Where is core.img?

Question

I'd like to verify that core.img lies between the MBR and the start of my first partition, /dev/sda1.

Using

sudo dd bs=512 if=/dev/sda skip=512 count=1540 | hexdump -C

I can see /sda1 referenced:

00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000c0400  00 e0 3f 00 00 1a ff 00  4c c1 0c 00 96 a3 21 00  |..?.....L.....!.|
000c0410  ed d7 3f 00 00 00 00 00  02 00 00 00 02 00 00 00  |..?.............|
000c0420  00 80 00 00 00 80 00 00  00 20 00 00 12 11 ac 5e  |......... .....^|
000c0430  70 30 ac 5e 47 00 ff ff  53 ef 01 00 01 00 00 00  |p0.^G...S.......|
000c0440  a6 15 01 5c 00 00 00 00  00 00 00 00 01 00 00 00  |...\............|
000c0450  00 00 00 00 0b 00 00 00  00 01 00 00 3c 00 00 00  |............<...|
000c0460  42 02 00 00 6b 04 00 00  e6 59 c6 2c f6 f1 4e 6f  |B...k....Y.,..No|
000c0470  b4 c4 ea 0f 76 73 e4 46  00 00 00 00 00 00 00 00  |....vs.F........|
000c0480  00 00 00 00 00 00 00 00  2f 73 64 61 31 00 00 00  |......../sda1...|
000c0490  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000c04c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 fc 03  |................|
000c04d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000c04e0  08 00 00 00 00 00 00 00  00 00 00 00 b0 a3 c5 a6  |................|
000c04f0  c6 67 43 54 92 58 c9 42  d2 f9 21 0d 01 01 00 00  |.gCT.X.B..!.....|
000c0500  0c 00 00 00 00 00 00 00  a6 15 01 5c 0a f3 02 00  |...........\....|
000c0510  04 00 00 00 00 00 00 00  00 00 00 00 00 80 00 00  |................|
000c0520  00 80 78 00 00 80 00 00  00 80 00 00 00 00 79 00  |..x...........y.|
000c0530  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000c0540  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 10  |................|
000c0550  00 00 00 00 00 00 00 00  00 00 00 00 20 00 20 00  |............ . .|
000c0560  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000c0570  00 00 00 00 04 01 00 00  a7 53 a4 1f 00 00 00 00  |.........S......|
000c0580  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000c07f0  00 00 00 00 00 00 00 00  00 00 00 00 79 1d 94 d7  |............y...|
000c0800

Is this part of core.img?

I was hoping for a magic number like MBR's 55 aa but couldn't find a mention whether core.img has that.

---

I'm using GRUB 2.02, uname -m gives x86_64.

Answer 1

Well, assume you have no further knowledge, you can search for it in raw data.

Some patterns to look for (and their byte offset):

# strings -n 6 -t d /boot/grub/i386-pc/core.img
    283 loading
    306  Error
   2622 RBRPQR
   3689 LH%N("
   4248 9dzj~)>
...

Repeating the same search on the boot drive:

# strings -n 6 -t d /dev/sda | grep -B 6 -F '9dzj~)>'
    395 Hard Disk
    410  Error
    795 loading
    818  Error
   3134 RBRPQR
   4201 LH%N("
   4760 9dzj~)>
--
2239165876 gcry_mpi_invm
2239165890 _gcry_mpi_alloc
2239168795 loading
2239168818  Error
2239171134 RBRPQR
2239172201 LH%N("
2239172760 9dzj~)>

So like wow, what is this stuff?

The string we searched for is 9dzj~)> which is at byte offset 4248 in the core.img file. There was a match for that at byte offset 4760 on my boot drive. That actually just happens to be the same offset + 512 bytes, so you can tell: core.img in my example is actually located at one sector offset. The boot drive in this example is still using an old-fashioned DOS partition table.

The second match at byte offset 2239171134 is just the regular file on the /boot partition.

---

The same exercise on another machine:

Look for some patterns in core.img:

# strings -n 6 -t d /boot/grub/i386-pc/core.img | head -n 6
    307 loading
    330  Error
   2638 RBRPQR
   3677 N8yf>W
   4228 ;E~T\4
   4367 8aDk B

grep for one of them on the boot drive:

# strings -n 6 -t d /dev/vda | grep -B 6 -F ';E~T\4'
1046494 &{$|zO.
1047463 Y 'lg4
1048883 loading
1048906  Error
1051214 RBRPQR
1052253 N8yf>W
1052804 ;E~T\4
--
141418559 normal
141418574 normal
141488435 loading
141488458  Error
141490766 RBRPQR
141491805 N8yf>W
141492356 ;E~T\4

That offset is a bit larger than 512 bytes, comes out as 1052804-4228 = 1048576 = 1 MiB ... why? Because this drive is GPT with a bios_grub partition at 1 MiB offset:

# parted /dev/vda unit b print
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 20401094656B
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start         End           Size         File system  Name            Flags
 1      1048576B      2097151B      1048576B                  grub            bios_grub

... and core.img is right at the start of it:

# cmp /boot/grub/i386-pc/core.img /dev/vda1
/boot/grub/i386-pc/core.img /dev/vda1 differ: byte 501, line 7
# cmp -l /boot/grub/i386-pc/core.img /dev/vda1
  501   2   1
  502   0  10
  509  62 145
  529   0  33
  530   0 146
cmp: EOF on /boot/grub/i386-pc/core.img after byte 26085

So, 26080 bytes were identical with core.img. It's something. \o/

---

This exercise might fail if the core.img file located in /boot is not actually the one that was installed. Some install CDs install a different version of GRUB; and some multi-boot systems, or updated instead of fresh installs, might be using a very different version of GRUB than what is installed on the filesystem (if you never re-run grub-install to actually update on-disk GRUB).