Convert testdisk CHS to linux sectors

How to use TestDisk to recover lost partition
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madady
Posts: 2
Joined: 01 Feb 2021, 15:11

Convert testdisk CHS to linux sectors

#1 Post by madady »

Hello,
First of all - thank you for a great tool! I've used it successfully in the past as well.

I'd like to try to recover a couple of partitions that got wiped in an android update (the partitions are on an EMMC card that can be accessed via a USB mass storage reader and usually lives in an Odroid N2 SBC).
My partitioning scheme was like this before the upgrade:
Image
The unpartitioned space before the first partition and following it is used by android. The first partition is android's /cache and the second one is USERDATA (which combines /data with /sdcard).

Sadly, I don't have their exact addresses.
Well, after android did an upgrade, it cleared the partitions from the MBR, and marked the second partition to extend until the end of the disk. But it did not resize the filesystem, so it still ends where it did before.
I've checked with testdisk and I can still find the missing BOOT and rootfs partitions and can browse their files, but I'd prefer not to let testdisk write them to disk, because, as far as I can tell, it gets tricked and can't reliably find the extended partition, and creates BOOT as a primary partition and rootfs inside an extended partition.
Here is the output log

Code: Select all

Using locale 'en_US.UTF-8'.


Mon Feb  1 16:24:26 2021
Command line: TestDisk /log /dev/sdb

TestDisk 7.1, Data Recovery Utility, July 2019
Christophe GRENIER <grenier@cgsecurity.org>
https://www.cgsecurity.org
OS: Linux, kernel 5.4.0-65-generic (#73-Ubuntu SMP Mon Jan 18 17:25:17 UTC 2021) x86_64
Compiler: GCC 9.2
ext2fs lib: 1.45.5, ntfs lib: libntfs-3g, reiserfs lib: none, ewf lib: none, curses lib: ncurses 6.1
Hard disk list
Disk /dev/sdb - 31 GB / 29 GiB - CHS 29820 64 32, sector size=512 - Generic MassStorageClass, FW:1538

Partition table type (auto): Intel
Disk /dev/sdb - 31 GB / 29 GiB - Generic MassStorageClass
Partition table type: Intel

Analyse Disk /dev/sdb - 31 GB / 29 GiB - CHS 29820 64 32
BAD_RS LBA=2185488 2048
check_FAT: Unusual number of reserved sectors 6 (FAT), should be 1.
check_FAT: Incorrect number of sectors per FAT
check_part_i386 failed for partition type 0C
BAD_RS LBA=7084304 2048
Current partition structure:
check_FAT: Unusual number of reserved sectors 6 (FAT), should be 1.
check_FAT: Incorrect number of sectors per FAT
Invalid FAT boot sector
 1 P FAT32 LBA             1067   8 17  1099   8 16      65536
 1 P FAT32 LBA             1067   8 17  1099   8 16      65536

Warning: Bad ending sector (CHS and LBA don't match)
 2 P Linux                 3459   8 17 29819  63 32   53987056

Warning: Bad ending sector (CHS and LBA don't match)
No partition is bootable

search_part()
Disk /dev/sdb - 31 GB / 29 GiB - CHS 29820 64 32
Filesystem created: Thu Aug  6 15:15:53 2020
Last mount time:    Tue Jan  1 02:00:20 2019
     Linux                  558   1  1  1582   0 32    2097152 [cache]
     ext4 blocksize=4096 Large_file Sparse_SB Recover, 1073 MB / 1024 MiB
Filesystem created: Tue Jan  1 02:01:36 2019
Last mount time:    Mon Feb  1 13:45:59 2021
     Linux                 3459   8 17 19658  63 32   33177328
     ext4 blocksize=4096 Large_file Sparse_SB Recover, 16 GB / 15 GiB
Filesystem created: Fri Dec 18 07:15:29 2020
Last mount time:    Wed Dec 30 14:03:25 2020
     Linux                19660   0  1 19915  63 32     524288 [BOOT]
     ext2 blocksize=4096 Large_file Sparse_SB, 268 MB / 256 MiB
Filesystem created: Fri Dec 18 07:15:30 2020
Last mount time:    Wed Dec 30 15:48:35 2020
     Linux                19917   0  1 29819  63 32   20281344 [rootfs]
     ext4 blocksize=4096 Large_file Sparse_SB, 10384 MB / 9903 MiB
get_geometry_from_list_part_aux head=64 nbr=5
get_geometry_from_list_part_aux head=8 nbr=5
get_geometry_from_list_part_aux head=16 nbr=5
get_geometry_from_list_part_aux head=32 nbr=5
get_geometry_from_list_part_aux head=64 nbr=5
get_geometry_from_list_part_aux head=128 nbr=3

interface_write()
 1 * Linux                  558   1  1  1582  63 32    2099168 [cache]
 2 P Linux                 3459   8 17 19658  63 32   33177328
 3 P Linux                19660   0  1 19915  63 32     524288 [BOOT]
 4 E extended LBA         19916   0  1 29819  63 32   20283392
 5 L Linux                19917   0  1 29819  63 32   20281344 [rootfs]
What I plan on doing is:
- use linux fdisk to print the partition offsets in sectors
- delete the second partition
- recreate the second partition with the same start offset, but with the original size (derived as BOOT start offset - 1MB)
- create a new extended partition
- create a new BOOT partition with the offsets retrieved by testdisk
- create a new rootfs partition with the offsets retrieved by testdisk

I was hoping testdisk would provide me (somehow) with the start/end sector of each partition, so I could feed this data into fdisk, but I get only CHS instead. I've looked online for ways to convert between the two, (even tried the code referenced here: https://superuser.com/questions/974581/ ... sk-storage), but I couldn't validate that the data reported by testdisk resembled the data in sectors reported by fdisk (for the first partition, which is unchanged).
So - I'd like to ask - is there a way to ask testdisk to report offsets in sectors, to be compatible with fdisk?
I also tried to use parted and use 'chs' unit, but the disk geometry was completely different than what testdisk reported, so the addresses were incompatible...

Plan b (in my case) is to use tune2fs:

Code: Select all

tune2fs -l /dev/sdb2
tune2fs 1.45.5 (07-Jan-2020)
Filesystem volume name:   <none>
Last mounted on:          /data
Filesystem UUID:          354d2ccc-bc98-4aba-bf75-1750d8d9d71d
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent sparse_super large_file huge_file uninit_bg dir_nlink extra_isize quota
Filesystem flags:         unsigned_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              1040384
Block count:              4147166
Reserved block count:     8192
Free blocks:              689526
Free inodes:              972071
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      1023
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Filesystem created:       Tue Jan  1 02:01:36 2019
Last mount time:          Mon Feb  1 13:45:59 2021
Last write time:          Mon Feb  1 13:45:59 2021
Mount count:              15
Maximum mount count:      -1
Last checked:             Mon Feb  1 10:36:07 2021
Check interval:           0 (<none>)
Lifetime writes:          40 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      1065 (group unknown)
First inode:              11
Inode size:	          256
Required extra isize:     32
Desired extra isize:      32
Journal inode:            8
Default directory hash:   half_md4
Directory Hash Seed:      5649bd68-5411-4b6c-865f-c03fe836fd90
Journal backup:           inode blocks
User quota inode:         3
Group quota inode:        4
The partition size should be Block count * block size / 512 = 33177328 sectors in size... But I'm not 100% certain my calculation is correct.
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recuperation
Posts: 3030
Joined: 04 Jan 2019, 09:48
Location: Hannover, Deutschland (Germany, Allemagne)

Re: Convert testdisk CHS to linux sectors

#2 Post by recuperation »

You can try out the GPT setting in Testdisk, do your calculation and compare.
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madady
Posts: 2
Joined: 01 Feb 2021, 15:11

Re: Convert testdisk CHS to linux sectors

#3 Post by madady »

Sorry, I went through with the recovery before finding your reply. As I suspected, it recovered the partitions, but added only the last partition in an extended partition. Well, I'll have to live with it :)

Regarding analysing as GPT - the partition table was MBR, but I'll try to remember it next time I'm in the same situation.

Regarding units for reporting data - isn't there some "gold standard" that can be understood by other tools as well? Like fdisk or dd, which can work on blocks? I think CHS, even though it's what the program uses internally, is dated and of limited use elsewhere...

Anyway, thank you for your help!
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recuperation
Posts: 3030
Joined: 04 Jan 2019, 09:48
Location: Hannover, Deutschland (Germany, Allemagne)

Re: Convert testdisk CHS to linux sectors

#4 Post by recuperation »

madady wrote: 02 Feb 2021, 15:39 Sorry, I went through with the recovery before finding your reply. As I suspected, it recovered the partitions, but added only the last partition in an extended partition. Well, I'll have to live with it :)

Regarding analysing as GPT - the partition table was MBR, but I'll try to remember it next time I'm in the same situation.

Regarding units for reporting data - isn't there some "gold standard" that can be understood by other tools as well? Like fdisk or dd, which can work on blocks? I think CHS, even though it's what the program uses internally, is dated and of limited use elsewhere...
I am not aware of a gold standard. You might try out the Testdisk standard. Run "Backup" after "analyze". Testdisk will write the structure of the selected drive at the end of the backup.log file or create a new backup.log file if it's not already there.
This log file is in text format. You should even run it on healthy drives before the mess occurs.
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