lfcode.ca notes compiled for future reference

Dell XPS 15: "I can't understand why some people _still_ think ACPI is a good idea.." -Linus Torvalds

I got my new machine in the mail, an XPS 15 bought on one of the numerous sales which pretty much happen every couple of days, and while most of the hardware is amazing compared to my previous machine (a beat-up X220), there are some significant hardware issues that need to be worked around. Besides, of course, the fact that the keyboard and lack of trackpoint is objectively inferior to the previous machine.

The first thing that many people may do after booting up a new machine on any operating system is to make sure they got what they paid for, and check detected hardware. So, naturally, I run lspci... and it hangs. I could change virtual console, but it said something about a watchdog catching a stalled CPU core. Fun! Off to Google, which states that it's the NVidia driver, specifically related to Optimus (which, by the way, this video remains an excellent description of). So I blacklist it, and lspci seems to work fine. Next, I install X and all the other applications I want to use, and being a sensible Arch user, I read the Arch wiki on the hardware, which states that the dedicated graphics card will use a lot of power if it isn't turned off.

So, I turn it off. For this, I use acpi_call with a systemd-tmpfiles rule to turn it off at boot. The setup is as follows:

~ » cat /etc/tmpfiles.d/acpi_call.conf
w /proc/acpi/call - - - - \\_SB.PCI0.PEG0.PEGP._OFF
~ » cat /etc/modules-load.d/acpi_call.conf

Next, I get to work doing some programming on it. It was a massive improvement on the previous hardware on account of having a 1080p screen instead of a 1366x768 device-usability-eliminator. However, my terminal-based vim sessions kept getting disturbed by messages such as the following:

kernel: pcieport 0000:00:1c.0: PCIe Bus Error: severity=Corrected, type=Data Link Layer, id=00e0(Transmitter ID)
kernel: pcieport 0000:00:1c.0:   device [8086:a110] error status/mask=00001000/00002000

After looking in the wiki again, I set pci=nommconf in the kernel options. At this point I was entirely unconvinced that the acpi_rev_override=1 stuff was necessary since I got rid of any NVidia software that could possibly break my machine.

Satisfied with my handiwork, I put the machine into service, and took it to school. Naturally, one may want to put a machine into sleep mode if it is not in use. Unfortunately, doing so was causing it to lock up upon any attempt at waking it. Another strange behaviour that I had been starting to notice at this point was that Xorg could not be started more than once a boot due to the same hard lock issue.

As it turns out, this was again the same issue as the sleep, which is fixed by the acpi_rev_override=1 in the kernel parameters. I had been dissuaded by the Arch developers disabling CONFIG_ACPI_REV_OVERRIDE_POSSIBLE at some point in the past, which was what was suggested by an outdated forum post (lesson learned: do more research on things which could easily change), but they reenabled it recently.

So, finally, the situation:

  • Power management appears to work correctly
  • Battery life is incredible (but could probably be hugely improved to "ridiculous")
  • The touchpad is a touchpad, which means it sucks, although it is one of the better ones
  • There is a significant and very annoying key-repeatt isssuee which happens on occasion, some users have reported it also occurs on Windows. It has happened at least 5 times while writing this post.
  • I hadn't noticed this earlier, but the keyboard has a tendency to scratch the screen while the laptop is closed. Since this is a thoroughly modern machine, there isn't really space to just shove a microfiber cloth between the screen and keyboard like I had done with my X220 with missing rubber standoffs.

Would I recommend buying one?

Maybe. For my use case, it made sense since I want to have a dedicated GPU which can be used in Windows for CAD work. The hardware with the exception of the keyboard and trackpad is very nice, especially for the price (a bit more than half what Apple charges for a similarly specced MacBook Pro 15"). If you don't need or want a dedicated GPU, buy another machine. NVidia still has awful Linux problems.

Which machine? Probably a ThinkPad since they have very good Linux support right out of the box. That being said, I acknowledge that Dell has a group dedicated to Linux support on their hardware, and both companies have similar complete lacks of desire to lift a finger with regards to pressuring their fingerprint reader vendor (the same one for both companies!) to release the driver spec.

Since Linus Torvalds provides such excellent material to quote,

The thing is, you have two choices:
 - define interfaces in hardware
 - not doing so, and then trying to paper it over with idiotic tables.

Sadly, Intel decided that they should do the latter, and invented ACPI.

There are two kinds of interfaces: the simple ones, and the broken ones.


The broken ones are the ones where hardware people know what they want to
do, but they think the interface is sucky and complicated, so they make it
_doubly_ sucky by then saying "we'll describe it in the BIOS tables", so
that now there is another (incompetent) group that can _also_ screw things
up. Yeehaa!

Tags: linux, arch-linux, hardware, laptop, dell-xps-15

SELinux notes

ausearch -m avc to find denials. If there are none, that's probably because some distro maintainer decided that the denial should be silent:

semodule -DB turns on dontaudit events, semodule -B turns them back off.

When trying to get things to work correctly with audit2allow, skip the 15 minutes of doing things over and over triggering different denials and running audit2allow -M mymodule < fails; semodule -i mymodule.pp by just doing a quick setenforce 0 before doing it once. All of the actions (AVCs?) in creating a file will show up in the log in one shot. Obviously turn on enforcing mode afterwards.

When in doubt, consult the colouring book. Yes, that's real.

Tags: linux, selinux

MS Documentation sucks (or how I got my VM hostnames to be set automatically from kickstart)

I wanted to automate my linux VM deployment on my Hyper-V based lab infrastructure. One small flaw: while DHCP does automatically update DNS, it does not do too much when your VM is named "localhost". I wanted to make the fedora deployment completely automated... which it is after I wrote a kickstart, except you can't get into the new box because you can't find its IP address.

I wrote a small tool to deal with this issue:

You want the variable VirtualMachineName in /var/lib/hyperv/.kvp_pool_3.

Documentation that took way too long to find:

Tags: hyper-v, linux

NUT not finding my UPS + fix

I use a CyberPower CP1500AVRLCD as a UPS in my lab. I'm just now getting more stuff running on it to the point that I want automatic shutdown (because it won't run for long with the higher power usage of more equipment). So, I plugged it into the pi that was running as a cups-cloud-print server and sitting on a shelf with my network equipment. The problem was that the driver for it in NUT didn't want to load. As is frighteningly common, it's a permissions problem:

Here's the log showing the issue:

Jul 09 16:49:58 print_demon upsdrvctl[8816]: USB communication driver 0.33
Jul 09 16:49:58 print_demon upsdrvctl[8816]: No matching HID UPS found
Jul 09 16:49:58 print_demon upsdrvctl[8816]: Driver failed to start (exit status=1)

Here's the udev rule that fixes it:


What this does is, when udev gets an event of the device with USB product id 0501 and vendor id 0764 being added to the system, it changes the permissions on the device files (think /dev/bus/usb/001/004 and /devices/platform/soc/20980000.usb/usb1/1-1/1-1.3) to allow group nut to read and write to it, allowing comms between the NUT driver and the device.

Tags: homelab, linux, raspberry-pi, udev

nftables: redirect not working + fix

Recently, I made the somewhat-rash decision to switch to nftables from ufw-managed iptables on this VPS.

It's been a fun ride. The man page doesn't even document the redirect feature. It doesn't even acknowledge its existence, nor what it really does.

That's irrelevant however, because it does the same thing as the REDIRECT target in iptables, documented in the iptables-extensions man page. This allows the functionality of redirect in nftables to be inferred as "change destination address to localhost, and change the destination port to the one specified after to".

I, however, was a bit too dense to go looking through there and didn't read the wiki too well about redirection. I figured "hey, just need to put redirect at the start of the chain hooked into nat prerouting to enable it, then add a rule specifically redirecting the port". Later, I wondered why it wasn't working. After some tcpdump, copious quantities of counters everywhere, and netcat instances, I figured that out.

Note that you need to allow the packets with dport 11113 in your filter. Your filter table will never see any packets on port 113 unless something has gone horribly wrong, as all of them will have dport changed to 11113 in the nat table. If, for some reason, you want to drop these, you probably can do it in a chain with type mangle hook prerouting priority 0, but I have no idea why you would want to do that.

Here's the functional config:

table ip nat {
  chain prerouting {
    type nat hook prerouting priority 0;
    tcp dport 113 counter redirect to 11113

  chain postrouting {
    type nat hook postrouting priority 0;

table ip6 nat {
  chain prerouting {
    type nat hook prerouting priority 0;
    tcp dport 113 counter redirect to 11113

  chain postrouting {
    type nat hook postrouting priority 0;

Tags: nftables, linux

How to have a functional dhcrelay

I'm dumb. Or ignorant. Or inexperienced. I haven't decided which.

dhcrelay only gets proper responses if it's listening on both the interface that it's actually listening on for requests and the one where it will get the responses.

My command line for it to forward dhcp requests to my Windows dhcp server in my virtual lab is:

/usr/bin/dhcrelay -4 -d -i eth1 -i eth2 10.x.x.x

eth1 is the interface with the Windows dhcp server on its subnet

eth2 is the interface with the clients on it

10.x.x.x is the address of the Windows dhcp server

This is run on my arch (yes, I know. Debian took longer than Windows to install. The only stuff on it is in base, vim, and dhcp) gateway VM. I could also stand up a Windows box and have it do NAT, but that doesn't use 512MB of RAM nearly as happily.

Tags: Windows Server, dhcp, linux, homelab