COMPAT_43 users?

[Konstantin Belousov]

On Tue, Jul 31, 2018 at 05:49:20PM -0400, Mark Johnston wrote:
> The COMPAT_43 kernel option, which enables syscall support for 4.3BSD
> binaries, hasn't been enabled in the standard kernel configs for well
> over a decade, and doesn't appear to be a dependency of any other kernel
> features.  Nonetheless, the kernel contains quite a bit of code to
> support this option.  Does anyone use it in modern versions of FreeBSD
> or have any arguments for keeping it?

COMPAT_43 means two things, the third part is a.out image activator.
First thing is the lcall $7.$0 syscall emulator, both on amd64 and
(surprisingly) i386, after 4/4 split.  Second thing is the syscalls
compat shims.

Together, all three things allow to run pre-3.x binaries on the modern
machines, including amd64. I think this is useful at least for 'waving
the flag' about our ABI compatibility guarantees, and for the historic
software reconstruction. I run 1.1.8 chroot and several old binaries
sometimes, I know that bde does, and there was at least one more user
some time ago.

What do you mean by a lot of code ?  Syscall compats is relatively easy.
lcall $7,$0 emulation is very non-trivial but tiny.  I do maintain this
code and do not want it to go away.