Re: [gentoo-user] Re: "loopback mount" hard-drive image created with dd?

[Tom <alanceil@gmx.net>]

Remy Blank wrote:
> Yahya Mohammad wrote:
>> # losetup /dev/loop0 /path/to/diskimage
>> # fdisk -l /dev/loop0
>> (example)
>> Disk /dev/sdb: 160.0 GB, 160041885696 bytes
>> 255 heads, 63 sectors/track, 19457 cylinders
>> Units = cylinders of 16065 * 512 = 8225280 bytes
>>
>>    Device Boot      Start         End      Blocks   Id  System
>> /dev/sdb1   *           1        1044     8385898+   c  W95 FAT32 (LBA)
>> /dev/sdb2            1045       19457   147902422+  83  Linux
> 
> Thanks for the tip! I didn't know you could mount a whole disk image as 
> loopback, and see the partition table with fdisk. I always printed the 
> partition table before making the disk image.
> 
>> I second that, I'm curious to know if it works
> 
> It does, I have done it before, but there's a special case for the first 
> partition of the disk. According to your output of fdisk, it is supposed 
> to start at cylinder 1. However, if you run fdisk with the -u option 
> (giving the positions and sizes in sectors), you get the following:
> 
> (example)
> # fdisk -lu /dev/loop0
> 
> Disk /dev/loop0: 10.0 GB, 10056130560 bytes
> 240 heads, 63 sectors/track, 1299 cylinders, total 19640880 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Disk identifier: 0xc1afc1af
> 
>       Device Boot      Start         End      Blocks   Id  System
> /dev/loop0p1   *          63     4097519     2048728+   b  W95 FAT32
> /dev/loop0p2         4097520     8195039     2048760    5  Extended
> /dev/loop0p5         4097583     8195039     2048728+   b  W95 FAT32
> 
> That is, the first partition starts at sector 63, i.e. at an offset 
> value of 63 * 512 = 32256.
> 
>> oh, and make a backup just in case :)
> 
> And mount the filesystem read-only with the "ro" mount option.
> 
> -- Remy
> 

Some time ago, I wrote a little script to automate that process, maybe 
it will help you also. (And yes, I know that the code isn't that pretty)

Direct copy & paste:

#!/bin/bash
INFILE="$1"
PARTITION="$2"
MOUNTPOINT="$3"
OPT="$4"

if [ "$INFILE" = "" ] ; then echo "Usage: $0 infile partition mountpoint 
<options>" ; exit ; fi
if [ "$PARTITION" = "" ] ; then echo "Usage: $0 infile partition 
mountpoint <options>" ; exit ; fi
if [ "$MOUNTPOINT" = "" ] ; then echo "Usage: $0 infile partition 
mountpoint <options>" ; exit ; fi

if [ "$OPT" != "" ] ; then OPT2=",${OPT}" ; fi

PSTARTB=`sfdisk -d "$INFILE" | grep "start=" | head -n"$PARTITION" | 
tail -n1 | sed 's/.*start=[ ]*//' | sed 's/,.*//'`
PSTART=$[ $PSTARTB * 512 ]
#PSIZEB=`sfdisk -d "$INFILE" | grep "size=" | head -n4 | tail -n1 | sed 
's/.*size=[ ]*//' | sed 's/,.*//'`
#PSIZE=`echo "$PSIZEB * 512" | bc`
mount "$INFILE" "$MOUNTPOINT" -o loop,offset="$PSTART""$OPT2"
#,sizelimit="$PSIZE""$OPT2"

- Tom
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