Fedora upgrading

_posts/2017-11-25-upgrade-fedora-online.md

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+---
+layout: post
+slug: upgrade-fedora-online
+status: published
+sitemap: true
+title: Upgrade your Fedora online with dnf-system-upgrade
+description: How to efficiently destroy your distribution
+disqus: false
+categories:
+tags:
+- linux
+- fedora
+---
+
+I have installed Fedora on my computer 3 or 4 years ago. Since this installation, I have installed many packages and upgraded it every 6 months. Unfortunately, Gnome Software has never worked for me, and I never took the time to debug it. Like many people, I am using the old method with `dnf-system-upgrade`, which worked pretty well. But this time, I had a segfault on dnf during the offline upgrade. The only information I got was the following lines:
+
+```
+kernel: show_signal_msg: 7 callbacks suppressed
+kernel: dnf[846]: segfault at 8 ip 00007f39b860f724 sp 00007ffd00588520 error 4 in libc-2.25.so[7f39b8586000+1cb000]
+kernel: audit: type=1701 audit(1511598827.366:96): auid=4294967295 uid=0 gid=0 ses=4294967295 subj=system_u:system_r:rpm_t:s0 pid=846 comm="dnf" exe="/usr/libexec/s
+systemd-coredump[869]: Failed to connect to coredump service: No such file or directory
+systemd[1]: dnf-system-upgrade.service: Main process exited, code=dumped, status=11/SEGV
+systemd[1]: Failed to start System Upgrade using DNF.
+```
+
+Just after the segfault, my computer rebooted on Fedora 26.
+
+## How dnf-system-upgrade works
+
+*Some information presented here could not be totally accurate. If you have a doubt, please follow the links.*
+
+`dnf-system-upgrade` use a feature of systemd described here: [Implementing Offline System Updates](https://www.freedesktop.org/software/systemd/man/systemd.offline-updates.html).
+By creating a symlink named `/system-update`, at the next reboot systemd will boot to a specific target named `system-update.target`
+
+But when will this symlink created ? When you run the following command:
+
+```
+# dnf system-upgrade reboot
+```
+
+We can see [in the source of the plugin](https://github.com/rpm-software-management/dnf-plugins-extras/blob/master/plugins/system_upgrade.py) that a symlink pointing to a dnf folder is created:
+
+```python
+DEFAULT_DATADIR = '/var/lib/dnf/system-upgrade'
+MAGIC_SYMLINK = '/system-update'
+
+# ...
+os.symlink(DEFAULT_DATADIR, MAGIC_SYMLINK)
+# ...
+reboot()
+```
+
+We can now investigate which services are triggered by this target:
+
+```raw
+$ ls /usr/lib/systemd/system/system-update.target.wants/
+dnf-system-upgrade.service  fwupd-offline-update.service  packagekit-offline-update.service
+```
+
+We are specifically interested by `dnf-system-upgrade.service` which basically run the following command:
+
+```raw
+# dnf --releasever=27 system-upgrade upgrade
+```
+
+
+At the end of this script, the symlink `/system-update` is destroyed and the computer rebooted.
+
+## Upgrading online to bypass the bug
+
+I didn't have any idea to debug this segfault in the offline mode.
+So I searched a way to run this command in an online system.
+If the command segfaults, I'll have some tools to investigate it.
+If the command works, I'll have an upgraded system.
+
+Before typing any command, you must know that this tool is not intended to be run this way.
+It may break your Fedora installation or kill your family.
+You really should run all your commands in a virtual terminal, as if Gnome crashes during the install, it will not kill dnf.
+
+First, you must have downloaded the update:
+
+```
+# dnf upgrade --refresh
+# dnf system-upgrade download --releasever=27
+```
+
+*For more information, you can refer to the article [Upgrading Fedora 26 to Fedora 27](https://fedoramagazine.org/upgrading-fedora-26-fedora-27/)*
+
+Then, you will need to update dnf-system-upgrade configuration stored in `/var/lib/dnf/system-upgrade.json` and set "upgrade\_status" to "ready":
+
+```json
+{
+  "download_status": null,
+  "upgrade_status": "ready", <--- this line
+  "exclude": [],
+  "allow_erasing": true,
+  "datadir": "/var/lib/dnf/system-upgrade",
+  "distro_sync": true,
+  "best": false,
+  "target_releasever": "27",
+  "system_releasever": "26",
+  "releasever": "23",
+  "install_packages": {},
+  "enable_disable_repos": []
+}
+```
+
+You must create the `/system-update` symlink, otherwise the upgrade command will fail:
+
+```raw
+# ln -s /var/lib/dnf/system-upgrade /system-update
+```
+
+And finally, you can run the upgrade command:
+
+```raw
+# dnf --releasever=27 system-upgrade upgrade
+```
+
+Your computer will automatically reboot at the end. Now you'll have either a working Fedora 27 or a borken Linux distribution. Good luck and have fun !