RFR(S) 8212200 assert when shared java.lang.Object is redefined by JVMTI agent

[Ioi Lam]

On 10/22/18 10:25 AM, Lois Foltan wrote:
> On 10/22/2018 1:49 AM, Ioi Lam wrote:
>
>> Hi David,
>>
>> Thanks for the review. Updated webrev:
>>
>> http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v04/ 
>>
>> http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v04.delta/ 
>>
>
> Hi Ioi,
>
> Looks good.  A couple of comments:
>
> classfile/systemDictionary.cpp
> - line#1975 - My preference is to declare the variable sid outside the 
> for statement and compare sid == 0 within the for loop conditional.
Do you mean this?

bool SystemDictionary::is_well_known_klass(Symbol* class_name) {
   int sid;
   for (int i = 0; (sid = wk_init_info[i]) != 0; i++) {
     Symbol* symbol = vmSymbols::symbol_at((vmSymbols::SID)sid);
     if (class_name == symbol) {
       return true;
     }
   }
   return false;
}

> - line#1981 - can you use class_name->fast_compare(symbol) for the 
> equality check?
>
The comments around fast_compare says it's for vtable sorting only.

> memory/heapShared.cpp
> - line#422 could be a ResourceMark rm(THREAD);
>
Fixed.

Thanks
- Ioi

> Thanks,
> Lois
>
>>
>> More comments below:
>>
>>
>>
>> On 10/21/18 6:57 PM, David Holmes wrote:
>>> Hi Ioi,
>>>
>>> Generally seems okay.
>>>
>>> On 22/10/2018 11:15 AM, Ioi Lam wrote:
>>>> Re-sending to the correct mailing lists. Please disregard the other 
>>>> email.
>>>>
>>>> http://cr.openjdk.java.net/~iklam/jdk12/8212200-cds-jvmti-clfh-critical-classes.v03/ 
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8212200
>>>>
>>>> Hi,
>>>>
>>>> CDS has various built-in assumptions that classes loaded by
>>>> SystemDictionary::resolve_well_known_classes must not be replaced
>>>> by JVMTI ClassFileLoadHook during run time, including
>>>>
>>>> - field offsets computed in JavaClasses::compute_offsets
>>>> - the layout of the strings objects in the shared strings table
>>>>
>>>> The "well-known" classes can be replaced by ClassFileLoadHook only
>>>> when JvmtiExport::early_class_hook_env() is true. Therefore, the
>>>> fix is to disable CDS under this condition.
>>>
>>> I'm a little unclear why we have to iterate JvmtiEnv list when this 
>>> has to be checked during JVMTI_PHASE_PRIMORDIAL?
>>>
>> I think you are asking about this new function? I don't like the name 
>> "early_class_hook_env()". Maybe I should change it to 
>> "has_early_class_hook_env()"?
>>
>>
>> bool JvmtiExport::early_class_hook_env() {
>>   JvmtiEnvIterator it;
>>   for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
>>     if (env->early_class_hook_env()) {
>>       return true;
>>     }
>>   }
>>   return false;
>> }
>>
>> This function matches condition in the existing code that would call 
>> into ClassFileLoadHook:
>>
>> class JvmtiClassFileLoadHookPoster {
>>  ...
>>   void post_all_envs() {
>>     JvmtiEnvIterator it;
>>     for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
>>         ..
>>         post_to_env(env, true);
>>     }
>>   }
>> ...
>>   void post_to_env(JvmtiEnv* env, bool caching_needed) {
>>     if (env->phase() == JVMTI_PHASE_PRIMORDIAL && 
>> !env->early_class_hook_env()) {
>>       return;
>>     }
>>
>>
>> post_all_envs() is called just before a class is about to be loaded 
>> in the JVM. So if *any* env->early_class_hook_env() returns true, 
>> there's a chance that it will replace a well-known class.So, as a 
>> preventive measure, CDS will be disabled.
>>
>>
>>
>>>> I have added a few test cases to try to replace shared classes,
>>>> including well-known classes and other classes. See
>>>> comments in ReplaceCriticalClasses.java for details.
>>>>
>>>> As a clean up, I also renamed all use of "preloaded" in
>>>> the source code to "well-known". They refer to the same thing
>>>> in HotSpot, so there's no need to use 2 terms. Also, The word
>>>> "preloaded" is ambiguous -- it's unclear when "preloading" happens,
>>>> and could be confused with what CDS does during archive dump time.
>>>
>>> A few specific comments:
>>>
>>> src/hotspot/share/classfile/systemDictionary.cpp
>>>
>>> + bool SystemDictionary::is_well_known_klass(Symbol* class_name) {
>>> +   for (int i = 0; ; i++) {
>>> +     int sid = wk_init_info[i];
>>> +     if (sid == 0) {
>>> +       break;
>>> +     }
>>>
>>> I take it a zero value is a guaranteed end-of-list sentinel?
>>>
>>
>> Yes. The array is defined just a few lines above:
>>
>> static const short wk_init_info[] = {
>>   #define WK_KLASS_INIT_INFO(name, symbol) \
>>     ((short)vmSymbols::VM_SYMBOL_ENUM_NAME(symbol)),
>>
>>   WK_KLASSES_DO(WK_KLASS_INIT_INFO)
>>   #undef WK_KLASS_INIT_INFO
>>   0
>> };
>>
>> Also,
>>
>> class vmSymbols: AllStatic {
>>   enum SID {
>>     NO_SID = 0,
>>     ....
>>
>>
>>
>>> + for (int i=FIRST_WKID; i<last; i++) {
>>>
>>> Style nit: need spaces around = and <
>>>
>>
>> Fixed.
>>
>>> ---
>>>
>>> test/hotspot/jtreg/runtime/SharedArchiveFile/serviceability/ReplaceCriticalClasses.java 
>>>
>>>
>>> New file should have current copyright year only.
>>>
>> Fixed.
>>
>>>  31  * @comment CDS should not be disabled -- these critical classes 
>>> will be replaced because JvmtiExport::early_class_hook_env() is true.
>>>
>>> Comment seems contradictory: if we replace critical classes then CDS 
>>> should be disabled right??
>>>
>> Fixed.
>>
>>> I expected to see tests that checked for:
>>>
>>> "CDS is disabled because early JVMTI ClassFileLoadHook is in use."
>>>
>>> in the output. ??
>>>
>> <rant>
>> It would have been easy if jtreg lets you check the output of @run 
>> easily. Instead, your innocent suggestion has turned into 150+ lines 
>> of new code :-( Maybe "let's write all shell tests in Java" isn't 
>> such a great idea after all.
>> </rant>
>>
>> Now the test checks that whether CDS is indeed disabled, whether the 
>> affected class is loaded from the shared archive, etc.
>>
>> Thanks
>> - Ioi
>>
>>> Thanks,
>>> David
>>>
>>>
>>>>
>>>>  > In early e-mails Jiangli wrote:
>>>>  >
>>>>  > We should consider including more classes from the default 
>>>> classlist
>>>>  > in the test. Archived classes loaded during both 'early' phase 
>>>> and after
>>>>  > should be tested.
>>>>
>>>> Done.
>>>>
>>>>
>>>>  > For future optimizations, we might want to prevent loading 
>>>> additional
>>>>  > shared classes if any of the archived system classes is changed.
>>>>
>>>> What's the benefit of doing this? Today we already stop loading a 
>>>> shared
>>>> class if its super class was not loaded from the archive.
>>>>
>>>>
>>>> Thanks
>>>> - Ioi
>>>>
>>
>