bug#34655: 26.1.92; Segfault in module with --module-assertions

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Thu, 21 Mar 2019 21:01:43 +0100
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, "Basil L. Contovounesios" <contovob@tcd.ie>, 34655@debbugs.gnu.org, 
> 	Daniel Colascione <dancol@dancol.org>
> 
> Am Do., 21. März 2019 um 20:50 Uhr schrieb Eli Zaretskii <eliz@gnu.org>:
> >
> > > From: Philipp Stephani <p.stephani2@gmail.com>
> > > Date: Thu, 21 Mar 2019 20:37:24 +0100
> > > Cc: Stefan Monnier <monnier@iro.umontreal.ca>, "Basil L. Contovounesios" <contovob@tcd.ie>, 34655@debbugs.gnu.org
> > >
> > > Let's go back to the known good state first, and then discuss how to
> > > go from there.
> >
> > I don't see why that is better than discuss first and then go to where
> > we decide to go.  It's not like Emacs 27 will be released any time
> > soon, so there's no rush.
> 
> For one, it becomes harder and harder to revert commits the older they
> get. Also such discussions tend to turn into endless debates about the
> "perfect" solution until one side gives up, without improving
> anything. I strongly prefer fixing actual bugs that affect users in
> practice and then discussing (or not discussing) the gold-plating
> steps later.

I also prefer fixing bugs (which is why I spent several hours looking
into Basil's crash, when no one else was replying to that bug report),
but this is a community project, so we should discuss first and act
later.  Especially when controversial issues are involved.

> > > We can't get stack marking to work, even theoretically.
> > >
> > > A module is free to do
> > >
> > > emacs_value x = ...;
> > > uintptr_t y = ((uintrptr_t) x) ^ 0x123456u;
> > > (garbage-collect)
> > > emacs_value z = (emacs_value) (y ^ 0x123456u);
> > > ... use z ...
> > >
> > > During the garbage collection, x isn't on the stack anywhere
> >
> > Why do you think x isn't on the stack in this case?
> 
> Because the compiler reused the stack slot for something else?

How can it?  You are using the same pointer later.  Garbage collection
cannot happen unless you call an Emacs function, such as Ffuncall.
Calling a function means that even if the pointer to a Lisp object was
in a register, it will be put on the stack when calling Emacs.

> Because the module is written in a language that doesn't use the stack
> in a way that the garbage collector expects?

Which language is that, and how can it use the emacs-module machinery?

> > Moreover, emacs_value is actually a pointer to a Lisp object, so this
> > object is also somewhere on the stack, right?

No answer?

> We do something very specific with the stack: we make sure that
> Lisp_Objects are never manipulated in a way similar to the above, and
> we use the C language.

If worse comes to worst, we can request module writers to adhere to
the same discipline.  We already request them to do/not to do quite a
few extraordinary things.

> All regression tests still pass after reverting the commit.

Didn't they also pass before?