[TUHS] Who's behind the UNIX filesystem permission

Doug McIlroy doug at cs.dartmouth.edu
Thu Aug 1 22:35:25 AEST 2019

Read and write permission were common ideas--even part of
the Atlas paging hardware that was described before 1960.
The original concept of time-sharing was to give a virtual
computer to each user. When it became clear that sharing
was an equally important aspect, owner/other permissions
arose. I believe that was the case with Multics.

Owner/other permissions were in PDP-11 Unix from the start.
Group permissions arose from the ferment of daily talk in
the Unix lab. How might the usual protections be extended
to collaborative projects? Ken and Dennis deserve credit
for the final implementation. Yet clean as the idea of groups
was, it has been used only sporadically (in my experience).

Execute permission (much overloaded in Unix) also dates
back to the dawn of paging. One Unix innovation, due to
Dennis, was the suid bit--the only patented feature in
the Research system. It was instantly adopted for 
maintaining the Moo (a game now sold under the name
"Master Mind") league standings table.

One trouble with full-blown ACLs as required by NSA's
Orange Book, is obscurity. It is hard (possibly NP-
complete) to analyze the actual security of an ACL

A common failing of Unix administration was a proliferation
of suid-root programs, e.g. mail(1). I recall one system
that had a hundred such programs. Sudo provided a way
station between suid and ACLs.


More information about the TUHS mailing list