Broken shell - I can't login at all

[Jeff BSD]

Hi-
I'm in the process of upgrading a 4.6 system to 5.3.  When I boot the
machine it gets to:

----
init: bin/sh on /etc/rc terminated abnormally, going to single user
mode
Enter root password, or ^D to go multi-user
Password:
----

I enter the password, then:

----
Enter full pathname of shell of RETURN for /bin/sh:
pid # (sh), uid 0: exited on signal 12
init: bin/sh on /etc/rc terminated abnormally, going to single user
mode
Enter root password, or ^D to go multi-user
Password:
----

Round and round I go.

Sounds like I broke /bin/sh to me.  I've messed around in safe mode
but I can't see how I can use it to possibly fix my problem, assuming
I did do anything to /bin/sh (which I don't think I did -
intentionally/directly that is).

How do I fix it so I can boot it?  A bit of the chicken and the egg,
what?

Jeff