ext2 read Block Group Descriptor

[onlshk]

Hello,

i have a disk which created with:

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	dd if=/dev/zero of=zerOS.img bs=1048576 count=10
	(echo "n"; echo "p"; echo ""; echo ""; echo ""; echo "w") | sudo fdisk -u -C10 -S63 -H16 zerOS.img

	sudo rm -rf /mnt
	sudo mkdir -p /mnt/fcd

	sudo losetup /dev/loop0 zerOS.img
	sudo mke2fs -b1024 /dev/loop0
	sudo mount -t ext2 /dev/loop0 /mnt/fcd

	sudo dd if=loader.bin of=zerOS.img count=1 conv=notrunc
	sudo dd if=stage1.bin of=zerOS.img count=1 conv=notrunc seek=1

	sudo cp kernel.bin /mnt/fcd

	sleep 0.1
	sudo umount /mnt/fcd
	sudo losetup -d /dev/loop0

I copied bootloader and kernel.bin there. Now i want to load kernel.bin. But before i must to find it's inode. First of all i need to load 2 inode for root directory. Trying to load block group descriptor from block group descriptor table, it is: 2 sectors (al), 0 cylinder (ch), 5 sector (cl), 0 head (dh) and buffer bx. After i read it, i get 'Number of directories in group' from this descriptor with

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mov ax, [buffer + 16]

, but everytime i'm getting 2 in ax, event i put 5 files/dirs there.

And i can't undersratnd, why it everytime shows 2. or i built image in wrong way or i'm reading it wrong way.

[Combuster]

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# create partition table with one partition
   (echo "n"; echo "p"; echo ""; echo ""; echo ""; echo "w") | sudo fdisk -u -C10 -S63 -H16 zerOS.img
# use entire disk
   sudo losetup /dev/loop0 zerOS.img
# destroy the partition table and use the entire disk as ext2
   sudo mke2fs -b1024 /dev/loop0

# ...
# profit

[onlshk]

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# create partition table with one partition
   (echo "n"; echo "p"; echo ""; echo ""; echo ""; echo "w") | sudo fdisk -u -C10 -S63 -H16 zerOS.img
# use entire disk
   sudo losetup /dev/loop0 zerOS.img
# destroy the partition table and use the entire disk as ext2
   sudo mke2fs -b1024 /dev/loop0

# ...
# profit

hmm... So how can i make this in right way?

[jnc100]

In addition to what Combuster posted above, you can quite easily tell whether its a problem with creating the image or reading the image in your OS by examining the image file with a hex editor and seeing if what you expect to be in a certain place is actually there.

Furthermore, I'd recommend against having your ext2 driver work with CHS addressing - have it use LBA instead and then either use the extended read BIOS functions or convert to CHS somewhere in your block layer.

Regards,
John.

[onlshk]

# create partition table with one partition
(echo "n"; echo "p"; echo ""; echo ""; echo ""; echo "w") | sudo fdisk -u -C10 -S63 -H16 zerOS.img

Am i right that this partition will start from 2048?

I created partition and write bootloader to disk with:

Code: Select all

dd if=/dev/zero of=zerOS.img bs=1048576 count=10
sudo losetup /dev/loop0 zerOS.img
(echo "n"; echo "p"; echo ""; echo ""; echo ""; echo "w") | sudo fdisk -u -C10 -S63 -H16 zerOS.img
sudo fdisk -u -C10 -S63 -H16 zerOS.img
# write bootloader
sudo dd if=loader.bin of=zerOS.img count=1 bs=512 conv=notrunc
sudo dd if=stage1.bin of=zerOS.img count=1 bs=512 conv=notrunc seek=1
sudo losetup -d /dev/loop0

Now i set up another loop device and mounted ext2 with offset:

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sudo losetup -o 1048576 /dev/loop1 zerOS.img

# 1024 block size
sudo mke2fs -b1024 /dev/loop1
sudo mount -t ext2 /dev/loop1 /mnt/fcd

# copy kernel
sudo cp kernel.bin /mnt/fcd

sudo umount /mnt/fcd
sudo losetup -d /dev/loop1

Now bootloader works, but now i can't read superblock. What's wrong here?

[Combuster]

Which superblocks were you unable to find? The Ext2 one or the partition table one?

I also wouldn't normally trust a power of two in partition table offsets.

[onlshk]

previously i read superblock at sector 3 with:

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mov ah, 0x02
mov al, 1
mov cx, 0x0003
xor dh, dh
mov bx, buffer
int 0x13

Now i can't understand where to read it.