linux-kernel - Re: v6.1-rc1: Regression in notification of sethostname changes

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Date:   Wed, 19 Oct 2022 14:50:41 +0200
From:   Torsten Hilbrich <torsten.hilbrich@...unet.com>
To:     Petr Vorel <pvorel@...e.cz>, <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: v6.1-rc1: Regression in notification of sethostname changes

On 19.10.22 14:31, Petr Vorel wrote:
>> Hi Torsten,
> 
>>> Hello Petr,
> 
>>> your commit
> 
>>> commit bfca3dd3d0680fc2fc7f659a152234afbac26e4d
>>> Author: Petr Vorel <pvorel@...e.cz>
>>> Date:   Thu Sep 1 21:44:03 2022 +0200
> 
>>>     kernel/utsname_sysctl.c: print kernel arch
> 
>>>     Print the machine hardware name (UTS_MACHINE) in /proc/sys/kernel/arch.
> 
>>>     This helps people who debug kernel with initramfs with minimal environment
>>>     (i.e.  without coreutils or even busybox) or allow to open sysfs file
>>>     instead of run 'uname -m' in high level languages.
> 
>>> broke the notification mechanism between the sethostname syscall and the pollers of /proc/sys/kernel/hostname.
> 
>>> The table uts_kern_table is addressed within uts_proc_notify by the enum value, however no new enum value was added in "enum uts_proc".
> 
>>> I noticed the problem when journald-systemd failed to detect hostname changes made with the sethostname syscall (as used by the hostname tool).
>>> When setting the hostname through /proc/sys/kernel/hostname the poll notification was working.
> 
>> Thanks a lot for your report, working on a fix!
>> Andrew, Greg, sorry for a regression.
> 
> Hi Torsten,
> 
> could you please post exact steps to reproduce the problem.
> Although the required fix to add new enum into enum uts_proc is trivial,
> I was not able to reproduce the problem with 6.1.0-rc1 (actually
> 6.1.0-rc1-4.g1d716d8-default which contains few extra patches).
> 
> # hostname; hostnamectl hostname; cat /proc/sys/kernel/hostname
> opensuse-tumbleweed.20221001
> opensuse-tumbleweed.20221001
> opensuse-tumbleweed.20221001
> 
> # hostnamectl set-hostname foo; echo $?
> 0
> # hostname; hostnamectl hostname; cat /proc/sys/kernel/hostname
> foo
> foo
> foo
> 
> # hostname bar; echo $?
> 0
> # hostname; hostnamectl hostname; cat /proc/sys/kernel/hostname
> bar
> bar
> bar
> 
> # echo "baz" > /proc/sys/kernel/hostname
> # hostname; hostnamectl hostname; cat /proc/sys/kernel/hostname
> baz
> baz
> baz
> 
> 
> # hostnamectl set-hostname foo; reboot
> After reboot it's 'foo'.
> What am I missing?
> 
> BTW I originally tested the feature only on dracut initramfs (with rapido [1]),
> which obviously bypass systemd. For a fix I'm creating rpm package (binrpm-pkg).

The problem is happening in the systemd-journald poll notification. I was checking for the problem by attaching gdb to the running systemd-journald and setting a breakpoint to the server_cache_hostname function. This function is triggered via dispatch_hostname_change whenever the hostname changes. This is done via sd_event API in systemd.

Here is an example program for this functionality without any further dependency:

#include <poll.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <unistd.h>

int main()
{
	struct pollfd info;

	info.fd = open("/proc/sys/kernel/hostname", O_RDONLY);
	info.events = 0;
	info.revents = 0;

	while (true) {
		int res = poll(&info, 1, -1);
		if (res > 0) {
			if (info.revents != 0) {
				char buffer[64];
				gethostname(buffer, sizeof(buffer));
				printf("Hostname has changed to: %s\n", buffer);
			}
		}
	}
}

I have also attached this program.

If you call this program and issue calls of the hostname utility to change the hostname some message should be printed.

	Torsten


View attachment "hostname-poll-test.c" of type "text/x-csrc" (450 bytes)