Ubuntu18.10:ppc64:s390x - Sysrq trigger disabled when writing to /proc/sysrq-trigger

[bugproxy]

== Comment: #0 - Praveen K. Pandey <email address hidden> - 2018-09-19 04:49:39 ==
---Problem Description---
 [Witherspoon-DD2.2][Ubu 18.10] [4.18.0-7-generic ] : Sysrq trigger disabled when writing to /proc/sysrq-trigger

while testing kdump trying trigger kdump panic via /proc/sysrq-trigger it failed as : SysRq : This sysrq operation is disabled
LOG:

root@test:~# cat /proc/sys/kernel/sysrq
176
root@test:~# echo c > /proc/sysrq-trigger
root@test:~# dmesg
[ 380.340051] mlx5_core 0000:01:00.1 enp1s0f1: Self test out: status flags(0x2)
[ 389.404239] sysrq: SysRq : This sysrq operation is disabled.
root@test~# cat /boot/config-4.18.0-7-generic | grep CONFIG_MAGIC_SYSRQ
CONFIG_MAGIC_SYSRQ=y
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x01b6
CONFIG_MAGIC_SYSRQ_SERIAL=y
root@test:~# cat /etc/sysctl.conf | grep kernel.sysrq
#kernel.sysrq=438
root@test:~# kdump-config show
DUMP_MODE: kdump
USE_KDUMP: 1
KDUMP_SYSCTL: kernel.panic_on_oops=1
KDUMP_COREDIR: /var/crash
crashkernel addr:
   /var/lib/kdump/vmlinuz: symbolic link to /boot/vmlinux-4.18.0-7-generic
kdump initrd:
   /var/lib/kdump/initrd.img: symbolic link to /var/lib/kdump/initrd.img-4.18.0-7-generic
current state: ready to kdump

kexec command:
  /sbin/kexec -p --command-line="root=UUID=c0302064-c5a3-49a7-8bd4-402283e6fcbe ro quiet splash nr_cpus=1 systemd.unit=kdump-tools.service irqpoll noirqdistrib nousb" --initrd=/var/lib/kdump/initrd.img /var/lib/kdump/vmlinuz
root@test:~#

Regards
Praveen

== Comment: #2 - Praveen K. Pandey <email address hidden> - 2018-09-19 05:16:11 ==
when i enable in /etc/sysctl.conf it works . i append below value in sysctl.conf

echo "kernel.sysrq = 1" >> /etc/sysctl.conf

I think this is should be enable by default in ubuntu 18.10 as earlier ubuntu distro it has , not sure why it got removed as kdump mechanism require this .

Praveen
Regards

[Frank Heimes (fheimes)]

This is not limited to ppc64el and at least applies to s390x, too.

[Thadeu Lima de Souza Cascardo (cascardo)]

The sysrq mechanism allows someone with physical access to the system to easily cause it to boot by just pressing some keys. That's why the sysrq default allows only very few commands to be issued. kdump is for real crashes. kdump testing may require using the sysrq-trigger mechanism, but that could just as well write to /proc/sys/kernel/sysrq before causing the crash. In fact, that's just what kdump-tools autopkgtest does.

echo 1 > /proc/sys/kernel/sysrq
echo c > /proc/sysrq-trigger

So, not a bug here.

[Dimitri John Ledkov (xnox)]

The default sysctl setting for kernel.sysrq in Ubuntu is 176 (and was in bionic and xenial) and it is shipped in /etc/sysctl.d/10-magic-sysrq.conf file. This file is provided by a required package from the minimal task, called `procps`.

176 enables:
128 - allow reboot/poweroff
32 - enable remount read-only
16 - enable sync command

Do you have `procps` package installed on your system?

Is anything else overriding that setting?

If that setting is not suitable for you, I recommend to either modify /etc/sysctl.d/10-magic-sysrq.conf or to add another .conf drop in, with a higher number in /etc/sysctl.d directory to suit your system better.