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<p>I like this anecdote because it points out the difference between
being able to handle and process bizarre conditions, as if they
were something that should work, which is maybe not that helpful,
vs. detecting them and doing something reasonable, like failiing
with a "limit exceeded" message. A silent, insidious failure down
the line because a limit was exceeded is never good. If "fuzz
testing" helps exercise limits and identifies places where
software hasn't realized it has exceeded its limits, has run off
the end of a table, etc., that seems like a good thing to me.<br>
</p>
<div class="moz-cite-prefix">On 05/21/2024 09:59 AM, Paul Winalski
wrote:<br>
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cite="mid:CABH=_VR9TEnPLtjexUKtpkfG-81bg=g1X2+0v7upN=f-sEkA4A@mail.gmail.com"
type="cite">
<div dir="ltr">On Tue, May 21, 2024 at 12:09 AM Serissa <<a
moz-do-not-send="true" href="mailto:stewart@serissa.com"
target="_blank">stewart@serissa.com</a>> wrote:
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<div dir="ltr">Well this is obviously a hot button
topic. AFAIK I was nearby when fuzz-testing for
software was invented. I was the main advocate for
hiring Andy Payne into the Digital Cambridge
Research Lab. One of his little projects was a
thing that generated random but correct C programs
and fed them to different compilers or compilers
with different switches to see if they crashed or
generated incorrect results. Overnight, his tester
filed 300 or so bug reports against the Digital C
compiler. This was met with substantial pushback,
but it was a mostly an issue that many of the
reports traced to the same underlying bugs.<br>
<div><br>
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Bill McKeemon expanded the technique and published
"Differential Testing of Software" <a
moz-do-not-send="true"
href="https://www.cs.swarthmore.edu/%7Ebylvisa1/cs97/f13/Papers/DifferentialTestingForSoftware.pdf"
target="_blank">https://www.cs.swarthmore.edu/~bylvisa1/cs97/f13/Papers/DifferentialTestingForSoftware.pdf</a><br>
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<div>In the mid-late 1980s Bill Mckeeman worked with DEC's
compiler product teams to introduce fuzz testing into our
testing process. As with the C compiler work at DEC
Cambridge, fuzz testing for other compilers (Fortran, PL/I)
also found large numbers of bugs.</div>
<div><br>
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<div>The pushback from the compiler folks was mainly a matter
of priorities. Fuzz testing is very adept at finding edge
conditions, but most failing fuzz tests have syntax that no
human programmer would ever write. As a compiler engineer
you have limited time to devote to bug testing. Do you
spend that time addressing real customer issues that have
been reported or do you spend it fixing problems with code
that no human being would ever write? To take an example
that really happened, a fuzz test consisting of 100 nested
parentheses caused an overflow in a parser table (it could
only handle 50 nested parens). Is that worth fixing?<br>
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<div><br>
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<div>As you pointed out, fuzz test failures tend to occur in
clusters and many of the failures eventually are traced to
the same underlying bug. Which leads to the
counter-argument to the pushback. The fuzz tests are
finding real underlying bugs. Why not fix them before a
customer runs into them? That very thing did happen several
times. A customer-reported bug was fixed and suddenly
several of the fuzz test problems that had been reported
went away. Another consideration is that, even back in the
1980s, humans weren't the only ones writing programs. There
were programs writing programs and they sometimes produced
bizarre (but syntactically correct) code.<br>
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<div>-Paul W.<br>
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