svn commit: r350764 - head/sys/arm64/arm64

[Warner Losh]

On Fri, Aug 9, 2019 at 12:57 AM Konstantin Belousov <kostikbel at gmail.com>
wrote:

> On Thu, Aug 08, 2019 at 07:38:28PM -0600, Warner Losh wrote:
> > On Thu, Aug 8, 2019, 4:59 PM Gleb Smirnoff <glebius at freebsd.org> wrote:
> >
> > >   Hi,
> > >
> > > why do we need COMPAT_43 for arm64 at all? I can't imagine an
> > > application that would require this compatibility.
> > >
> > > A more general question is how far in the future are we going
> > > to carry COMPAT_43 for i386/amd64?
> > >
> >
> > COMPAT_43 is a weird option. It's a combo of both sys calls and kernel
> > behavior modifications. Before we thinned the ABIs we supported, it was
> > necessary for them as well. The biggest behavior change is around
> signals.
> > It is weird to sort out and nobody has done the deep analysis to see what
> > is truly unused and what is there for compat with Linux and other SysV
> > systems...
> I am not aware of any changes that COMPAT_43 provides for the signal
> handling semantic, except a minor adjustment for interpretation of
> zero-sized stack for sigaltstack(2).
>

The onstack stuff was what I was thinking about, but we also have code in
sys_getpid() that returns the ppid in the second retval register, and
similar things for getuid and getgid,  It also allows ioctl numbers that
have IOC_IN set, but size == 0 (these would otherwise return ENOTTY). It
also turns on the COMPAT_OLDSOCK code which generally only kicks in when
compat bits are set, but in one place it allows a shorter unix domain
socket path length to be compatible unconditionally. The compatibility TTY
stuff, at least is under COMPAT_43TTY, but that's purely ioctl translation
code.

The COMPAT_43 option indeed enables lcall 7,0 syscall entry emulation,
> on both i386 and amd64.  We are able to run FreeBSD 1.1.8 (i386) on amd64
> kernel in chroot this way.  Since sometimes I get bug reports about this
> stuff, there are some users of it.  I believe it is important to be able
> to run any FreeBSD binary for PR purposes, to wave the flag of excellent
> binary compatibility we offer.
>
> COMPAT_43 is there to stay as far as there are people willing to maintain
> it.  There are more than one.
>

I think it's safe to retain on i386. amd64 is less clear to me, but I'd
lean yes. All the other platforms I'd agree with gleb: why do we need it in
the kernels by default (and maybe why do we need to support it at all)?

Warner