RFR(XS): 8193308: Disallow installing user signal handlers for SIGBUS on OSX

[Roger Riggs]

Hi,

I can see allowing applications to register for synchronous signals 
(SIGBUS) in the case
where a handlers actions can affect the execution flow.

But for JVM_RegisterSignal, the signal is queued to a separate thread 
which is in turn
dispatched to yet another thread to notify the Java handler.

What is the behavior of the original thread?  If it was an instruction 
that caused SIGBUS,
does it get repeated?  Does the thread get killed, etc.

I can't see a handler for SIGBUS or SIGSEGV being make correct or useful 
to a java handler.
Maybe it is up to sun.misc.Signal to refuse to register those handlers, 
independent of
JVM_RegisterSignal.

Thanks, Roger


On 2/11/2018 8:36 AM, David Holmes wrote:
> On 11/02/2018 3:37 AM, mandy chung wrote:
>> On 2/10/18 3:24 AM, David Holmes wrote:
>>> On 10/02/2018 5:18 PM, mandy chung wrote:
>>>>>>
>>>>>> I think JVM_RegisterSignal should disallow installing a user 
>>>>>> handler for one of the implementation-specific signals for all 
>>>>>> platforms. That'll simplify your fix.  I would think the 
>>>>>> compatibility risk should be low as rare cases would want to 
>>>>>> intercept SIGBUS which is also clearly documented in [1].
>>>>>
>>>>> Each platform defines its own set of signals that are used by the 
>>>>> VM on that platform. While I've never thought it made any sense to 
>>>>> attempt to install user "handlers" for SIGBUS or SIGSEGV (not that 
>>>>> we allow the latter) others have argued that unless we use it then 
>>>>> we should allow it. Hence no "lowest common denominator" approach 
>>>>> - though that certainly would be simpler.
>>>>>
>>>>> In this case I believe it is simply another case where the OS X 
>>>>> port copied the Linux code and it wasn't realized that OS X would 
>>>>> generate SIGBUS when SIGSEGV is "expected" and so didn't list it 
>>>>> as excluded.
>>>>
>>>> Are we agreeing on disallowing SIGBUS for all platforms?
>>>
>>> No. If the VM doesn't use SIGBUS on a platform then it shouldn't 
>>> disallow it.
>>>
>>
>> JVM_RegisterSignal is the VM support for sun.misc.Signal.   As it 
>> does not make sense to install user handlers for SIGBUS, regardless 
>> whether the VM uses it or not, I suggest to disallow it from 
>> JVM_RegisterSignal for consistency.
>
> I've argued this in the past and was shot down. If the VM doesn't use 
> a signal then it's fair game for end-user use.
>
> The only thing broken here was OS X and that is now fixed.
>
> David
>
>> Mandy