Re: [Fedora-livecd-list] Is automounting of LVM volumes by a livecd dangerous?

[Antonio Olivares]

>  > Dear all,
>  > 
>  > I have asked the following question on
>  fedora-list(a)redhat.com.
>  > 
>  > But then it clicked to me that I should have directed
>  the question here on livecd list.
>  > 
>  > Is automounting of LVM volumes by a livecd dangerous?
>  
>  Probably you've heard this before, but the short answer
>  is 'it depends'.
>  
>  If you believe in the philosophy that a default livecd boot
>  should be 
>  guaranteed to not write any bits on system disk/storage,
>  then yes, it is 
>  dangerous.  I'll even go one further- it's even
>  dangerous to mount ext3 
>  filesystems ""read-only"".
>  
>  When implementing liveusb-persistence, an early variation
>  of my 
>  implementation would attempt to readonly mount every disk,
>  looking for 
>  persistent overlay files to utilize.  Ultimately, for the
>  first liveusb 
>  persistence release, I backed off of the flexibility that
>  doing so would 
>  enable, and now the current fedora liveusb mechanism will
>  only by 
>  default look at the booting media (e.g. usbstick) for the
>  persistence 
>  file which is by definition already mounted.
>  
>  One thing I noticed in that earlier implementation was that
>  if you did a 
>  'blockdev --setro' on devices before attempting to
>  mount them readonly 
>  (because like me, you are ultra paranoid, and believe that
>  users should 
>  be confident that by default no bits on their disks will be
>  twiddled)... 
>    Anyway, if you do that, and then try to mount readonly an
>  ext3 device, 
>  you'll be confounded by the fact that the mount now
>  fails, because for 
>  some reason mounting an ext3fs readonly is not a purely
>  read-only 
>  operation.  I think maybe in some instances it really wants
>  to replay 
>  the journal.  I vaguely recall also trying to mount an ext3
>  as readonly 
>  as an ext2 filesystem, but I vaguely recall that not
>  working.
>  
>  Ultimately, for another tool I was working on (viros.org),
>  I ended up 
>  implementing a devicemapper solution.  I.e. to be truly
>  paranoid, you 
>  can blockdev --setro, then build up a devicemapper snapshot
>  to ram to 
>  get a virtually writable device, which you can then mount
>  readonly (and 
>  amusingly, get some visibility into which bits get written
>  in such a case).
>  
>  But back to your question-  Another very good reason to be
>  this 
>  paranoid, is whether or not you want to support the
>  following use-case:
>  
>  - user has ubuntu(or any distro) installed on their system
>  disk.
>  - user hibernates
>  - user boots your livecd
>  - user reboots, and wants to unhibernate
>  - user is hosed if you mounted filesystems that were
>  mounted in the 
>  hibernated OS
>  
>  -dmc
>   
By default slax mounts all devices rw including NTFS partitions.  Maybe a warning should
be placed on the use.  Anyhow, I have not seen it be dangerous with regular partitions.  

Thank you for your input,  I will relay the information to Tomas.

Regards,

Antonio