I got the error when trying to run "who" from musl compiled coreutils
package. I assume there's a good reason but I have to ask...

Why not set _PATH_UTMP to /var/run/utmp instead? Or set it at build time
based on actual utmp location?

./include/paths.h:#define _PATH_UTMP    "/dev/null/utmp"
./include/utmp.h:#define _PATH_UTMP "/dev/null/utmp"

John