Bug#427559: problems with your proposal of suid root backend runner

[Jerome Alet <alet@librelogiciel.com>]

Martin, you suggest a suid root backend to launch other CUPS
backends as root.

This is not really possible as far as I know, for several reasons :

  - Your suid backend MUST be present in /usr/lib/cups/backend
    for CUPS to accept to launch it. (unless you modify CUPS
    heavily)
    
  - This suid wrapper would need to obey the backend arguments
    handling (man backend), and so would probably have to
    autodetect existing backends, the way PyKota, Tea4CUPS, or 
    Jasmine do. As an alternative you could do as accsnmp does,
    (no autodetection) and rely to the end user to manually
    specify the Device URIs.
    So your suid wrapper would have to output, for example, a list
    of Device uris like : yourwrapper://cups-pdf:// for each backend
    in /usr/lib/cups/backend/ otherwise you wouldn't have any way
    to tell your wrapper which real backend to run (the number and
    value of a backend's arguments is fixed and defined in CUPS'
    documentation).
    
  - There's a 30 seconds timeout during the launch of a backend  
    in autodetection mode. When a lot of backends exist in
    /usr/lib/cups/backend, this may not be sufficient to
    autodetect them all (let's say cupspykota://tea4cups://ipp://
    and the like, and the other way around tea4cups://cupspykota://ipp//
    and for each backend).
          
the easiest way was certainely to 

        chown root.root mybackend          
        chmod 700 mybackend
        
as specified in CUPS' documentation.        

don't forget that many CUPS backends are written in scripting languages,
and won't support being suid root.

hoping this helps the discussion

bye

Jerome Alet