> +    if (!lock_user_struct(VERIFY_WRITE, frame, frame_addr, 0)) {
> +        goto give_sigsegv;

FreeBSD for Arm (haven't checked other BSDs or other archs)
gives a SIGILL for the "can't write signal frame to stack"
case, I think:
https://github.com/freebsd/freebsd-src/blob/main/sys/arm/arm/exec_machdep.c#L316
I don't understand why they picked SIGILL, SIGSEGV seems much more
logical to me, but we should follow the kernel behaviour.

This is a good thing to find. I'm going to have to study all the architectures, but

the first 5 I looked at all returned SIGILL, so this code has to change to reflect

that...