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C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)

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Operating Systems Linux C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)
# 15  
Old 01-27-2011
Yes, that's what I mean, you have to understand how ext3 works to mess with it raw. With the bits of it you've overwritten now, it probably needs a reformat to be valid ext3 again.

Do you need to use ext3? If not, I'd reformat it with /sbin/fsck.msdos -F 16 filename /sbin/mkfs.msdos -F 16 filename as the MSDOS filesystem is far, far simpler: just a big table, instead of a tree.

You might find the hexdump utility handy to show you what data you should expect to find in what places. Here's part of a hex dump of a flash drive I had lying around:

Code:
$ hexdump -C 8gflash-2011-01-27.vfat | head
00000000  eb 58 90 29 5e 76 56 4b  49 48 43 00 02 08 90 08  |.X.)^vVKIHC.....|
00000010  02 00 00 00 00 f8 00 00  20 00 10 00 80 1f 00 00  |........ .......|
00000020  80 40 ef 00 b8 3b 00 00  00 00 00 00 02 00 00 00  |.@...;..........|
00000030  01 00 08 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 29 53 6a af d5 20  20 20 20 20 20 20 20 20  |..)Sj..         |
00000050  20 20 46 41 54 33 32 20  20 20 fa 33 c9 8e d1 bc  |  FAT32   .3....|
00000060  f8 7b 8e c1 bd 78 00 c5  76 00 1e 56 16 55 bf 22  |.{...x..v..V.U."|
00000070  05 89 7e 00 89 4e 02 b1  0b fc f3 a4 8e d9 bd 00  |..~..N..........|
00000080  7c c6 45 fe 0f 8b 46 18  88 45 f9 38 4e 40 7d 25  ||.E...F..E.8N@}%|
00000090  8b c1 99 bb 00 07 e8 97  00 72 1a 83 eb 3a 66 a1  |.........r...:f.|

If you want to use a proper Linux filesystem, try ext2 instead of ext3, it's pretty much the same as ext3 but with a lot of journalling fluff cut out.

These links are good starting points on FAT and ext2.
Last edited by Corona688; 01-27-2011 at 01:30 PM.. Reason: collosal thinko
# 16  
Old 01-27-2011
Hi Corona688,

I think going with the MS-DOS filesystem (FAT-16) is a much better Idea. I formatted my current loop back device using the command you gave as follows :

Code:
$ sudo fsck.msdos -F 16 /dev/loop0

But I get the following error.
Code:
 
fsck.msdos: invalid option -- 'F'

Could you please verify how can I do this correctly ?. So I could format my loop back device with FAT-16 (for DOS file system) mode ?.
# 17  
Old 01-27-2011
Did I say fsck!? Smilie Sorry! I meant /sbin/mkfs.msdos -F 16 filename
# 18  
Old 01-27-2011
Hi Corona688,

I found the following article through a Google search(while awaiting your reply) on creating a floppy drive with the loop back device and got it mounted.


CREATING A Virtual MS-DOS Floppy Inside Linux.

Is the above approach correct ?.
Anyways now I'm back where I began as of how can I create files & folders inside the loop back device. Now with the MS-DOS FAT-16 file system. If you have any direct pointers to get me started on implementing this it would be a great help Smilie.
# 19  
Old 01-27-2011
Floppies use the FAT12 filesystem, which is good at not wasting space, but very hard to use. Its FAT entries are, as the name suggests, twelve bits in size. That's one and a half bytes, like this:
Code:
00 10 15 05 80 90

You can't make an array of that. You'd have to tease the data out the hard way, figuring out where the entry starts, which whole bytes and which half bytes should be retrieved, etc, etc.

You don't need to use the loop device at all to make the DOS filesystem, just to mount it. mkfs.msdos will warn you a file's not a device but write to it anyway.

Be sure to run losetup -d /dev/loop0 when you're done with that tutorial, or you'll find the loop device still busy when you try to use it for something else.

The instructions I posted in the post before yours should work fine for creating a FAT16 filesystem on a file 500 megabytes or smaller. FAT16 entries fit nicely in an array of 16-bit integers.




You should start making data structures to hold information so you don't have to find absolutely everything by byte offsets all the time:

Code:
#include <stdint.h> // standard sized integer types
// to make sure the compiler doesn't insert extra space in the structure
#pragma pack(push,1)
typedef struct bootsector
{
        uint8_t jump[3];
        uint8_t oemname[8];
        uint16_t bytes_per_sector;
        uint8_t bytes_per_cluster;
        uint16_t reserved;
        uint8_t fats;
        uint16_t root_entries;
        uint16_t total_sectors;
        uint8_t media_descriptor;
        uint16_t sectors_per_fat;
        uint16_t sectors_per_track;
        uint16_t heads;
        uint32_t hidden;
        uint32_t total;
} bootsector;
// reset structure definitions to normal
#pragma pack(pop);

...

bootsector bs;
read(fd, &bs, sizeof(bs));

fprintf(stderr, "%d bytes per sector\n", bs.bytes_per_sector);

All that's gleaned from information on the wiki, and it appears to match the dump I posted above(the OEM name is indeed filled with garbage for some reason!)

Look if the values you get in that structure seem normal. Try to use the information in it to find other structures. See if you can find the information you need to piece the partition together. You have to understand what's going on in those structures before you can alter them properly.

I hope this gives you the idea as I don't have the time to do absolutely everything for you.
Last edited by Corona688; 01-27-2011 at 02:17 PM..
This User Gave Thanks to Corona688 For This Post:
shen747
# 20  
Old 01-27-2011
Hi Corona688,

Thank you very very much for the immense support you've given me throughout this thread. I've learned a lot from you since I started this thread Smilie. I understand what I should do and how I should proceed. Thanks you very much sir.

I've tried the HexDump Command you gave me on my Virtual MS-DOS(FAT-16) Floppy Drive & as follows :

Code:
sudo hexdump -C /dev/loop0 | head

and following is the output I got :

Code:
00000000  eb 3c 90 6d 6b 64 6f 73  66 73 00 00 02 01 01 00  |.<.mkdosfs......|
00000010  02 e0 00 40 0b f0 09 00  12 00 02 00 00 00 00 00  |...@............|
00000020  00 00 00 00 00 00 29 26  7a d7 50 20 20 20 20 20  |......)&z.P     |
00000030  20 20 20 20 20 20 46 41  54 31 32 20 20 20 0e 1f  |      FAT12   ..|
00000040  be 5b 7c ac 22 c0 74 0b  56 b4 0e bb 07 00 cd 10  |.[|.".t.V.......|
00000050  5e eb f0 32 e4 cd 16 cd  19 eb fe 54 68 69 73 20  |^..2.......This |
00000060  69 73 20 6e 6f 74 20 61  20 62 6f 6f 74 61 62 6c  |is not a bootabl|
00000070  65 20 64 69 73 6b 2e 20  20 50 6c 65 61 73 65 20  |e disk.  Please |
00000080  69 6e 73 65 72 74 20 61  20 62 6f 6f 74 61 62 6c  |insert a bootabl|
00000090  65 20 66 6c 6f 70 70 79  20 61 6e 64 0d 0a 70 72  |e floppy and..pr|

I hope this information will also be useful as I proceed as you've mentioned earlier. Thanks again for all your help & time Smilie.
# 21  
Old 01-27-2011
Quote:
Originally Posted by shen747
Hi Corona688,

Thank you very very much for the immense support you've given me throughout this thread. I've learned a lot from you since I started this thread Smilie. I understand what I should do and how I should proceed. Thanks you very much sir.

I've tried the HexDump Command you gave me on my Virtual MS-DOS(FAT-16) Floppy Drive & as follows :
Your virtual floppy disk, like any floppy disk, is FAT12, not FAT16. Look closely at your dump:
Code:
00000000  eb 3c 90 6d 6b 64 6f 73  66 73 00 00 02 01 01 00  |.<.mkdosfs......|
00000010  02 e0 00 40 0b f0 09 00  12 00 02 00 00 00 00 00  |...@............|
00000020  00 00 00 00 00 00 29 26  7a d7 50 20 20 20 20 20  |......)&z.P     |
00000030  20 20 20 20 20 20 46 41  54 31 32 20 20 20 0e 1f  |      FAT12   ..|
00000040  be 5b 7c ac 22 c0 74 0b  56 b4 0e bb 07 00 cd 10  |.[|.".t.V.......|
00000050  5e eb f0 32 e4 cd 16 cd  19 eb fe 54 68 69 73 20  |^..2.......This |
00000060  69 73 20 6e 6f 74 20 61  20 62 6f 6f 74 61 62 6c  |is not a bootabl|
00000070  65 20 64 69 73 6b 2e 20  20 50 6c 65 61 73 65 20  |e disk.  Please |
00000080  69 6e 73 65 72 74 20 61  20 62 6f 6f 74 61 62 6c  |insert a bootabl|
00000090  65 20 66 6c 6f 70 70 79  20 61 6e 64 0d 0a 70 72  |e floppy and..pr|

To make a FAT16 partition you might want to use the instructions I gave you on a blank file of a couple megabytes in size.

You don't need to use the loop device for anything but mounting the partition (something you intend to avoid doing anyway). As far as mkfs.msdos, read(), and write() are concerned they act the same.
Last edited by Corona688; 01-27-2011 at 02:44 PM..
This User Gave Thanks to Corona688 For This Post:
shen747
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