ugh.book

“It Can’t Be a Bug, My Makefile Depends on It!” 193

bookkeeping information that the C runtime system stores on the stack,
such as addresses for returning from subroutine calls. At best, corrupting
this information will probably cause a program to crash.

We say “probably” because you can corrupt the runtime stack to achieve an
effect that the original programmer never intended. Imagine that our func-
tion was called upon to read a really long line, over 2,000 characters, and
that this line was set up to overwrite the bookkeeping information on the
call stack so that when the C function returns, it will call a piece of code
that was also embedded in the 2,000 character line. This embedded piece of
code may do something truly useful, like exec a shell that can run com-
mands on the machine.

Robert T. Morris’s Unix Worm employed exactly this mechanism (among
others) to gain access to Unix computers. Why anyone would want to do
that remains a mystery.

Date: Thu, 2 May 91 18:16:44 PDT

From: Jim McDonald <jlm%[email protected]>

To: UNIX-HATERS

Subject: how many fingers on your hands?

Sad to say, this was part of a message to my manager today:

The bug was that a program used to update Makefiles had a

pointer that stepped past the array it was supposed to index and

scribbled onto some data structures used to compute the

dependency lists it was auto-magically writing into a Makefile.

The net result was that later on the corrupted Makefile didn’t

compile everything it should, so necessary .o files weren’t

being written, so the build eventually died. One full day wasted

because some idiot thought 10 includes was the most anyone

would ever use, and then dangerously optimized code that was

going to run for less than a millisecond in the process of

creating X Makefiles!

The disadvantage of working over networks is that you can’t so eas-

ily go into someone else's office and rip their bloody heart out.

Exceptional Conditions

The main challenge of writing robust software is gracefully handling errors
and other exceptions. Unfortunately, C provides almost no support for han-
dling exceptional conditions. As a result, few people learning program-
ming in today’s schools and universities know what exceptions are.