Bug#589854: Bug#589873: grub-pc: fails to install in mbr during postinstall

[Mario 'BitKoenig' Holbe]

On Sat, Jul 24, 2010 at 06:36:01AM +0200, Mario 'BitKoenig' Holbe wrote:
> On Thu, Jul 22, 2010 at 09:37:15PM +0100, Colin Watson wrote:
> > Could you try 1.98+20100722-1 once it's available?  I think it may fix
> > this.
> Sad to say: nope, it doesn't...
> 
> /usr/sbin/grub-probe: info: opening md0.
> kern/disk.c:245: Opening `md0'...
> disk/raid.c:134: md0: total_devs=3, disk_size=481664
> disk/raid.c:163: md0: level=1, total_sectors=481664
> ...
> kern/fs.c:54: Detecting ext2...
> kern/fs.c:60: ext2 detection failed.

Did a little more debugging:

kern/fs.c:54: Detecting ext2...
fs/ext2.c:855: grub_ext2_dir(): grub_ext2_mount()
fs/ext2.c:541: grub_ext2_mount()
fs/ext2.c:547: grub_ext2_mount(): grub_disk_read()
kern/disk.c:396: grub_disk_read(disk = md0, sector = 2, offset = 0, size = 336)
kern/disk.c:407: grub_disk_read() after grub_disk_adjust_range(): sector = 2, offset = 0)
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:571:  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
fs/ext2.c:575: grub_ext2_mount(): EXT2_MAGIC: 0
kern/fs.c:60: ext2 detection failed.

The dump is data->sblock in grub_ext2_mount(). There are no real read()s
in strace around this, so the data seems to come from grub's cache.
But there's data on disk:

# hexdump -C /dev/md0
...
00000200  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000400  50 eb 00 00 c0 ac 03 00  09 2f 00 00 31 f7 02 00  |Pë..À¬.../..1÷..|
00000410  65 ea 00 00 01 00 00 00  00 00 00 00 00 00 00 00  |eê..............|
00000420  00 20 00 00 00 20 00 00  d8 07 00 00 68 6b 47 4c  |. ... ..Ø...hkGL|
00000430  42 13 4b 4c 06 00 14 00  53 ef 00 00 02 00 00 00  |B.KL....Sï......|
00000440  f3 0d 42 4c 00 4e ed 00  00 00 00 00 01 00 00 00  |ó.BL.Ní.........|
00000450  00 00 00 00 0b 00 00 00  80 00 00 00 38 00 00 00  |............8...|
00000460  02 00 00 00 01 00 00 00  d0 fc 44 a3 cd 4a 42 79  |........ÐüD£ÍJBy|
00000470  b5 88 ce ef 6f 45 e1 81  62 6f 6f 74 00 00 00 00  |µ.ÎïoEá.boot....|
00000480  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
# hexdump -C /dev/sda2
...
00000200  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000400  50 eb 00 00 c0 ac 03 00  09 2f 00 00 31 f7 02 00  |Pë..À¬.../..1÷..|
00000410  65 ea 00 00 01 00 00 00  00 00 00 00 00 00 00 00  |eê..............|
00000420  00 20 00 00 00 20 00 00  d8 07 00 00 68 6b 47 4c  |. ... ..Ø...hkGL|
00000430  42 13 4b 4c 06 00 14 00  53 ef 00 00 02 00 00 00  |B.KL....Sï......|
00000440  f3 0d 42 4c 00 4e ed 00  00 00 00 00 01 00 00 00  |ó.BL.Ní.........|
00000450  00 00 00 00 0b 00 00 00  80 00 00 00 38 00 00 00  |............8...|
00000460  02 00 00 00 01 00 00 00  d0 fc 44 a3 cd 4a 42 79  |........ÐüD£ÍJBy|
00000470  b5 88 ce ef 6f 45 e1 81  62 6f 6f 74 00 00 00 00  |µ.ÎïoEá.boot....|
00000480  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|


regards
   Mario
-- 
Das Gegenteil einer trivialen Wahrheit ist falsch;
das Gegenteil einer grossen Wahrheit ist ebenfalls wahr.
                                           -- Niels Bohr