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

From: Noah Lavine <[email-masked]>
Date: Tue, 11 Dec 2012 08:32:26 -0500

On Tue, Dec 11, 2012 at 4:42 AM, Ludovic Courtès <[email-masked]> wrote:
>
> 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.
>

Okay.

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

Yes, "easily". :-)

> 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.
>

I loaded a procedure on the stack, used the new-frame instruction, and then
the call instruction. When I switched the order of the first two things,
the problem went away. I must have been using uninitialized stack space.

Noah