[CRIU] Can't launch unshare with -U or -r as unprivileged user

[Allan Cecil]

Greetings,

I am still working toward using CRIU for the nethack-tas-tools project 
but I'm continuing to run into various problems when running CRIU as an 
unprivileged user.  With the fixes from earlier it is now possible for 
me to use the Python RPC script I mentioned before to checkpoint a 
screen session.  Unfortunately, restoring it is hit and miss, and often 
fails with various problems such as the PID not being the expected PID.  
Cyril suggested compiling and using the latest unshare and passing in 
something along the lines of this: unshare -uimpn -- bash -c "(bash)"

Unfortunately, this fails on unshare from util-linux 2.24.903-56ce 
because virtually all of the flags require root privileges.  The manpage 
talks about using -U and -r instead to allow unshare to run without 
resorting to sudo (which is not available to the unprivileged user 
account in use).  The issue is unshare -U or unshare -r always fails 
with the following error message: unshare: unshare failed: Invalid 
argument

My question is, do I need to compile unshare (util-linux from git) with 
a particular flag to allow the -U option to work?  Should I be doing 
something else entirely, such as figuring out some way to carefully 
craft a sudo exception to allow an unprivileged user to do this (and if 
I go that direction, is there a way to do it and maintain a sense of 
security in the system)?  Better question - will the -U -r flags allow 
an unprivileged user to execute at least the -p flag without being in 
sudoers, or am I trying something impossible?

I have lots of questions but I'll keep investigating in the meantime.  
Thanks in advance,

A.C.
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