JDK-8230517 : UnifiedOop invokes undefined behavior
  • Type: Bug
  • Component: hotspot
  • Sub-Component: jfr
  • Affected Version: 14
  • Priority: P4
  • Status: Resolved
  • Resolution: Duplicate
  • Submitted: 2019-09-04
  • Updated: 2019-12-04
  • Resolved: 2019-12-04
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JDK 14
14Resolved
Related Reports
Duplicate :  
JDK-8235174 - ZGC: Add support for JFR leak profiler
Relates :  
JDK-8230729 - UnifiedOop uses wrong Access barriers
Relates :  
JDK-8229437 - assert(is_aligned(ref, HeapWordSize)) failed: invariant
Relates :  
JDK-8230520 - Incorrect UnifiedOop::is_narrow assertions
Description
UnifiedOop may attempt to construct oop* values that are misaligned.  That has unspecified behavior.  Any further manipulation of the value (such as attempting to convert back to a narrowOop*) can't remove that.  And dereferencing a pointer with unspecified value is undefined behavior.

It is generally a bad idea to lie to the type system the way this class is doing. An alternative would be to have a distinct type that carried around the internal (tagged) pointer as a void* or the like.  Make it a thin, trivially copyable class, and there's no additional overhead.
Comments
We will address this as part of JDK-8235174.
04-12-2019
ILW = MLM = P4
10-09-2019