RFR 8146984: SIGBUS: bool Method::has_method_vptr(const void*)+0xc

[Coleen Phillimore]

On 1/29/16 11:02 AM, Daniel D. Daugherty wrote:
> On 1/29/16 5:46 AM, Coleen Phillimore wrote:
>> Summary: Add address check and use SafeFetchN for Method* vptr access 
>> when Method* may be bad pointer.
>>
>> Tested with RBT and failing test case (reproduced 1 in 100 times) 
>> with fatal in the 'return's in the change to verify.
>>
>> open webrev at http://cr.openjdk.java.net/~coleenp/8146984/
>
> This one caught my eye because it has to do with sampling...

I should mention sampling in all my RFRs then!
>
> src/share/vm/oops/method.cpp
>     The old code checked "!is_metaspace_object()" and used
>     has_method_vptr((const void*)this).
>
>     The new code skips the "!is_metaspace_object()" check even after 
> sanity
>     checking the pointer, but you don't really explain why that's OK.

is_metaspace_object is a very expensive check.   It has to traverse all 
the metaspace mmap chunks.   The new code is more robust in that it 
sanity checks the pointer first but uses Safefetch to get the vptr.


>
>     The new code also picks up parts of Method::has_method_vptr() which
>     makes me wonder if that's the right place for the fix. Won't other
>     callers to Method::has_method_vptr() be subject to the same crashing
>     mode? Or was the crashing mode only due to the 
> "!is_metaspace_object()"
>     check...

I should have moved the SafeFetch in to the has_method_vptr.  I can't 
remember why I copied it now.  It crashed because the pointer was in 
metaspace (is_metaspace_object returned true) but wasn't aligned, but 
the pointer could come from anywhere.

Thanks,   I'll test out this fix and resend it.
Coleen

>
> Dan
>
>
>> bug link https://bugs.openjdk.java.net/browse/JDK-8146984
>>
>> Thanks,
>> Coleen
>>
>