Re: bpfilter blocks root unmount during shutdown

From: Andrei Borzenkov <arvidjaar@gmail.com>
To: Lennart Poettering <lennart@poettering.net>
Cc: "initramfs@vger.kernel.org" <initramfs@vger.kernel.org>,
	"systemd-devel@lists.freedesktop.org"
	<systemd-devel@lists.freedesktop.org>
Subject: Re: bpfilter blocks root unmount during shutdown
Date: Mon, 24 Sep 2018 19:30:02 +0300	[thread overview]
Message-ID: <d4aa5a74-d3a2-0865-5afd-7c15cef679f2@gmail.com> (raw)
In-Reply-To: <20180924132047.GD31443@gardel-login>

24.09.2018 16:20, Lennart Poettering пишет:
> On So, 23.09.18 10:38, Andrei Borzenkov (arvidjaar@gmail.com) wrote:
> 
>> Dracut /shutdown script first tries to kill all processes still running
>> off old root. Unfortunately this fails for special user process that
>> runs bpfilter because it does not include reference to /oldroot in
>> places where dracut looks for in kilall_proc_mountpoint()
> 
> Hmm, when we invoke the /shutdown executable we already executed our
> process killing spree as part of systemd-shutdown. How come your
> processes even survive that long?

        p = procfs_file_alloca(pid, "cmdline");
        f = fopen(p, "re");
        if (!f)
                return true; /* not really, but has the desired effect */

        count = fread(&c, 1, 1, f);

        /* Kernel threads have an empty cmdline */
        if (count <= 0)
                return true;

This process is spawned as special kernel thread, even though it is
otherwise normal user process.

net/bpfilter/bpfilter_kern.c:load_umh():

        /* fork usermode process */
        err = fork_usermode_blob(&bpfilter_umh_start,
                                 &bpfilter_umh_end - &bpfilter_umh_start,
                                 &info);
        if (err)
                return err;
        pr_info("Loaded bpfilter_umh pid %d\n", info.pid);
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