RFR: 8278602: CDS dynamic dump may access unloaded classes

[Coleen Phillimore]

On Thu, 16 Dec 2021 03:46:10 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Cause of crash:
> 
> When dumping a CDS archive,  while iterating over entries of the `SystemDictionaryShared::_dumptime_table`, we do not check whether the classes are already unloaded. In the crash, we are trying to call `InstanceKlass::signer()` but the class has already been unloaded.
> 
> Fix:
> 
> Override the  template function `DumpTimeSharedClassTable::iterate` to ensure iteration safety.  Do not iterate over a class if its `class_loader_data` is no longer alive.
> 
> The assert in `DumpTimeSharedClassTable::IterationHelper` found another existing bug -- we were calling `SystemDictionaryShared::is_dumptime_table_empty()` without holding the `DumpTimeTable_lock`. I delayed the call until we have grabbed the lock.
> 
> Testing:
> 
> I have attached a test case into the bug report. Without the fix, it would reproduce the same crash in less than a minute. With the fix, the crash is no longer reproducible.
> 
> Unfortunately, the test case requires a ZGC patch (thanks to @stefank) that adds delays to  increase the likelihood of seeing unloaded classes inside the `_dumptime_table`. Therefore, I cannot integrate the test as a jtreg test. I'll mark the bug as **noreg-hard**

Could you also add your unloads a lot test even though it doesn't reproduce this particular error without the ZGC change?  It might find a similar bug under stress conditions.

src/hotspot/share/cds/dumpTimeClassInfo.inline.hpp line 53:

> 51:     assert_lock_strong(DumpTimeTable_lock);
> 52:     if (k->is_loader_alive()) {
> 53:       assert(k->is_loader_alive(), "must be");

This does seem a bit paranoid and redundant here.

src/hotspot/share/cds/dumpTimeClassInfo.inline.hpp line 58:

> 56:       return result;
> 57:     } else {
> 58:       if (!SystemDictionaryShared::is_excluded_class(k)) {

I thought this was the original bug?  is_excluded_class() looks at mirror->signers() which if the class isn't alive, mirror->signers() will crash.  This has to be in the k->is_loader_alive() too.

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Changes requested by coleenp (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6859