This is a neat trick, and so I thought worth reblogging. I was handed a corrupt SD memory card yesterday and asked if I could try and get some Christmas photos off it.
Typically, the first thing I did was image the card with ddrescue (package gddrescue in Debian/Ubuntu), confirming that the partition table was intact, it looks like the problem was a corrupted FAT, so I wanted to run fsck on the partition.
Back in the day this would have involved either dd-ing the partition out of the image, or re-imaging the card, both slow, or screwing around with loopback offsets. Instead I discovered a tool called kpartx which maps the partitions from the image into the device mapper, giving a device like /dev/mapper/loop0p1, which can be used like a regular block device.
Unfortunately while different, both FATs were damaged, and has been incorrectly repaired by something, and I ended up recovering the files with photorec instead.