Mixing tied variables with goto label

Limbic~Region has asked for the wisdom of the Perl Monks concerning the following question:

All,
Every now and then I try to think of ideas for Obfuscated code. I seldom follow through, but it is often a great learning exercise. Consider the following code:

#!/usr/bin/perl
use strict;
use warnings;

sub jump {
    my $place = 'START';
    goto $place;
}
START:
print "Hello, world\n";
jump();
[download]

As you might expect, this creates an infinite loop. I wondered what might happen if $place were a tied variable that instead of fetching a value - did a goto elsewhere. Would the original goto ever get called?

#!/usr/bin/perl
use strict;
use warnings;

sub jump {
    tie my $place, 'main';
    goto $place;    
}
START:
print "Hello, world\n";
jump();
print "I am jumping over this spot\n";
END:
print "Goodbye, cruel world\n";

sub TIESCALAR { bless {}, $_[0] }
sub STORE     { print "Read only\n"; }
sub FETCH     { goto END; }
[download]

This dies yelling about Can't find label END at ... First, I know that Perl supports goto label out of a sub as demonstrated by the first snippet. Second, I can't find anything in goto docs that would suggest this shouldn't work. Can anyone shed some light?

Cheers - L~R

Comment on Mixing tied variables with goto label Select or Download Code

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Re: Mixing tied variables with goto label by dave_the_m (Monsignor) on Nov 16, 2005 at 00:27 UTC
goto only searches the immediate call stack for labels, ie where perl code has called a function. Functions that are called indirectly, like signal handlers, tie handlers, sort functions etc don't get searched. I don't know whether this is documented. Dave.	[reply]
Re^2: Mixing tied variables with goto label by Limbic~Region (Chancellor) on Nov 16, 2005 at 00:50 UTC
dave_the_m, I couldn't find it documented anywhere. So it is how the function is called that is causing the problem. I can still produce a fair amount of misdirection. I am not sure I have the time or energy to actually make the obfu, but I would be happy and impressed if someone did. Like most magic and misdirection, the most important part is the setup. You will need to hide the tie, sub TIESCALAR, and friends. This should be possible using evil eval. The FETCH needs to be special too. Instead of using an internal goto to produce the magic, it will return the current value in the STORE the first time, but the second time it will return a different label. `my $label; # magic setup happens here $label = 'START'; print "$label\n"; # prints START goto $label; exit; START: print "Hello, world\n"; END: print "Goodbye, cruel world\n";` [download] When it is run, the output will be unexpected: `START Goodbye, cruel world` [download] In any case, if you happen to find some documentation explaining this behavior I would appreciate it. Cheers - L~R	[reply] [d/l] [select]
Re^2: Mixing tied variables with goto label by Steve_p (Priest) on Nov 16, 2005 at 21:07 UTC
Where do you think would be a good place to document this? perlfunc doesn't seem like the right place, so perlsyn maybe?	[reply]
Re^3: Mixing tied variables with goto label by dave_the_m (Monsignor) on Nov 16, 2005 at 22:02 UTC
It should go in perlfunc, in the section on goto Dave.	[reply]
Re: Mixing tied variables with goto label by jeffa (Bishop) on Nov 15, 2005 at 21:32 UTC
Try this: `sub FETCH { 'END' }` [download] This is kinda neat. :) jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re^2: Mixing tied variables with goto label by Limbic~Region (Chancellor) on Nov 16, 2005 at 00:07 UTC
jeffa, Try this: Yeah, but there is nothing special about that behavior. `goto $var` treats $var as the label as I showed in my first example. What I am trying to do is a little misdirection. Imagine that I could make you forget that I had a tied variable and that storing a value didn't appear to do anything abnormal: `my $label = 'START'; goto $label; START: print "Hello, world\n"; END: print "Goodbye, cruel world\n";` [download] When you run the code, instead of going to START it actually goes to END. The trouble is that the label goes away inside the tied FETCH sub with no explanation I can find. Cheers - L~R Added a code example to make it clearer what is being attempted.	[reply] [d/l] [select]


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