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Old 12-24-2020, 01:58 PM   #1
gglq000
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ext2 inode table does not seem to be pointing to correct block


Hi all, topic of this post does not exactly align it is about file system (ext2) question but looking through all other sub-threads, i think "Linux - Enterprise" may be best fit since it is more of an adv. topic.

Here is the story, I brushed up with my ext file system structure knowledge and needed to code to locate arbitrary file from /dev/sda1's root directory. File name is boot.bin.

So good place to start is from superblock. The /dev/sda1 in this case is about 5G volume with only primary partition formatted with ext2.

dumpe2fs /dev/sda1 shows first group block's following info:
block size: 4096
inode table start: 323 (th block)
first inode 11.

I found through stat boot.bin that inode No. is 0xC (12d) and zeroed onto inode's entry in the inode table as following:

hexdump -C /dev/sda/ -n 512 -s $((323 * 4096)) = 0x143100.
Since it is 12th inode, and located boot.bin's inode at 0x143100
..

00143100 ed 41 00 00 00 10 00 00 50 0e e4 5f ed ff e3 5f |.A......P.._..._|
00143110 ed ff e3 5f 00 00 00 00 00 00 03 00 08 00 00 00 |..._............|
00143120 00 00 00 00 03 00 00 00 43 03 00 00 00 00 00 00 |........C.......|
00143130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*

Here, couple of checks look at i.e. No. of blocks=8 dates etc, but at 0x28th offset the block address shows 43 03 00 00 which is 0x334 corresponding to 825th block?

When I dump that it does not show the boot.bin's file content at all:
hexdump -C /dev/sda1 -n 4096 -s $((835*4096))
00343000 02 00 00 00 0c 00 01 02 2e 00 00 00 02 00 00 00 |................|
00343010 0c 00 02 02 2e 2e 00 00 0b 00 00 00 14 00 0a 02 |................|
00343020 6c 6f 73 74 2b 66 6f 75 6e 64 00 00 0c 00 00 00 |lost+found......|
00343030 d4 0f 08 01 62 6f 6f 74 2e 62 69 6e 0d 00 00 00 |....boot.bin....|
00343040 c4 0f 0d 01 2e 62 6f 6f 74 2e 62 69 6e 2e 73 77 |.....boot.bin.sw|
00343050 70 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |p...............|
00343060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*

I can't discern why because stat of boot.bin from debugfs shows blocks being used 8 and 1st block is 1029 vs. 835 which is what its inode table points to?

However if I use stat's output to dump 1029th block, then yes it prints out the boot.bin's content exactly:
# hexdump -C /dev/sda -n 4096 -s $((1029*4096)):
00405000 0a 38 38 37 37 36 36 35 35 34 34 33 33 32 32 31 |.887766554433221|
00405010 31 38 38 37 37 36 36 35 35 34 34 33 33 32 32 31 |1887766554433221|

Conclusion is, of all those, everything makes sense except boot.bin's inode table entry just pointing to 835th block vs actual block is 1029th block.
Can anyone see what I am looking wrong at???

THanks.,
 
Old 12-24-2020, 03:21 PM   #2
shruggy
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If it's the 12th inode, and inode table starts at 323th block, shouldn't you seek to 323*4096+11*inode_size=0x143B00? (if inode_size=256)

I mean inode 1 is the bad blocks inode, inode 2 is the root directory inode (which is probably what you were seeing above), and so on. I don't remember what the rest of them are for, but the first ten inodes are special. Inode 11 is the first non-reserved inode.

Last edited by shruggy; 12-25-2020 at 01:56 AM.
 
Old 12-24-2020, 04:05 PM   #3
gglq000
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Registered: Mar 2012
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Quote:
Originally Posted by shruggy View Post
If it's the 12th inode, and inode table starts at 323th block, shouldn't you seek to 323*4096+11*inode_size=0x143B00? (if inode_size=256)

I mean inode 1 is the bad blocks inode, inode 2 is the root directory inode (which is probably what you were seeing above) and so on, ..., inode 11 is the first non-reserved inode.
Yup, not sure why I did not think about it before it works now, thanks!!!

# hexdump -C sda1.bin -n 4096 -s $((323*4096+11*256))
00143b00 a4 81 00 00 21 00 00 00 72 2e e4 5f 92 19 e4 5f |....!...r.._..._|
00143b10 92 19 e4 5f 00 00 00 00 00 00 01 00 08 00 00 00 |..._............|
00143b20 00 00 00 00 01 00 00 00 05 04 00 00 00 00 00 00 |................|
00143b30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|

# hexdump -C sda1.bin -n 512 -s $((0x405*4096))
00405000 0a 38 38 37 37 36 36 35 35 34 34 33 33 32 32 31 |.887766554433221|
00405010 31 38 38 37 37 36 36 35 35 34 34 33 33 32 32 31 |1887766554433221|
00405020 31 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |1...............|
00405030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
 
  


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