RFC: 8139864: Improve handling of stack protection zones.

[Dean Long]

What if we defined the units of StackRedPages, StackYellowPages and 
StackShadowPages to be 4K,
instead of the platform-specific page size determined at runtime?

dl

On 10/28/2015 2:01 PM, Coleen Phillimore wrote:
>
>
> On 10/19/15 3:35 AM, Lindenmaier, Goetz wrote:
>>
>> Hi,
>>
>> I opened this issue because I think the lower bounds for StackRedPages,
>>
>> StackYellowPages and StackShadowPages introduced in 8078556 cause
>>
>> problems on systems with page sizes > 8K.
>>
>> For a more detailed description of the problems to expect see
>>
>> https://bugs.openjdk.java.net/browse/JDK-8139864.
>>
>> I see a row of possible solutions to this.
>>
>> 1.) Just set the lower bounds to '1'.  That's what I proposed before.
>>
>>     I can imagine setups where one wants to reduce the size of these 
>> zones to a minimum.
>>
>>     If an application starts a lot of threads, and is known not to 
>> overflow stacks, this
>>
>>     might be desirable to save memory.
>>
>
> Yes, I think 1 would be an acceptable minimum.  It doesn't matter for 
> yellow and red pages, but for shadow pages, setting 1 may cause 
> customers to get stack overflow SEGVs with normal page sizes but so 
> would setting a higher value (like 3).   There really isn't a good 
> minimum so I think 1 might be something a customer might want to set 
> so we shouldn't disallow it.
>
>
>> 2.) Implement constraint methods that assure the zones never get 
>> smaller than DEFAULT_STACK_*_PAGES * default_page_size.
>>
>>     This is necessary if you want to make sure the zones never can 
>> get smaller than that.
>>
>> 3.) Introduce new flags StackRedZoneSize, StackYellowZoneSize and 
>> StackShadowZoneSize.
>>
>>     Set the default values to those of StackRedPages *4K etc, which 
>> is the most common page size I guess.
>>
>>     During startup, round these to page size.
>>
>>     Use Stack*ZoneSize in the code.
>>
>>     If Stack*Pages flags are set on the command line, convert them to 
>> Stack*ZoneSize by multiplying with the
>>
>>     current page size.
>>
>
> People have suggested adding these parameters, but they seem like more 
> work for us.   Plus rounding to page size, you might as well specify 
> the value in pages.   And adding parameters gives us more things to 
> test and maintain.
>
> Were you going to make this change?
>
> Thanks,
> Coleen
>
>
>> I'm happy to make a webrev for the favored solution.
>>
>> Best regards,
>>
>>   Goetz
>>
>