On Wed, 8 Feb 2023 06:58:52 GMT, Thomas Stuefe <stu...@openjdk.org> wrote:
> I read through your explanation, and through the [design > docs](https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizerDesignDocument), > but my question remains unanswered. See below. > > > > Metaspace objects hold pointers to malloced memory. Metaspace Objects can > > > leak, that's accepted. So these pointers would leak too. Would that not > > > give us tons of false positives? > > > > > > No. LSan only cares that it can find pointers to malloc'ed memory, not that > > it can find pointers to Metaspace objects. The Metaspace is registered as a > > root region. It does however take into account ASan poisoning, which we > > added earlier to Metaspace. LSan will not scan poisoned memory regions that > > are part of root regions. We only poison in Metaspace when deallocation > > occurs and unpoison when allocation occurs. > > But they are not poisened. > > Upon JVM end, the heap still exists, holding class loaders, which hold > references to CLDs. Those hold metaspace arenas (living in Metaspace), which > hold Metaspace chunks, which are registered LSAN root areas, which hold > C-allocated sub structures. These chunks are not poisened since they have > never been returned to the chunk pool. They are still alive. > > To put it in other words: even I as a human cannot determine that this is not > a C-heap leak. Technically, it is. But it is accepted since we accept these > things leaking. Since we don't support unloading and re-loading the JVM, we > accept that to have a faster shutdown. Let's ignore ASan/poisoning for now, its an optimization. I shouldn't have brought it up. LSan leak checking, as implemented for Hotspot, happens before the JVM has actually shutdown. It happens immediately after one thread wins the right to begin shutting down and other threads are either waiting or doing other work. Everything is still alive, including CollectedHeap, Metaspace, and CodeHeap. We ignore leaks as a result of shutdown this way, because as you mentioned we are not that concerned. Outside of Hotspot, LSan leak checking is typically done at program exit. If you do that with Hotspot you will definitely get false positives. Note: If you want, feel free to run a build with `--enable-asan` and `--enable-lsan` and run some of the test tiers. Some tests will fail due to some existing incompatibilities with ASan (working on fixing those separately), but some will fail with real leaks. I need to report these, just have not gotten around to it yet. Plan was to do so after this integrated. Most of the remaining ones are minor. The only time false positives can happen is some of the tests which do some interesting things and use a custom launcher, these don't inherit our options and will attempt to leak check at exit. I am slowly going through and correcting these. They should be rare at this point. On a side note, once this discussion is complete and in favor of this change, I should probably add some documentation related to sanitizers in Hotspot including ASan, LSan, and UBSan. ------------- PR: https://git.openjdk.org/jdk/pull/12229
