$ rpm -q mock dnf yum
mock-1.2.13-2.fc23.noarch
dnf-1.1.2-4.fc23.noarch
yum-3.4.3-507.fc23.noarch


Most package fail to 'fedpkg mockbuild' for me with f23 host in f23 chroot. Original example is qemu, but a simpler example is parted. I suspect this is reproducible for anyone on an f23 host:

fedpkg co parted
cd parted
fedpkg switch-branch f23
fedpkg mockbuild
...
Total                                              567 kB/s | 8.1 MB  00:14     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction (shutdown inhibited)
  Installing : crontabs-1.11-11.20150630git.fc23.noarch                    1/59 
  Installing : cronie-anacron-1.5.0-3.fc23.x86_64                          2/59 
warning: %post(cronie-anacron-1.5.0-3.fc23.x86_64) scriptlet failed, exit status 126
...


There's more complete output in my posting to fedora devel:

https://lists.fedoraproject.org/pipermail/devel/2015-September/215183.html
https://lists.fedoraproject.org/pipermail/devel/2015-October/215341.html

I've played with it a bit since then, and if I edit /etc/mock/site-defaults.cfg and add

config_opts['yum_command'] = '/usr/bin/yum-deprecated'

Builds start working again, at least it did for my parted test case. So maybe this is a dnf issue but I'll let yall sort it out since I don't really know how to debug further