cifs root fails with read-only share

[Luiz Angelo Daros de Luca]

If someone uses this kernel cmdline with casper:

 ip=dhcp maybe-ubiquity initrd=initrd netboot=cifs nfsroot=//myserver/share/ubuntu22.10/

It will fail if //myserver/share is marked as readonly. It fails on the losetup step:

 losetup /dev/loop0 /cdrom/casper/filesystem.squashfs

Because losetup will try to open that file as read/write and samba denies it. It looks like mount.cifs does not know/care about share read-only state and mount it as rw.

There are two ways to make losetup work:

1) mount the samba share as ro adding "NFSOPTS=-oro,user=root,password=" to the kernel command line
2) add '-r' to that losetup call

The first option is already possible without any code changes. However, I do believe there is no reason to try to access a squashfs file as read/write. It should somehow propagate to the function that calls losetup that it is dealing with a file that will not be written and losetup can use '-r'.

Tested with ubuntu 22.10 media