Bug#543924: "error: invalid magic number" when booting from .iso loopback

[Marc Haber]

Package: grub-pc
Version: 1.96+20090808-1
Severity: normal

I have an USB stick which is partitioned like this:

$ sudo fdisk -lu /dev/sdb

Disk /dev/sdb: 261 MB, 261095424 bytes
64 heads, 32 sectors/track, 249 cylinders, total 509952 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x3d6e2e09

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          32      509951      254960   83  Linux

/grub holds a normal grub 1.96 installation.

/grml has a grml.iso, and the linux26 file extracted from there:

$ ls -l /media/disk-1/grml/
-rw-r--r-- 1 root root  99M 27. Aug 15:33 grml.iso
-r--r--r-- 1 root root 2,9M 27. Aug 15:55 linux26

When I try loading the linux26 file that was extracted, everything is
fine:

sh:grub> linux /grml/linux26
   [Linux-bzImage, setup=0x3000, size=0x2e18f0]

But when I try loading the linux26 file from the .iso, it doesn't work:

sh:grub> loopback loop /grml/grml.iso
sh:grub> linux (loop)/boot/grmlsmalldailysid/linux26
error: invalid magic number

When I try to continue with the directly loaded kernel, loading the
initrd from loopback fails:
sh:grub> linux /grml/linux26
   [Linux-bzImage, setup=0x3000, size=0x2e18f0]
sh:grub> loopback loop /grml/grml.iso
sh:grub> initrd (loop)/boot/grmlsmalldailysid/initrd.gz
error: ext2fs doesn't support triple indirect blocks

When the initrd is directly on the stick's fs, it works fine, of
course, but I don't need grub2 for that.

What might be going on here?

Greetings
Marc