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

[Kamil Rytarowski]

On 11.10.2018 23:20, Brad Smith wrote:
> On Thu, Oct 11, 2018 at 09:31:23PM +0200, Kamil Rytarowski wrote:
>> On 11.10.2018 16:25, Brad Smith wrote:
>>> 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 <address@hidden> 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?
>>>
>>
>> Why do we want it only on OpenBSD and its either unneeded or meaning
>> something else on other OSes.
>>
>> #ifdef __OpenBSD__
>> flags |= MAP_STACK;
>> #endif
> 
> How about the following?
> 

It looks good to me.

Reviewed-by: Kamil Rytarowski <address@hidden>

> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index fbd0dc8c57..7814e61114 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -596,6 +596,7 @@ pid_t qemu_fork(Error **errp)
>  void *qemu_alloc_stack(size_t *sz)
>  {
>      void *ptr, *guardpage;
> +    int flags;
>  #ifdef CONFIG_DEBUG_STACK_USAGE
>      void *ptr2;
>  #endif
> @@ -610,8 +611,15 @@ void *qemu_alloc_stack(size_t *sz)
>      /* allocate one extra page for the guard page */
>      *sz += pagesz;
>  
> -    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,
> -               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +    flags = MAP_PRIVATE | MAP_ANONYMOUS;
> +#if defined(MAP_STACK) && defined(__OpenBSD__)
> +    /* Only enable MAP_STACK on OpenBSD. Other OS's such
> +       as Linux/FreeBSD/NetBSD have a flag with the same
> +       name but have differing functionality. */
> +    flags |= MAP_STACK;
> +#endif
> +
> +    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);
>      if (ptr == MAP_FAILED) {
>          perror("failed to allocate memory for stack");
>          abort();
>

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