Hello, recently I found that "guix system" makes a "broken" system for
me.  When I boot a freshly created system, I get something like this:

  In procedure <...> at ./gnu/build/file-systems.scm:282:4 (device)
  In procedure fport_seek: Invalid argument

and I'm thrown at the Guile promt.

The same problem (well, I think it's the same) was also reported by
roptat on #guix:

  https://gnunet.org/bot/log/guix/2017-01-28#T1277485

After bisecting the guix git checkout, I found that commit b1a505baf6¹
was the first one where my system starts to fail.  And indeed when I
reverted this commit on the latest master, guix built a working system.
Moreover, the following simple diff (it's a partial revert of that
commit) "fixes" guix for me: