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

[Petr Vorel]

> On 19.10.22 14:31, Petr Vorel wrote:
> >> Hi Torsten,

> >>> Hello Petr,

> >>> your commit

> >>> commit bfca3dd3d0680fc2fc7f659a152234afbac26e4d
> >>> Author: Petr Vorel <pvorel@xxxxxxx>
> >>> 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);
> 			}
> 		}
> 	}
> }

Rigth, poll() is broken. Thanks a lot for a simple reproducer!
I'll send a fix shortly.

Kind regards,
Petr

> 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);
> 			}
> 		}
> 	}
> }