blktrace doesn't work, time to debug. Things that don't fix it include upgrading from 4.15 to 5.3 HWE kernel and disabling apparmor. Next up is disabling kernel lockdown via mokutil --disable-validation. Alas, blktrace still fails at startup.
After running mokutil and then rebooting there are still a few messages in dmesg output about lockdown so I wonder whether it was fully disabled.
Lockdown: Hibernation is restricted; see man kernel_lockdown.7OK, lets read the man page. Great, it doesn't exist -- not for Ubuntu nor for other distros. There is a draft but I am starting to get the impression that lockdown wasn't ready for prime time. And Linus had a strong opinion about it in 2018.
Lockdown: /dev/mem,kmem,port is restricted; see man kernel_lockdown.7
Next up is a strong opinion from Brendan Gregg.
Many distros are enabling lockdown, breaking BPF.OK, maybe my problem is lockdown and mokutil wasn't sufficient. Time to try:
echo 1 > /proc/sys/kernel/sysrq; echo x > /proc/sysrq-triggerAnd now blktrace works. Well, until I reboot. I already have a script to run after reboot to reduce security so that PMP can run. That script just got larger:
echo -1 > /proc/sys/kernel/perf_event_paranoid
echo 0 > /proc/sys/kernel/yama/ptrace_scope
sudo sh -c " echo 0 > /proc/sys/kernel/kptr_restrict"
echo 1 > /proc/sys/kernel/sysrq
echo x > /proc/sysrq-trigger