bug#29773: urandom-seed-service should run earlier in the boot process

[Leo Famulari]

The urandom-seed-service preserves some of the Linux cryptographic
random number generator's (CRNG) pool across reboots so that freshly
booted systems have access to unpredictable psuedorandom numbers. It
also tries to take advantage of whatever is backing /dev/hwrng.

However, the urandom-seed-service may not be started before certain
applications that assume a good source of randomness.

In some cases, the applications require some random data before any
services are started, during activation. For example, our OpenSSH
service generates its host keys during activation. And even if it
generated host keys during the start of the OpenSSH service, that
service does not depend on urandom-seed-service. [0]

In systemd, there is an abstract sysinit "target" that basically serves
as a checkpoint. All the lower-level system initialization is required
before the sysinit.target is met, and the rest of the services depend on
sysinit. The random seeding is part of sysinit. I've reproduced a graph
of this in [1].

In practice, I'm not sure if it matters. I'd appreciate if GuixSD users
could check /var/log/messages for warnings like this one and report
them:

random: application: uninitialized urandom read (16 bytes read) 

And I'd also appreciate any feedback or ideas for improvement in this
area.

[0] See the attached service graph of a bare-bones system with OpenSSH.
[1] from: <https://www.freedesktop.org/software/systemd/man/bootup.html>

local-fs-pre.target
         |
         v
(various mounts and   (various swap   (various cryptsetup
 fsck services...)     devices...)        devices...)       (various low-level  
 (various low-level
         |                  |                  |             services: udevd,   
  API VFS mounts:
         v                  v                  v             tmpfiles, random   
  mqueue, configfs,
  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)  
    debugfs, ...)
         |                  |                  |                    |           
         |
         \__________________|_________________ | 
___________________|____________________/
                                              \|/
                                               v
                                        sysinit.target
                                               |
          
____________________________________/|\________________________________________
         /                  |                  |                    |           
         \
         |                  |                  |                    |           
         |
         v                  v                  |                    v           
         v
     (various           (various               |                (various        
  rescue.service
    timers...)          paths...)              |               sockets...)      
         |
         |                  |                  |                    |           
         v
         v                  v                  |                    v           
   rescue.target
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         v                  \_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                
 emergency.service
         /                  |                  |                                
         |
         |                  |                  |                                
         v
         v                  v                  v                                
 emergency.target
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           multi-user.target
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  graphical.target

signature.asc
Description: PGP signature