RFR: JDK-8147388: Add diagnostic commands to attach JVMTI agent.

David Holmes david.holmes at oracle.com
Tue Jan 19 01:25:11 UTC 2016 

On 19/01/2016 9:13 AM, Yasumasa Suenaga wrote:
> Hi David, Dmitry,
>
>> This reports they are not available in Visual Studio C++:
>>
>> https://msdn.microsoft.com/en-us/library/zb1574zs.aspx
>
> Thanks David,
> Should I use malloc/free at this patch?

I think so - but you need to check for a NULL from malloc and have some 
means of error propagation.

David
-----

> (I think that we should NOT use VLA because VS2015 does not support it.)
>
>
> Thanks,
>
> Yasumasa
>
>
> On 2016/01/19 8:06, David Holmes wrote:
>> On 19/01/2016 7:26 AM, Dmitry Samersoff wrote:
>>> David,
>>>
>>> On 2016-01-18 23:47, David Holmes wrote:
>>>> On 18/01/2016 11:20 PM, Dmitry Samersoff wrote:
>>>>> Yasumasa,
>>>>>
>>>>>> Can we use VLA (Variable Length Arrays) ?
>>>>>
>>>>> Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
>>>>> Target: x86_64-apple-darwin14.5.0
>>>>> Thread model: posix
>>>>>
>>>>> Compiles it just fine.
>>>>
>>>> Are we using variable length arrays anywhere else in the VM yet?
>>>
>>> Probably not.
>>>
>>> But personally, I see no reason to don't use it for simple cases like
>>> this one.
>>
>> Anytime we start to use a language feature for the first time we need
>> to be extra diligent to make sure there are no unintended side-effects
>> and that all our supported compilers (and probably a few others used
>> in the community) do the right thing. A bit of googling seems to
>> indicate that variable length arrays are part of C99 but are not
>> allowed in C++ - though gcc has an extension that does allow them.
>>
>> This reports they are not available in Visual Studio C++:
>>
>> https://msdn.microsoft.com/en-us/library/zb1574zs.aspx
>>
>> David
>> -----
>>
>>>> What
>>>> are the implications for allocation and in particular allocation
>>>> failure?
>>>
>>> This allocation just reserves some space on the stack[1], so it can
>>> cause stack overflow if we attempt to allocate two much bytes.
>>
>>
>>
>>> 1. Listing fragment (extra labels are removed)
>>>
>>>     3                    .Ltext0:
>>>     5                    .LC0:
>>>
>>>    14:test.cxx      **** void testme(int n) {
>>>    15:test.cxx      ****   char m[n];
>>>    25 0000 4863FF                movslq  %edi, %rdi
>>>    28 0003 55                    pushq   %rbp
>>>    37 0004 BE000000              movl    $.LC0, %esi
>>>    41 0009 4883C70F              addq    $15, %rdi
>>>    46 000d 31C0                  xorl    %eax, %eax
>>>    50 000f 4883E7F0              andq    $-16, %rdi
>>>    54 0013 4889E5                movq    %rsp, %rbp
>>>    59 0016 4829FC                subq    %rdi, %rsp
>>>    64 0019 BF010000              movl    $1, %edi
>>>    65 001e 4889E2                movq    %rsp, %rdx
>>>    66 0021 E8000000              call    __printf_chk
>>>    16:test.cxx      ****   printf("%s", m);
>>>    17:test.cxx      **** }
>>>
>>> -Dmitry
>>>
>>>
>>>>
>>>> David
>>>> -----
>>>>
>>>>> -Dmitry
>>>>>
>>>>> On 2016-01-18 16:09, Yasumasa Suenaga wrote:
>>>>>> Hi Dmitry,
>>>>>>
>>>>>>> 1. It might be better to have one jcmd to run both java and
>>>>>>> native java
>>>>>>> agent. If agent library name ends with ".jar" we can assume it's
>>>>>>> java
>>>>>>> agent.
>>>>>>
>>>>>> Okay, I'll try it.
>>>>>>
>>>>>>> if (_libpath.value() == NULL) {
>>>>>>>       error ...
>>>>>>> }
>>>>>>
>>>>>> I will add it.
>>>>>> However, I note you that _libpath is given mandatory flag.
>>>>>> http://mail.openjdk.java.net/pipermail/serviceability-dev/2016-January/018661.html
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>> char options[option_len];
>>>>>>
>>>>>> Can we use VLA (Variable Length Arrays) ?
>>>>>> I'm worried that several C++ compiler cannot compile this code.
>>>>>> http://clang.llvm.org/compatibility.html#vla
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Yasumasa
>>>>>>
>>>>>>
>>>>>> On 2016/01/18 19:38, Dmitry Samersoff wrote:
>>>>>>> Yasumasa,
>>>>>>>
>>>>>>> 1. It might be better to have one jcmd to run both java and
>>>>>>> native java
>>>>>>> agent. If agent library name ends with ".jar" we can assume it's
>>>>>>> java
>>>>>>> agent.
>>>>>>>
>>>>>>> 2. Please get rid of malloc/free and check _libpath.value() for
>>>>>>> NULL at
>>>>>>> ll. 295 and below.
>>>>>>>
>>>>>>>
>>>>>>> if (_libpath.value() == NULL) {
>>>>>>>       error ...
>>>>>>> }
>>>>>>>
>>>>>>> if (_option.value() == NULL) {
>>>>>>>       JvmtiExport::load_agent_library("instrument",
>>>>>>>                    "false", _libpath.value(), output());
>>>>>>>      return;
>>>>>>> }
>>>>>>>
>>>>>>> size_t option_len = \
>>>>>>>       strlen(_libpath.value()) + strlen(_option.value()) + 1;
>>>>>>>
>>>>>>> char options[option_len];
>>>>>>>
>>>>>>> ....
>>>>>>>
>>>>>>> -Dmitry
>>>>>>>
>>>>>>>
>>>>>>> On 2016-01-15 16:33, Yasumasa Suenaga wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> I added permissions and tests in new webrev:
>>>>>>>>      http://cr.openjdk.java.net/~ysuenaga/JDK-8147388/webrev.01/
>>>>>>>>
>>>>>>>> Two tests (LoadAgentDcmdTest, LoadJavaAgentDcmdTest) work fine.
>>>>>>>>
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Yasumasa
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2016/01/15 17:20, Staffan Larsen wrote:
>>>>>>>>> This is a good improvement overall.
>>>>>>>>>
>>>>>>>>> The new diagnostic commands need to have proper permissions set:
>>>>>>>>>
>>>>>>>>>        static const JavaPermission permission() {
>>>>>>>>>           JavaPermission p =
>>>>>>>>> {"java.lang.management.ManagementPermission",
>>>>>>>>>                              “control", NULL};
>>>>>>>>>          return p;
>>>>>>>>>       }
>>>>>>>>>
>>>>>>>>> And as David said: tests! See
>>>>>>>>> hotspot/test/serviceability/dcmd/jvmti.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> /Staffan
>>>>>>>>>
>>>>>>>>>> On 14 jan. 2016, at 15:00, Yasumasa Suenaga <yasuenag at gmail.com>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>> Hi all,
>>>>>>>>>>
>>>>>>>>>> We can use Attach API to attach JVMTI agent to live process.
>>>>>>>>>> However, we have to write Java code for it.
>>>>>>>>>>
>>>>>>>>>> If we can attach JVMTI agents through jcmd, it is very useful.
>>>>>>>>>> So I want to add two new diagnostic commands:
>>>>>>>>>>
>>>>>>>>>> * JVMTI.agent_load: Load JVMTI native agent.
>>>>>>>>>> * JVMTI.javaagent_load: Load JVMTI java agent.
>>>>>>>>>>
>>>>>>>>>> I maintain two JVMTI agents - HeapStats [1] and JLivePatcher [2].
>>>>>>>>>> [1] is native agent, [2] is java agent.
>>>>>>>>>> They provide a program for attaching to live process.
>>>>>>>>>>
>>>>>>>>>> I guess that various JVMTI agents provide own attach mechanism
>>>>>>>>>> like
>>>>>>>>>> them.
>>>>>>>>>> I think that we should provide general way to attach.
>>>>>>>>>>
>>>>>>>>>> I've uploaded webrev. Could you review it?
>>>>>>>>>>      http://cr.openjdk.java.net/~ysuenaga/JDK-8147388/webrev.00/
>>>>>>>>>>
>>>>>>>>>> I'm jdk9 committer, however I cannot access JPRT.
>>>>>>>>>> So I need a sponsor.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>>
>>>>>>>>>> Yasumasa
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> [1] http://icedtea.classpath.org/wiki/HeapStats
>>>>>>>>>> [2] https://github.com/YaSuenag/jlivepatcher  (in Japanese)
>>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>
>>>>>
>>>
>>>

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