Messages in this thread | | | From | Torsten Hilbrich <> | Subject | Re: v6.1-rc1: Regression in notification of sethostname changes | Date | Wed, 19 Oct 2022 14:50:41 +0200 |
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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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