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Sudden “operation not permitted”

(Apologies if this mail comes through poorly formatted for the list; my main machine is unavailable due to this problem and I’m writing on an iPad...)

Running Stretch on a circa-2009 self-built machine which has run happily without serious issues since it was built, apart from the odd annoyance with Bluetooth audio which the list has already had the pleasure of hearing about.

This morning I unlocked it before leaving home, and noticed that load was fairly constant at about 1.0 when it should have been at 0 as the machine should have been idle. I listed processes with top and noticed that upowerd was taking up a whole CPU to itself. Normally I wouldn’t notice this daemon doing its thing.

Google turned up nothing relevant.

I decided to try a reboot, which cleared the upowerd problem and returned load to 0 or close to it. But now, network activity is not working. Any attempt to ping an IP address (eg my router) results in “Operation not permitted” even when run as root. Attempt to access any web page results in failing to find the site. Attempting to ping a text domain (eg www.google.com) results in an error message (instantly) saying could not resolve...

It seems like networking is bejiggered suddenly on this machine. I did not install updates before rebooting, last time updates were installed was Sunday, and all has been well since then until this morning, although I did not reboot during that period until this morning. The machine is attached to my network via an Ethernet cable running to a WiFi+wired router. That is obviously working as the machine was able to get an IP address by DHCP after the reboot (ip route after reboot showed IP address correctly assigned) but unable to resolve any address and unable to ping an IP address of the form 192.168.xx.yy with the “Operation not permitted” error.

All the pinging I’ve been trying worked without issues before this problem occurred, both as root and as an unprivileged user.

Looking through the journalctl since my reboot, I do not see anything that obviously points to the problem. Network Manager seems to start OK, as far as I can tell. I don’t see any significant errors except postgreSQL failing to start, which is normal and I don’t use it. The first sign of trouble (to my eye, anyway) in the boot log is when services that want the network eg ntp start trying to interact with it, and failing.

A second reboot produced exactly the same result. Other devices on my network are working fine. 

Putting the upowerd behavior together with the suddenness of this problem, I’m very afraid that this isn’t really permissions and is in fact some sort of hardware issue — the machine is 10 years old, was built by me, and has been in continuous use since it was built... Any suggestions for what I can do to diagnose?

Thanks in advance