Cleaning up numbering for new x86 syscalls?

[Andy Lutomirski]

Hi all-

We currently have some giant turds in the way that syscalls are
numbered.  We have the x86_32 table, which is totally sane other than
some legacy multiplexers.  Then we have the x86_64 table, which is,
um, demented:

 - The numbers don't match x86_32.  I have no idea why.

 - We use bit 30, which triggers in_x32_syscall().  It should have
been bit 31, bit I digress.

 - We have this weird set of extra x32 syscalls that start at 512.
Who wants to bet whether we have no bugs if someone does syscall with,
say, nr == 512 (i.e. not 512 | BIT(30)) or nr == (16 | BIT(30))?  The
latter would be non-compat ioctl with in_x32_syscall() set and hence
in_compat_syscall() set.

 - Bloody restart_syscall() has a different number on x86_64 and
x64_32, which is a big mess.

I propose we consider some subset of the following:

1. Introduce restart_syscall_2().  Make its number be 1024.  Maybe
someday we could start using it instead of restart_syscall().  The
only issue I can see is programs that allow restart_syscall() using
seccomp but don't allow the new variant.

2. Introduce an outright ban on new syscalls with nr < 1024.

3. Introduce an outright ban on the addition of new __x32_compat
syscalls.  If new compat hacks are needed, they can use
in_compat_syscall(), thank you very much.

4. Modify the wrappers of the __x32_compat entries so that they will
return -ENOSYS if in_x32_syscall() returns false.

5. Adjust the scripts so that we only have to wire up new syscalls
once.  They'll have a nr above 1024, and they'll have the same nr on
all x86 variants.

Thoughts?