Question about stack overflows in native code

[Thomas Stüfe]

Hi David,

On Mon, Apr 3, 2017 at 11:02 PM, David Holmes <david.holmes at oracle.com>
wrote:

> Just to follow up on what Fred responded ...
>
> On 4/04/2017 4:42 AM, Thomas Stüfe wrote:
>
>> Hi Fred,
>>
>> thanks! Some more questions inline.
>>
>> On Mon, Apr 3, 2017 at 8:29 PM, Frederic Parain <
>> frederic.parain at oracle.com>
>> wrote:
>>
>> When the yellow zone is hit and the thread state is not in
>>> _thread_in_java (which means thread state is _thread_in_native or
>>> _thread_in_vm), the yellow zone is silently disabled and the thread
>>> is allowed to resume its execution.
>>>
>>>
>>> Disabled by whom exactly?
>>
>> Normally, this would be done in the signal handler, but that requires
>> enough stack space to run. AFAIK jitted or interpreted code does stack
>> banging in order to trigger the yellow-page-segfault at a point where
>> there
>> are enough pages left on the stack to invoke the signal handler (n shadow
>> pages before), but that is not guaranteed to work with native C-compiled
>> code, no?
>>
>
> The stack banging is done to ensure the stackoverflow is hit before we
> start doing the actual operation. The size of the yellow and red zones are
> supposed to be sufficient to allow the respective signal processing and
> response to be executed.


And the size of the shadow pages should be sufficient to invoke initial
signal handler which will unprotect the yellow or red zone, right?
So, back to my original question, if native C code does not bang the stack
but simply runs into the yellow zone, process will simply die, or?


> But that assumes you simply advance into the guard zones - if your native
> code suddenly jumped to the end of the yellow zone for example, then signal
> processing would hit the red zone; similarly if you jump to the end of the
> red zone then signal processing will hit the OS guard page. If you jump
> past all guard pages you simply die.
>
>
Thank you!

See also my response to Fred. We wondered whether exporting a simple JNI
helper function to check the stack size on behalf of the native code would
be something helpful, for cooperative native code at least.

Kind Regards, Thomas


> David
>
>
> (not just a theory, we have a test case here where a stack overflow in
>> native code just silently kills the process.)
>>
>> I guess it may work accidentally if the C-compiled code itself does some
>> form of stack banging when establishing frames, in order to detect OS
>> stack
>> overflows? Very fuzzy here. But whatever the C-compiled code does, it has
>> no notion about how much space we need to invoke the signal handler and
>> handle stack overflows, no?
>>
>> When the red zone is hit, what ever the current thread state is,
>>
>>> the red zone is disabled and VMError::report_and_die() is called,
>>> which should generate a hs_err file unless the generation of the
>>> error file requires more memory than the red zone provides.
>>>
>>> Fred
>>>
>>>
>>> Thanks, Thomas
>>
>>
>>
>>
>>> On 04/03/2017 02:08 PM, Thomas Stüfe wrote:
>>>
>>> Hi,
>>>>
>>>> Today we wondered what would happen when a stack overflow occurs in
>>>> native
>>>> code running in a java thread (an attached thread or one created by the
>>>> VM).
>>>>
>>>> In that case yellow and red pages are in place, but this would not help
>>>> much, would it not, because the native code would not do any stack
>>>> banging?
>>>>
>>>> So, native code would hit the yellow page, and then there would probably
>>>> not be enough space left on the stack to invoke the signal handler. The
>>>> result would be immediate VM death - not even an hs-err file - is that
>>>> correct?
>>>>
>>>> Also, we would hit the our own yellow page, not the guard page the OS
>>>> may
>>>> or may not have established, so - on UNIX - this would show up as
>>>> "Segmentation Fault", not "Stack Overflow", or?
>>>>
>>>> Thank you,
>>>>
>>>> Thomas
>>>>
>>>>
>>>>