Re: Use PROT_NONE for unused virtual stack memory

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

On 2016-10-12 23:11, Bill Allombert wrote:
> >   static void *
> >   pari_mainstack_malloc(size_t size)
> >   {
> >     void *b = mmap(NULL, size, PROT_READ|PROT_WRITE,
> > -                             MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE,-1,0);
> > +                             MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
> >     return (b == MAP_FAILED) ? NULL: b;
> >   }
>
> Is it correct ? It seems to me that we want to use PROT_NONE here as
> well, as you do in config/has_mmap.c.

Well, in some sense both PROT_READ|PROT_WRITE as well as PROT_NONE are 
correct. It just depends what you want.

I chose PROT_READ|PROT_WRITE here to check that it is possible to 
allocate memory up to size parisizemax. With vm.overcommit=2, this 
limits parisizemax to the amount of memory that is available at the time 
of setting parisizemax. I think it's better to fail the allocation early 
(when setting parisizemax) instead of when actually using the memory.