RFR: JDK-8147388: Add diagnostic commands to attach JVMTI agent.
Yasumasa Suenaga
yasuenag at gmail.com
Mon Jan 18 23:13:45 UTC 2016
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 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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