in reply to Re: Re: flattening a list-of-lists
in thread flattening a list-of-lists

I wanted to make a point. Show what Perl does when you feed it code, by writing code that writes code and feeds it back to Perl, as if it were written by you :) . Of course, the obvious solution is to just take away recursion from broquaint's version: 

sub flatten { map { ref $_ eq 'ARRAY' ? @$_ : $_ } @_ }

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This is an extremely scary solution. So scary that I can only assume you are joking. Please dont suggest things like this without a security disclaimer. A fresh newbie might think its a good plan then end up having their hard drive deleted or other nasty tings.

This sounds freaky. I'm curious, can you post an exploit against flatten_stringeval?. The only thing interpolated is $_, which iterates through (0..$#_). Everything else is quoted. Also, it checks for hard array ref before putting @{...}, so one can't play games with symbolic references (one doesn't need string eval for that, just no strict 'refs'). Maybe I'm missing something...

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Re: Re: Re: Re: flattening a list-of-lists
by demerphq (Chancellor) on Nov 21, 2003 at 09:06 UTC

    This sounds freaky. I'm curious, can you post an exploit against flatten_stringeval?.

    No. Your code actually is safe. (I have to admit it took some time for me to convince myself of this.) However the point I make is valid. Using eval for things like this is an extremely dangerous thing. A very subtle error could open an exploit. (Have a hunt around the archives, merlyn has stung people a few times this way, as have others.)

    As you point out here, everything is quoted correctly, and you arent actually interpolating values in the eval. However only slight changes to your code, an omitted backslash, etc could cause huge problems. If you had said "Note particularly that we dont allow interpolation of raw data to avoid potential security problems" or something to that effect I wouldnt have said a thing. Sometimes its not necessarily sufficient to post something that is correct. If its dangerous but correct you should IMO explain how to do it safely. (And why you are posting it.)

    However I do feel I owe you an apology. This is the second time in a short time that I have misread one of your posts. I promise ill read your nodes more closely, but do you think you could be a little clearer? Even realizing that you have in fact avoided the eval trap I would have replied pointing out how thin the ice is in this area. (As you didnt.)

    Anyway, cheers. :-)


    ---
    demerphq

      First they ignore you, then they laugh at you, then they fight you, then you win.
      -- Gandhi

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