[PATCH v4 1/5] support: Add support_stack_alloc

[Florian Weimer]

* Adhemerval Zanella:

> On 10/03/2021 13:10, Florian Weimer wrote:
>> * Adhemerval Zanella via Libc-alpha:
>> 
>>> +  /* Use MAP_NORESERVE so that RAM will not be wasted on the guard
>>> +     bands; touch all the pages of the actual stack before returning,
>>> +     so we know they are allocated.  */
>>> +  void *alloc_base = xmmap (0,
>>> +                            alloc_size,
>>> +                            PROT_READ|PROT_WRITE,
>>> +                            MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|MAP_STACK,
>>> +                            -1);
>>> +  xmprotect (alloc_base, guardsize, PROT_NONE);
>>> +  xmprotect (alloc_base + guardsize + stacksize, guardsize, PROT_NONE);
>> 
>> The usual pattern is to map with PROT_NONE and then use
>> PROT_READ|PROT_WRITE with mprotect.
>> 
>> Rest looks okay, thanks.
>> 
>> Florian
>> 
>
> Ok, I will fix it and push upstream.

Just to be clear, it avoids the need for MAP_NORESERVE.

Thanks,
Florian