[Bug 1997511] [NEW] cifs root fails with read-only share

[Luiz Angelo Daros de Luca]

Public bug reported:

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

** Affects: casper (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1997511

Title:
  cifs root fails with read-only share

Status in casper package in Ubuntu:
  New

Bug description:
  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

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