isolinux.bin changes

randysparks · 01-03-2007, 09:34 AM

I've made my own DVD using the Ubuntu Edgy 6.10 ISO image as a base.

I've not modified *any* files contained within the original ISO, apart from the isolinux.cfg file -- the boot menu. I commented-out one of the entries.

I also added a new directory containing some files.

To make the DVD, I mounted the Edgy ISO image and copied the files to a new directory. Then I added my additional directory, and created a new ISO for the DVD using the following command:

mkisofs -r -V "Custom Ubuntu" -cache-inodes -J -l -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -o /tmp/DVD.iso .

Once the ISO is burned to DVD disc, it works fine. Edgy boots and installs. All files are accessible.

But... for some reason, one byte of the isolinux.bin file is different. In addition, its time/date stamp of the file is updated. In other words, mkisofs has modified it. Why? When I did exactly the same thing with the Dapper ISO, the isolinux.bin file wasn't touched.

Like I say, this isn't a problem. But I'm curious as to what's happening. Why is mkisofs modifying the isolinux.bin file? What's it doing?

win32sux · 01-03-2007, 12:08 PM

Quote:

Originally Posted by randysparks

I've made my own DVD using the Ubuntu Edgy 6.10 ISO image as a base.

I've not modified *any* files contained within the original ISO, apart from the isolinux.cfg file -- the boot menu. I commented-out one of the entries.

I also added a new directory containing some files.

To make the DVD, I mounted the Edgy ISO image and copied the files to a new directory. Then I added my additional directory, and created a new ISO for the DVD using the following command:

mkisofs -r -V "Custom Ubuntu" -cache-inodes -J -l -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -o /tmp/DVD.iso .

Once the ISO is burned to DVD disc, it works fine. Edgy boots and installs. All files are accessible.

But... for some reason, one byte of the isolinux.bin file is different. In addition, its time/date stamp of the file is updated. In other words, mkisofs has modified it. Why? When I did exactly the same thing with the Dapper ISO, the isolinux.bin file wasn't touched.

Like I say, this isn't a problem. But I'm curious as to what's happening. Why is mkisofs modifying the isolinux.bin file? What's it doing?

my guess is it's your "-r" option... try a _test_ using "-R" instead and see if that leaves your isolinux.bin untouched...

Code:

-r     This is like the -R option, but file ownership and modes are set
              to more useful values.  The uid and gid are set to zero, because
              they  are  usually  only  useful on the author’s system, and not
              useful to the client.  All the file read bits are set  true,  so
              that  files and directories are globally readable on the client.
              If any execute bit is set for a file, set  all  of  the  execute
              bits, so that executables are globally executable on the client.
              If any search bit is set for a directory, set all of the  search
              bits, so that directories are globally searchable on the client.
              All write bits are cleared, because the CD-Rom will  be  mounted
              read-only in any case.  If any of the special mode bits are set,
              clear them, because file locks are not  useful  on  a  read-only
              file  system, and set-id bits are not desirable for uid 0 or gid
              0.  When used on Win32, the execute bit is  set  on  all  files.
              This  is  a  result of the lack of file permissions on Win32 and
              the  Cygwin  POSIX  emulation  layer.   See  also   -uid   -gid,
              -dir-mode, -file-mode and -new-dir-mode.

andygoth · 06-06-2021, 12:10 PM

Quote:

Originally Posted by randysparks

I've made my own DVD using the Ubuntu Edgy 6.10 ISO image as a base. [...] But... for some reason, one byte of the isolinux.bin file is different. [...] In other words, mkisofs has modified it. Why? When I did exactly the same thing with the Dapper ISO, the isolinux.bin file wasn't touched.

Pardon the thread necromancy, but even though it's been fourteen years, I do have a real answer to share, and I want to link it here in case anyone else comes along searching. I mean, I was one such person, and since no answers were readily available, I had to dive into source code and work it out for myself.

mkisofs/xorriso indeed does (and must) modify isolinux.bin, or whichever El Torito bootloader is used, to patch in its LBA. That's Logical Block Address, or the raw location of the file on the disc divided by 2048. This is done because during early boot, the first 2048 bytes of isolinux.bin get loaded into memory at a known location before being executed, and it then has to find the rest of itself. Rather than require each bootloader to implement a full ISO9660 driver inside that 2K, El Torito tries to make things easy by just putting the location in a known spot.

Your experience with Dapper suggests that your changes did not result in isolinux.bin actually moving to a new location on the disc. Perhaps you added files that got sorted after it, perhaps you modified files without changing their size (measured in LBAs), etc.

Read my analysis here.