`disk` level IO: how it works?

[Oleg Strikov]

Good day!
I'm trying to look into grub low-level disk IO.
I've started exploration with my own  low-level FS block reader, but it works too strange:

/* I know that such code is incorrect due to low-level FS operations but i'm trying to understand some principals of low level IO :) */

file = grub_file_open (FILE);
fs_data = (struct grub_ext2_data *) file->data;
disk = fs_data->disk;

fsize = grub_file_size(file);
block_num = (fsize >> LOG2_BLOCK_SIZE (fs_data));
if (((grub_off_t) block_num << LOG2_BLOCK_SIZE (fs_data)) != fsize)
    block_num++;

log2_blksz = LOG2_EXT2_BLOCK_SIZE (fs_data);

blocks = grub_malloc (sizeof (int) * block_num);

fshelp_node = &fs_data->diropen;

for (i = 0; i < block_num; i++)
  {
    blocks[i] = grub_ext2_read_block_wrapper (fshelp_node, i);
  }

grub_disk_read(fs_data->disk,                                                  \
           grub_le_to_cpu32 (blocks[i]) << log2_blksz,               \
           0,                                                                                        \
           sizeof (struct my_entry),                                                 \
           (char *) entry);

grub_ext2_read_block_wrapper its my exported wrapper from ext2.c that calls original grub_ext2_read_block with the same parameters.

Problem: I have extended partition with two logical (hd0,5) & (hd0,6)
If I place FILE on (hd0,5) (at the begining of my physical disk) - all is OK
If I place FILE on (hd0,6) i get `out of partition` at grub_disk_read.

I run grub2 using qemu and gdb block num seems to be correct (smth like 4500), but i don't pass grub_ext_read_block range check :(

P.S. I'm right that `disk` represents one partition (logical or primary), not a group?

Thanks!