rdist/setreuid

[Guy Harris]

>We ran into a problem while trying to port rdist from 4.3bsd to sysVr3.
>rdist uses setreuid to switch back and forth between root and the user,
>but sysVr3 (and even sysVr4 apparently) only has setuid; ie, once
>you're the user, there's no going back to root.  Are we overlooking
>something?  It's particularly surprising that sysVr4 also seems to
>have this limitation.

S5R4 may have, as an undocumented feature, a "seteuid()" call that, even
if you're root, sets *only* the effective UID and leaves the
saved-set-user ID alone.  I think the next POSIX 1003.1 supplement will
mandate "seteuid()", so it shouldn't remain undocumented forever....

(S5R4 apparently has a "setreuid()" call in the BSD compatibility
library; whether any particular vendor supplies that library is up to
them.  I don't know how complete it is, and I think it's implemented, in
part, atop "seteuid()"....)