`mount -o loop` changes mounted ISO image file

checksumisomount

It looks like mount -o loop changes the mounted image file. I downloaded the ISO image file and checked its SHA-1 checksum. Then I mounted that ISO file and the checksum changed. Here are the exact steps I made, a copy-paste from my console, CentOS 7 x64 (note, that ISO file is read-only):

[mbartnicki@89-78-33-81 Downloads]$ ls -lh
total 3.1G
-r--r--r--. 1 mbartnicki mbartnicki 3.1G Mar 31 18:28 X17-58997.iso
[mbartnicki@89-78-33-81 Downloads]$ sha1sum X17-58997.iso 
6c9058389c1e2e5122b7c933275f963edf1c07b9  X17-58997.iso
[mbartnicki@89-78-33-81 Downloads]$ sudo mount -o loop X17-58997.iso /mnt/iso/
mount: /dev/loop0 is write-protected, mounting read-only
[mbartnicki@89-78-33-81 Downloads]$ sha1sum X17-58997.iso 
994944df320e9e03c032263d7b9e36157a0a587c  X17-58997.iso

I was so surprised, that I repeated above steps on another machine (Ubuntu 14.04 x64), just to get the same result: file checksum changed after mount -o loop. Unmounting ISO image doesn't return its checksum to original value; file change is persistent. Moreover, next mount change the checksum again to some new value. How can it be explained?

Mounting explicitly read-only by adding ro mount option prevents file from being changed, so that if I use:

sudo mount -o ro,loop X17-58997.iso /mnt/iso

then everything is OK, and ISO image file lasts unchanged after mount. The wrong behaviour occurs only when ro option is not specified, even if the ISO image file is read-only on the file system level. I tried on three different ISO images: Windows 7 installation DVD (official), SystemRescueCD (small rescue Linux) and hand-made ISO created by genisoimage tool.

I checked all that ISO files with file utility, and file said that all of them are ISO 9660 CD-ROM filesystem data. Files inside ISO images are not changed – I also checked it – it looks like the faulty mount scenario adds some meta-data or just garbage.

Best Answer

Iso isn't synonym for read only, under some circunstances, you may wish to mount an iso read/write (when creating a boot media).

If you want read only, use option

 mount -o ro,iso

Related Solutions

Mount – How to Mount an Image File Without Root Permission

You can't mount anything that the administrator hasn't somehow given you permission to mount. Only root can call the mount system call. The reason for this is that there are many ways to escalate privileges through mounting, such as mounting something over a system location, making files appear to belong to another user and exploiting a program that relies on file ownership, creating setuid files, or exploiting bugs in filesystem drivers.

The mount command is setuid root. But if you aren't root, it only lets you mount things that are mentioned in fstab.

The fusermount command is setuid root. It only lets you mount things through a FUSE driver, and restricts your abilities to provide files with arbitrary ownership or permissions that way (under most setups, all files on a FUSE mount belong to you).

Your best bet is to find a FUSE filesystem that's capable of reading your disk image. For ISO 9660 images, try both fuseiso and UMfuse's ISO 9660 support (available under Debian as the fuseiso9660 package).

Mount: could not find any free loop device

Take a look and see if there are any mounts using any of the above loopback devices. You can use the mount command to see this:

$ mount

If they are mounted, they you'll likely need to unmount (umount) them prior to getting losetup -d <loopdevice> to detaching them.

$ umount /dev/some/mount

As to if it's safe or not, that really depends on what these are being used for. I'd probably hold off till I had a better grasp of what these loop devices are for, before I started unmounting them. Just a guess but they might have something to do with an encrypted drive.

Therefore I'd create another one just to be safe.

making another loop device

Here are the steps:

$ sudo mknod -m640 /dev/loop8 b 7 8

-m640 define the permission of the device
/dev/loop8 define the name of the device
b for the creation of the special block device
7 8 the number 7 AND 8 define the MAJOR AND the MINOR

Check if the loop is created:

$ ls -l /dev/loop8
brw-r----- 1 root root 7, 8 Oct 3 14:54 /dev/loop8

Now set ownership on the device:

$ sudo chown root:disk /dev/loop8

References

Howto create Loop Device on Linux