[11] RFR(XS): MallocSiteTable::initialize() doesn't take function descriptors into account

[Volker Simonis]

Thank you Zhengyu!

On Tue, Jul 3, 2018 at 2:38 PM, Zhengyu Gu <zgu at redhat.com> wrote:
> BTW, patch looks good as short term fix.
>
> Thanks,
>
> -Zhengyu
>
>
> On 07/03/2018 07:47 AM, Zhengyu Gu wrote:
>>
>>
>>
>> On 07/03/2018 03:03 AM, Volker Simonis wrote:
>>>
>>> On Tue, Jul 3, 2018 at 1:09 AM, Zhengyu Gu <zgu at redhat.com> wrote:
>>>>
>>>> Hi Volker,
>>>>
>>>> fp[0] should work with little endian as well, right?
>>>>
>>>
>>> Yes, of course. But little endian doesn't use function descriptors. On
>>> little endian the new ABI ELFv2 version is used which uses plain
>>> function pointers like on x86.
>>
>>
>> Ah, thanks for explaining.
>>
>>>
>>>> I wonder if change
>>>>
>>>> pc[2] = (address)MallocSiteTable::allocation_at;
>>>>
>>>> to
>>>>
>>>> pc[2] = (address)(&MallocSiteTable::allocation_at);
>>>>
>>>> will work.
>>>>
>>>
>>> No it won't! You really have to manually decompose the function
>>> descriptor. You also have to be careful when trying things in gdb
>>> because inside gdb you may manage to cast the function descriptor to
>>> the corresponding function pointer. But the C/C++ compiler only gives
>>> you access to the function descriptor.
>>
>>
>>
>> I agree with Thomas, ABI issues should be addressed at os level. The
>> comments in elfFuncDescTable.hpp mentions "and other architectures like for
>> example IA64 ...", not sure if "other" will pop up at some point.
>>
>> Thanks,
>>
>> -Zhengyu
>>
>>
>>>
>>> Regards,
>>> Volker
>>>
>>>> Thanks,
>>>>
>>>> -Zhengyu
>>>>
>>>>
>>>> On 07/02/2018 01:27 PM, Volker Simonis wrote:
>>>>>
>>>>>
>>>>> Hi,
>>>>>
>>>>> can I please have a review for this trivial change which fixes a
>>>>> problem on Linux/ppc64
>>>>>
>>>>> http://cr.openjdk.java.net/~simonis/webrevs/2018/8206173/
>>>>> https://bugs.openjdk.java.net/browse/JDK-8206173
>>>>>
>>>>> MallocSiteTable::initialize() creates a pseudo call stack for
>>>>> hashtable entry allocations which is populated with the addresses of
>>>>> some well-known static methods.
>>>>>
>>>>> However, taking the address of a function or static method in C/C++
>>>>> isn't required to return the actual code address where that function
>>>>> lives in memory. On ppc64 (i.e. big-endian, ABI ELFv1) applying the
>>>>> address operator on a function will return the functions "function
>>>>> descriptor" (see
>>>>>
>>>>>
>>>>> http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#FUNC-DES)
>>>>> which is a struct of three pointers where the first one points to the
>>>>> actual function address.
>>>>>
>>>>> In order to get a correct stack trace on ppc64 as well, we therefore
>>>>> have to dereference the function descriptor before we can store the
>>>>> real function address in the stack trace.
>>>>>
>>>>> In order to make the shared code fix less intrusive, I've added a pair
>>>>> of LITTLE_ENDIAN_ONLY/BIG_ENDIAN_ONLY macros to macros.hpp.
>>>>>
>>>>> This bug is the reason why runtime/ElfDecoder/TestElfDirectRead.java
>>>>> fails on Linux/ppc64.
>>>>>
>>>>> Thank you and best regards,
>>>>> Volker
>>>>>
>>>>
>