Subject: Re: bug#13074: VM Segfaults with Bad `Call' Instruction

From: [email-masked] (Ludovic Courtès)
Date: Tue, 11 Dec 2012 10:42:09 +0100

Hi!

Noah Lavine <[email-masked]> skribis:

> In general, I do think there should at least be an option for having full
> error-checking in the VM. It would have been much, much harder for me to
> find this without having patched the VM, because it would have taken me a
> very long time to try each new thing I tried, because I would have had to
> restart Guile. I am happy for it not to be on the regular code-path,
> though. I also realize that writing a compiler is an unusual application,
> so maybe it should even be a compile-time option for users who prefer their
> Guile slow. How does that sound?

The VM does full error checking.  But there’s a difference between
checking whether an object has the expected type, and checking whether
an object is a well-formed ‘SCM’ object (and NULL is not a valid ‘SCM’
object.)

Guile never does the latter, and as a rule of thumb I would keep things
this way.

The brave hacker working on a compiler can easily figure out what how to
debug all sorts of crazy things.  :-)

So I’m closing it for now.

Thanks,
Ludo’.

PS: It’s still unclear to me how you ended up forging an invalid SCM
    object.  I think you either have to generate invalid bytecode, or to
    use (pointer->scm %null-pointer), or variants thereof.