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SubjectRe: v6.1-rc1: Regression in notification of sethostname changes
Date
On 19.10.22 14:31, Petr Vorel wrote:
>> Hi Torsten,
>
>>> Hello Petr,
>
>>> your commit
>
>>> commit bfca3dd3d0680fc2fc7f659a152234afbac26e4d
>>> Author: Petr Vorel <pvorel@suse.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

#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);
}
}
}
}
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