Nullfs Allows Jailbreaking

Jason C. Wells jcw at
Thu Dec 23 03:41:17 UTC 2010

On 12/22/10 19:00, Chad Leigh -- Shire.Net LLC wrote:
> I have been doing this for years with great success.   I don't understand your question.   How does it look like everything is read only from inside the jail?  The fact that a "df" only shows the root filesystem and not all your others file systems? (assuming that is still the truth -- my jails do this on older FBSD systems
Your report of great success is encouraging.  I've never done this and 
sometimes you don't see the full consequences when you haven't done 
something before.

I think you understand my question.  It's the fact that mount(8) report 
read only. If you looked at that, you would conclude that you had no 
write access at all.  Well, I set up the jail, so I can see behind the 
curtain.  A downstream user that never saw behind the curtain wouldn't 
know where they could write a file unless they simply guessed.

That is why I asked about jailbreaking.  There is information crossing 
the jail in a mysterious way.  The jail reports that everything under 
root is read only, but the jail can still see the read-writeness of the 
invisible (to mount) null filesystems.

 From the sound of it, this is expected behavior.

Jason C. Wells

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