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

Ioi Lam ioi.lam at oracle.com
Mon Oct 22 05:49:03 UTC 2018

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::resolvewellknownclasses must not be replaced by JVMTI ClassFileLoadHook during run time, including - field offsets computed in JavaClasses::computeoffsets - the layout of the strings objects in the shared strings table The "well-known" classes can be replaced by ClassFileLoadHook only when JvmtiExport::earlyclasshookenv() 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 JVMTIPHASEPRIMORDIAL? 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::iswellknownklass(Symbol* classname) { +   for (int i = 0; ; i++) { +     int sid = wkinitinfo[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=FIRSTWKID; 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::earlyclasshookenv() 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. ?? 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.

Now the test checks that whether CDS is indeed disabled, whether the affected class is loaded from the shared archive, etc.

Thanks

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

More information about the hotspot-runtime-dev mailing list