netdev - Re: bpfilter causes a leftover kernel process

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Date:   Wed, 5 Sep 2018 17:52:43 +0200
From:   Olivier Brunel <jjk@...cky.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     netdev@...r.kernel.org, daniel@...earbox.net
Subject: Re: bpfilter causes a leftover kernel process

Hi,

Quick follow-up on this:

- first off, Arch devs have updated their kernel config so the next
  kernel will not have bpfilter enabled anymore, thus avoiding any
  issue.

- having said that, I've found a neasy way to reproduce it in an Arch
  VM, in case you're interested :

Boot the latest Arch ISO[1] which now contains a kernel 4.18.5 and do a
very basic installation, pretty much just:

# pacstrap /mnt base
# genfstab -U /mnt >> /mnt/etc/fstab

And of course install your boot loader of choice.

Then boot the brand new system, log in and make sure the helper is
actually started, i.e. `modprobe bpfilter` -- Now halt.

You'll see in the end that systemd complains that it can't
unmount /oldroot (EBUSY), aka the root fs; and that's because of the
bpfilter helper, which wasn't killed because it's seen as a kernel
thread due to its empty command line and therefore not signaled.

Cheers,

[1] https://www.archlinux.org/download/