It just occurs to me that the simple way to implement
procfd_sigqueueinfo info is like:

int copy_siginfo_from_user_any(kernel_siginfo_t *info, siginfo_t *uinfo)
{
#ifdef CONFIG_COMPAT
	if (in_compat_syscall)
		return copy_siginfo_from_user32(info, uinfo);
#endif
	return copy_siginfo_from_user(info, uinfo);                        
}

long procfd_sigqueueinfo(int fd, siginfo_t *uinfo)
{
	kernel_siginfo info;

        if (copy_siginfo_from_user_any(&info, uinfo))
        	return -EFAULT;
	...;                
}

It looks like there is already a place in ptrace.c that already
hand rolls copy_siginfo_from_user_any.

So while I would love to figure out the subset of siginfo_t tha we can
just pass through, as I think that would make a better more forward
compatible copy_siginfo_from_user32.  I think for this use case we just
add the in_compat_syscall test and then we just need to ensure this new
system call is placed in the proper places in the syscall table.

Because we will need 3 call sights: x86_64, x32 and ia32.  As the layout
changes between those three subarchitecuters.

Eric