I actually believe it sends a SIGINFO, not a SIGUSR1. It's unfortunate that SIGINFO never made it to Linux, it's an incredibly useful feature.

Indeed, but on GNU dd you can send SIGUSR1 to do the same as what SIGINFO would do on BSD.

Downside of SIGUSR is that it will kill the process if there's no handler, rather than ignoring it, so "just try it" is always risky.

I used to have a habit of running 'killall -USR1 dd' to get the status of a dd process.

Then I switched to using sddm for my display manager/login screen. It didn't have a USR1 handler. If you kill your display manager, you're forcefully and suddenly logged out of your session.

I stopped using 'killall -USR1 dd' for dd status.

You could make that a bit safer by first checking for SIGUSR1 in SigCgt mask in /proc/pid/status

It's SIGINFO, and this is a common thing on the BSDs.