Re: [Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD

[Brad Smith <brad@comstyle.com>]

On 10/11/2018 5:41 AM, Kamil Rytarowski wrote:

> On 11.10.2018 11:36, Peter Maydell wrote:
>> On 11 October 2018 at 00:55, Brad Smith <brad@comstyle.com> wrote:
>>> And from FreeBSD...
>>>
>>>       MAP_STACK MAP_STACK implies MAP_ANON, and offset of 0.  The fd
>>> argument must be -1 and prot must include at least
>>> PROT_READ and PROT_WRITE.
>>>
>>> This option creates a memory region that grows to at
>>> most len bytes in size, starting from the stack top
>>> and growing down.  The stack top is the starting
>>> address returned by the call, plus len bytes.  The
>>> bottom of the stack at maximum growth is the starting
>>> address returned by the call.
>>>
>>> Stacks created with MAP_STACK automatically grow.
>>> Guards prevent inadvertent use of the regions into
>>> which those stacks can grow without requiring mapping
>>> the whole stack in advance.
>> Hmm. That "automatically growing" part sounds like
>> behaviour we definitely do not want for our use case.
>> So we're going to need to make this OS-specific :-(
>>
> I propose to restrict MAP_STACK it to OpenBSD (with a comment in the
> code). Once it will be needed by someone else will be able to enable it
> for other OSes.

I was going to propose doing something like that but you had replied 
before I did.
What sort of comment did you have in mind?