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

[David Holmes]

Hi Ioi,

Sorry for all the extra work in the test. ;-)

The updates look good to me (and I've seen follow up comments).

Thanks,
David

On 22/10/2018 3:49 PM, 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/ 
> 
> 
> 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
>>>
>