Re: [Caml-list] a bad value detector

["Perry E. Metzger" <perry@piermont.com>]

On Wed, 2 May 2018 18:19:39 +0200 Frederic Perriot
<fperriot@gmail.com> wrote:
> Hello caml-list,
> 
> I'd like to propose a detector to help in the detection of
> incorrect C bindings that do not follow the GC rules.

Once, I built a precise garbage collector for a language I'd
implemented in C. It was a simple Cheney collector with two
semispaces. I munmapped the unused semispace after each gc. In order
to debug C code that didn't properly register or protect GC roots, I
ran tests during every code change where I forced a collection after
every alloc -- this caused an almost immediate segfault in any C code
that was still pointing at the wrong semispace after the collection
because it would try to touch unmapped pages.

Generally I think tricks like this are a good idea. They allow one to
rapidly find code that doesn't correctly follow the rules at a fairly
low price. The only thing is you can only run such things while
testing because otherwise the performance hit is too high.

Perry

> 
> The idea is rather simple:
> 
> 1. after a minor collection, mprotect the pages of the minor heap to
> disallow reads and writes
> 2. install a SEGV handler to catch the ensuing faults
> 3. if the faulting address is above caml_young_ptr -
> Max_young_whsize, unprotect the page and carry on
> 4. otherwise, the program has no business accessing a value in the
> unallocated part of the minor heap, so let it crash
> 
> I've hacked up a prototype that protects a single page at
> caml_young_start, and it catches the bug I mention in my other
> message entitled "an implicit GC rule".
> 
> Such a change surely degrades performance, but maybe it would be
> useful as a runtime option available through CAMLRUNPARAM, to detect
> misbehaved C bindings.
> 
> Does it sound like a viable technique?
> 
> I'm curious to hear what you think.
> 
> thanks,
> Frédéric Perriot
> 



-- 
Perry E. Metzger		perry@piermont.com

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