quentin.dufour.io/_posts/2017-11-25-upgrade-fedora-online.md

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---
layout: post
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slug: online-upgrade-fedora
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status: published
sitemap: true
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title: Online Fedora Upgrade
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description: How to efficiently destroy your distribution
disqus: false
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category: operation
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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:
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<pre style="white-space: pre">
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kernel: dnf[846]: segfault at 8 ip 00007f39b860f724 sp 00007ffd00588520 error 4 in libc-2.25.so[7f39b8586000+1cb000]
systemd[1]: dnf-system-upgrade.service: Main process exited, code=dumped, status=11/SEGV
systemd[1]: Failed to start System Upgrade using DNF.
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</pre>
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Just after the segfault, my computer rebooted on Fedora 26.
## How dnf-system-upgrade works
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*Some information presented here could be not totally accurate. If you have a doubt, please follow the links.*
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`dnf-system-upgrade` uses 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`. That's what we name "an offline upgrade" contrary to an upgrade run in the `default.target` which I call "an online upgrade".
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But when will this symlink created ? When you run the following command:
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```raw
dnf system-upgrade reboot
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```
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
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dnf --releasever=27 system-upgrade upgrade
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```
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.
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So I searched a way to run this command on an online system.
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If the command segfaults, I'll have some tools to investigate it.
If the command works, I'll have an upgraded system.
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In my case, once run online, everything went well.
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Before typing any command, you must know that this tool is not intended to be run this way.
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You may break your Fedora installation or kill your family.
You really should run all your commands in a virtual terminal as it will not kill dnf if Gnome crashes during the install.
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First, you must have downloaded the update:
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```raw
dnf upgrade --refresh
dnf system-upgrade download --releasever=27
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```
*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
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ln -s /var/lib/dnf/system-upgrade /system-update
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```
And finally, you can run the upgrade command:
```raw
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dnf --releasever=27 system-upgrade upgrade
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```
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Your computer will automatically reboot at the end. Now you'll have either a working Fedora 27 or a broken Linux distribution. Good luck and have fun !