xorriso -as mkisofs \
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-o new_image.iso \
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-b boot/isolinux/isolinux.bin -c boot/boot.cat \
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-no-emul-boot -boot-load-size 4 -boot-info-table \
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-isohybrid-mbr RIPLinux-9.3-non-X.sysarea \
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-partition_offset 16 \
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/mnt
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}}}
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The image was copied onto a USB stick
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{{{
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dd if=new_image.iso of=/dev/sdc
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}}}
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and plugged into a Debian system.
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{{{
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fdisk -lu /dev/sdb
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}}}
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yields
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{{{
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Device Boot Start End Blocks Id System
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/dev/sdb1 * 64 119215 59576 17 Hidden HPFS/NTFS
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}}}
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I can mount /dev/sdb and /dev/sdb1 alike:
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{{{
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mount /dev/sdb1 /mnt1
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mount -o loop /dev/sdb /mnt
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}}}
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-o loop avoids failure with "mount: /dev/sdb already mounted or /mnt busy".
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A comparison by
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{{{
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diff -r /mnt /mnt1
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}}}
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reports no difference.
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Human readable files look ok.
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Test-reading all content by
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{{{
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tar cf - /mnt | wc
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}}}
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yields a reasonable byte count of 60743680 and no errors.
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The machine boots RIPLinux from this USB stick with no visible problems.
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It can then mount /dev/sdb as well as /dev/sdb1.
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The ISO image boots from CD too.
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Mounting the partition can be simulated with an image file on hard disk by
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cutting off the first partition_offset blocks of 2 KB:
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{{{
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dd if=new_image.iso of=partition_image.iso bs=2048 skip=16
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mount -o loop partition_image.iso /mnt1
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}}}
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--------------------------------------------------------------------------
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Another test was made with GRUB 2 by downloading
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{{{
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bzr branch http://bzr.savannah.gnu.org/r/grub/trunk/grub
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}}}
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Before building GRUB 2, the file
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{{{
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util/grub-mkrescue.in
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}}}
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was edited to replace in the options of the xorriso command:
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{{{
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--protective-msdos-label
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}}}
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by
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{{{
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-partition_offset 16 -no-pad --no-emul-toc
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}}}
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Then GRUB 2 was built and installed.
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The resulting image from
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{{{
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./grub-mkrescue -o image.iso
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}}}
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was put onto USB stick. It passed the same tests on Debian
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as above RIPLinux example. It boots to a GRUB prompt.
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Due to options -no-pad --no-emul-toc the image is about 300 kB smaller than
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the image produced by original grub-mkrescue. Else it would have grown by
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about 130 kB.
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--------------------------------------------------------------------------
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Open questions:
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- Shall the partition of an isohybrid image be marked bootable ?
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Currently xorriso keeps the 0x80 mark of an eventually imported MBR
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resp. the 0x80 mark which xorriso eventually sets by its own MBR
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preparations.
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- If not to be marked bootable:
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What equipment would the partition need to justify having the mark ?
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- I am still puzzled by the cylinder-head-sector ambiguity. How does a reader
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of the partition table determine the numbers for heads/cylinder and
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sectors/head ?
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fdisk tells about my /dev/sdb
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{{{
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105 heads, 17 sectors/track, 2193 cylinders, total 3915776 sectors
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}}}
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My partiton table rather was computed with 255 heads/cylinder, 63 sectors/head
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Debian does not mind the difference. (It uses the 32 bit LBAs, i assume.)
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------------------------------------------------------------------------
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Application:
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The partition offset feature can be controlled by libisofs API call
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{{{
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int iso_write_opts_set_part_offset(IsoWriteOpts *opts,
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uint32_t block_offset_2k,
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int secs_512_per_head,
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int heads_per_cyl);
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}}}
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resp. by libisoburn calls
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{{{
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int isoburn_igopt_set_part_offset(struct isoburn_imgen_opts *opts,
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uint32_t block_offset_2k,
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int secs_512_per_head, int heads_per_cyl);
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int isoburn_igopt_get_part_offset(struct isoburn_imgen_opts *opts,
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uint32_t *block_offset_2k,
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int *secs_512_per_head, int *heads_per_cyl);
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}}}
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resp. by xorriso options
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{{{
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-boot_image any partition_offset=(2kb_block_adr)
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-boot_image any partition_sec_hd=(number)
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-boot_image any partition_hd_cyl=(number)
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-as mkisofs ... -partition_offset (2kb_block_adr) ...
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}}}
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As stated above, an offset larger than 16 would expose vital parts of the
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ISO image as unclaimed space. Values smaller than 16 are not accepted.
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So use either an offset of 16 blocks or keep the feature disabled by
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offset 0.
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