[Caml-list] a bad value detector

[Frederic Perriot <fperriot@gmail.com>]

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.

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

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