Explaining the partition offset feature

libisoburn/trunk/Makefile.am

@@ -228,6 +228,7 @@ EXTRA_DIST = \
 	version.h.in \
 	doc/comments \
 	doc/doxygen.conf.in \
+	doc/partition_offset.wiki \
 	README \
 	AUTHORS \
 	CONTRIBUTORS \

libisoburn/trunk/doc/partition_offset.wiki

@@ -0,0 +1,159 @@
+
+The partition offset feature of libisofs can produce ISO 9660 images which bear
+a quite conventional partition table if copied onto a USB stick. The first
+partition marks the size of the ISO image but starts at a non-zero address.
+Thus it marks a small part of the device as unclaimed by partitions and
+available for storing boot loader code.
+
+Nevertheless the USB stick is mountable via its overall device file as well as
+via the partition device file. E.g. on GNU/Linux: /dev/sdb and /dev/sdb1.
+This is achieved by two distinct sets of meta-data which refer to the same
+file content.
+
+The dual-mount feature supports Rock Ridge and eventually Joliet too.
+It is capable of multi-session.
+
+Currently only offset 32 kB seems to make sense. Smaller offsets are prohibited
+by fundamental assumptions of libisofs and libisoburn. Larger offsets would
+extend the unclaimed area into vital blocks of the ISO image.
+
+--------------------------------------------------------------------------
+
+Example:
+
+Testing mountability and ISOLINUX bootability from USB stick and CD.
+
+Overview:
+
+The test image was derived from one year old RIPLinux-9.3-non-X.iso which
+has an isohybrid MBR. Syslinux version seems to be 3.82. That MBR and the file
+tree from the mounted RIPLinux image was used to build a new ISO image
+with 16 * 2kB partition offset. Isohybrid MBR patching was done by xorriso.
+
+Details:
+
+The first 32 kB of an ISO 9660 image are called System Area and may host any
+byte pattern. In the case of RIPLinux-9.3-non-X.iso only the first 512 bytes
+are non-zero. But to avoid any assumptions, all 32 kB get copied here.
+{{{
+  dd if=RIPLinux-9.3-non-X.iso bs=1K count=32 of=RIPLinux-9.3-non-X.sysarea
+}}}
+The files of the image are made accessible for reading
+{{{
+  mount -o loop RIPLinux-9.3-non-X.iso /mnt
+}}}
+
+A new ISO image gets composed. The first three lines of arguments are taken
+from the prescriptions of ISOLINUX wiki and adapted to the names used in
+RIPLinux-9.3-non-X.iso.
+Option -isohybrid-mbr imports the copied System Area and patches the MBR
+according to rules published by hpa on Syslinux mailing list.
+Option -partition_offset 16 causes the first partition to start at 2 kB block
+number 16. It also prepares the image to be mountable by this partition, too.
+{{{
+  xorriso -as mkisofs \
+     -o new_image.iso \
+     -b boot/isolinux/isolinux.bin -c boot/boot.cat \
+     -no-emul-boot -boot-load-size 4 -boot-info-table \
+     -isohybrid-mbr RIPLinux-9.3-non-X.sysarea \
+     -partition_offset 16 \
+     /mnt
+}}}
+The image was copied onto a USB stick
+{{{
+  dd if=new_image.iso of=/dev/sdc
+}}}
+and plugged into a Debian system.
+{{{
+  fdisk -lu /dev/sdb
+}}}
+yields
+{{{
+   Device Boot      Start         End      Blocks   Id  System
+/dev/sdb1   *          64      119215       59576   17  Hidden HPFS/NTFS
+}}}
+
+I can mount /dev/sdb and /dev/sdb1 alike:
+{{{
+  mount /dev/sdb1 /mnt1
+  mount -o loop /dev/sdb /mnt
+}}}
+-o loop avoids failure with "mount: /dev/sdb already mounted or /mnt busy".
+A comparison by
+{{{
+  diff -r /mnt /mnt1
+}}}
+reports no difference.
+Human readable files look ok.
+Test-reading all content by
+{{{
+  tar cf - /mnt | wc
+}}}
+yields a reasonable byte count of 60743680 and no errors.
+
+The machine boots RIPLinux from this USB stick with no visible problems.
+It can then mount /dev/sdb as well as /dev/sdb1.
+The ISO image boots from CD too.
+
+Mounting the partition can be simulated with an image file on hard disk by
+cutting off the first partition_offset blocks of 2 KB:
+{{{
+  dd if=new_image.iso of=partition_image.iso bs=2048 skip=16
+  mount -o loop partition_image.iso /mnt1
+}}}
+
+--------------------------------------------------------------------------
+
+Open questions:
+
+- Shall the partition of an isohybrid image be marked bootable ?
+  Currently xorriso keeps the 0x80 mark of an eventually imported MBR
+resp. the 0x80 mark which xorriso eventually sets by its own MBR
+preparations.
+  - If not to be marked bootable:
+    What equipment would the partition need to justify having the mark ?
+
+- I am still puzzled by the cylinder-head-sector ambiguity. How does a reader
+of the partition table determine the numbers for heads/cylinder and
+sectors/head ?
+fdisk tells about my /dev/sdb
+{{{
+    105 heads, 17 sectors/track, 2193 cylinders, total 3915776 sectors
+}}}
+My partiton table rather was computed with 255 heads/cylinder, 63 sectors/head
+Debian does not mind the difference. (It uses the 32 bit LBAs, i assume.)
+
+------------------------------------------------------------------------
+
+Application:
+
+The partition offset feature can be controlled by libisofs API call
+{{{
+int iso_write_opts_set_part_offset(IsoWriteOpts *opts,
+                                   uint32_t block_offset_2k,
+                                   int secs_512_per_head,
+                                   int heads_per_cyl);
+}}}
+resp. by libisoburn calls
+{{{
+int isoburn_igopt_set_part_offset(struct isoburn_imgen_opts  *opts,
+                                  uint32_t block_offset_2k,
+                                  int secs_512_per_head, int heads_per_cyl);
+
+int isoburn_igopt_get_part_offset(struct isoburn_imgen_opts *opts,
+                                  uint32_t *block_offset_2k,
+                                  int *secs_512_per_head, int *heads_per_cyl);
+}}}
+resp. by xorriso options
+{{{
+  -boot_image any partition_offset=(2kb_block_adr)
+  -boot_image any partition_sec_hd=(number)
+  -boot_image any partition_hd_cyl=(number)
+  -as mkisofs ... -partition_offset (2kb_block_adr) ...
+}}}
+
+As stated above, an offset larger than 16 would expose vital parts of the
+ISO image as unclaimed space. Values smaller than 16 are not accepted.
+So use either an offset of 16 blocks or keep the feature disabled by
+offset 0.
+

libisoburn/trunk/xorriso/make_xorriso_standalone.sh

@@ -204,6 +204,9 @@ copy_files xorriso/xorriso_buildstamp_none.h \
 copy_files xorriso/xorriso_buildstamp_none.h \
            "$lone_dir"/xorriso/xorriso_buildstamp_none.h
 
+create_dir "$lone_dir"/doc
+copy_files doc/partition_offset.wiki "$lone_dir"/doc
+
 create_dir "$lone_dir"/test
 copy_files \
  test/compare_file.c \
@@ -214,7 +217,6 @@ copy_files \
 
 create_dir "$lone_dir"/libisofs
 create_dir "$lone_dir"/libisofs/filters
-create_dir "$lone_dir"/doc
 goto_dir   "$current_dir"/nglibisofs-develop
 copy_files libisofs/*.[ch] "$lone_dir"/libisofs
 copy_files libisofs/filters/*.[ch] "$lone_dir"/libisofs/filters

libisoburn/trunk/xorriso/xorriso_timestamp.h

@@ -1 +1 @@
-#define Xorriso_timestamP "2010.09.21.150325"
+#define Xorriso_timestamP "2010.09.23.135551"