Loopback problems in 2.4.x kernels

[davidsen@tmr.com (bill davidsen)]

I regularly get CDs written with binary data at the front followed by an
iso9660 image. And I have always read then by:

  mount -o ro,offset=17000 /dev/cdrom /cdrom

which works in 2.0 and 2.2 kernels. However, it doesn't work in 2.4.x
kernels, at least anything recent enough for me to try, and certainly
not in 2.4.[67].

I have already tried:

- boot a 2.2 kernel on the same hardware. WORKS
- dd the data to a file and offset mount that. WORKS
- try both atapi and ide-scsi for device. FAILS
- use losetup. FAILS

I think the success at mounting the copy of the file shows that the
kernels are built correctly, the loop module loads if I build as a
module, no errors from the kernel in terms of operation, just an unknown
file type.

Based on looking at the data, I think the offset is being moved to some
multiple of a sector or page size. It's not what I see in the copy on
disk in a file.

I have been doing this since 2.0.33 and it's been working. I think I
remember early 2.4 kernels, or late test releases, working fine, but I
don't have a machine to test until the weekend at the earliest.

Unless someone has some trick to get by this, consider it a warning and
bug report.

REPEAT BY:

  mkisofs -dDRNLJ -o /tmp/small.iso /etc
  dd if=/dev/random bs=6 count=1 of=/tmp/rxx
  cat /tmp/rxx /tmp/samll.iso | cdrecord dev=0,0,4 -pad
  mount -o ro,offset=6 /dev/cdrom /cdrom

or whatever is similar on your system.

-- 
  bill davidsen <davidsen@tmr.com>  CTO, TMR Associates, Inc
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