[Tutor] (no subject)

Dave Angel davea at ieee.org
Sat Oct 30 13:08:25 CEST 2010

On 2:59 PM, Alan Gauld wrote:
> "Steven D'Aprano" <steve at pearwood.info> wrote
>> is actually being executed. There are very few programming tasks 
>> harder than trying to debug code that doesn't actually contain any 
>> bugs, or contains bugs different from the ones you are seeing, 
>> because the code you are actually executing is something different 
>> from what you think you are executing.
> To illustrate with a true story (back in the days when you had to build
> and maintain your own compilers!):
> Take a C compiler source code and modify it so it produces faulty
> executable code but does not crash or otherwise report an error.
> Compile the now faulty compiler source code with the old (ie working)
> compiler.
> Fix the source code bug.
> Use the new (now broken) compiler to compile the now perfect source
> code to produce a broken compiler with a slightly different defect.
> Now use the resulting compiler to recompile the 'perfect' source code.
> Now figure out why none of your executables work as expected.
> That took us nearly 2 weeks to figure out... :-(
> (And made us very thankful for source code version control!)
> Alan G.
That sounds remarkably parallel to a speech given by Ken Thompson:

though there the point was that a trojan could be inserted into a 
compiler (or other programming tool) that would survive repair of the 
source code.  There has been at least one viruse which has been credited 
to this approach.



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