eighty-twenty

Recently on HN, rbanffy brought up a form of the old chestnut about crashing the image by simply executing true become: false.

It turns out it’s no longer true!

In the latest Squeak,

True := False.

doesn’t work – the compiler complains that you can’t assign into a read-only variable. So let’s try this:

Smalltalk at: #True put: False.

But now the metaprogramming system complains you’re trying to modify a read-only binding! So we view source on ClassBinding»value:, and see that a resumable exception is being used to guard the modification, so let’s explicitly signal that we really want to modify that binding:

[Smalltalk at: #True put: False]
  on: AttemptToWriteReadOnlyGlobal
  do: [:ex | ex resume: true].

Finally! Now, evaluating True yields False.

But the image keeps running! Use of the literal class True seems to be rare enough that things are OK for at least several minutes after the change.

Doing this, however, definitely should immediately torpedo things:

true become: false.

… huh. It didn’t work. It used to! Again, on this current Squeak version, we see a different behaviour. This time, it says Cannot execute #elementsExchangeIdentityWith: on read-only object #(false).

So we’ll have to try harder:

true becomeForward: false.

That doesn’t work either! Same error as for #become:.

Welp, I’m actually all out of ways to crash this modern image in analogous ways to the easy pitfalls of images of yesteryear…

P.S. Running the following command,

ln -sf /dev/zero /lib/x86_64-linux-gnu/libc.so.6

analogous to True := False, definitely does have an analogous effect on a similarly-“alive” Unix image :-) . I just tried it on a scratch VM; the results are pretty intimidating!