Re: [Caml-list] Native code stack overflow detection guarantees - followup

["Mark Shinwell" <mshinwell@janestreet.com>]

On Fri, Jul 09, 2010 at 09:39:06AM +0100, Mark Shinwell wrote:
> On Thu, Jul 08, 2010 at 08:59:30PM -0400, Michael Ekstrand wrote:
> > Therefore, I am wondering: are there documented guarantees on which the
> > native code stack overflow behavior rests?  Linux processes usually receive
> > a segmentation fault when they run out of stack space; is that guaranteed,
> > or is it simply the usual convenient behavior?  What about for other
> > systems?
> 
> I am not an expert on all the various cases which might arise during a case
> of stack overflow, but I believe on Linux it is exposed to userland as a
> segmentation fault.  Exactly how this is exposed shouldn't be any different
> when executing Caml native-compiled code or C code, for example.  In terms of
> Caml-specific behaviour (and assuming that the user's code does not itself
> alter the signal handling behaviour) then the runtime should catch the stack
> overflow via the SIGSEGV handler.  The intuition behind what is supposed to
> happen next is as follows: if the faulting address was in the stack, and the
> program counter (PC) was "in your Caml program", then we produce a
> Stack_overflow exception.  Otherwise we will just invoke the default signal
> action for the segmentation fault, which on Linux will terminate the program.
> 
> (There are lots of functions in the runtime which could cause a stack
> overflow in the case of a bug in their own code; and it would probably be bad
> if those ended up with a Stack_overflow exception rather than a segfault.
> This seems to me to be at least one reason why you probably don't want to
> turn every segfault in the stack into a Stack_overflow exception; instead, we
> try to distinguish based on the PC.)

I should add that what I wrote was for Linux/x86.  On other platforms the
behaviour may differ depending on what system support is available.
You need HAS_STACK_OVERFLOW_DETECTION (see the Caml configure script) set
to get any of this at all; and to have the distinguishing based on the
program counter location, you need CONTEXT_PC to have been defined
(see asmrun/signals_osdep.h in the Caml source).

Mark