UML failing at "Failed to initialize default registers" on kernel 5.10

[Glenn Washburn]

Hi Benjamin,

After applying the close_range patch, I'm now getting a failure at
runtime where the last line printed from UML is "Failed to initialize
default registers". The host is on Debian 11 at kernel 5.10.216. I've
bisected the issue and it looks like the bad commit is: 3f17fed2149
("um: switch to regset API and depend on XSTATE").

On another machine which is on Debian 12 and kernel 6.1.119 the UML
binary compiled from v6.13-rc1 runs fine.

I notice that the bad commit appears to be using ptrace in a different
way. Perhaps there is a difference in how ptrace operate from 5.10 to
6.1 kernels. I've tried debugging this without much success, as GDB
can't be used. Any idea on what's going on here?

Glenn