Segfault in libQt5Gui.so.5 when qview is compiled with -stdlib=libc++

[telans]

UPDATE: Fixed by using -stdlib=libstdc++ instead of -stdlib=libc++

---

Environment:

• OS: Gentoo ~amd64
• qView version: b662db3
• Qt version: 5.15.2

clang -v:

clang version 12.0.0
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm/12/bin
Selected GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

CFLAGS="-march=znver2 -O3 -glldb -pipe"
CXXFLAGS="-march=znver2 -O3 -glldb -pipe"

Information:
When qview is compiled with Clang the program fails to start with a segfault. Below is a backtrace of the segfault, not sure how useful it is. To me it looks like this might be a Qt issue though I haven't seen this issue in any other Qt programs.

The rest of Qt is also compiled with Clang. I will test and update whether or not building qtcore etc with GCC fixes the issue.

Using a different Clang version (i.e 11) has no effect.

Backtrace:

Process 291548 launched: '/usr/bin/qview' (x86_64)
Only C and default locale supported with the posix collation implementation
Only C and default locale supported with the posix collation implementation
Numeric mode unsupported in the posix collation implementation
libQt5Gui.so.5 was compiled with optimization - stepping may behave oddly; variables may not be available.
Process 291548 stopped
* thread #1, name = 'qview', stop reason = signal SIGSEGV: invalid address (fault address: 0x14)
    frame #0: 0x00007ffff747cf00 libQt5Gui.so.5`QApplicationStateChangeEvent::applicationState(this=0x0000000000000000) const at qevent.cpp:5223:12 [opt]
   5220	*/
   5221	Qt::ApplicationState QApplicationStateChangeEvent::applicationState() const
   5222	{
-> 5223	    return m_applicationState;
   5224	}
   5225	
   5226	/*!
(lldb) bt
* thread #1, name = 'qview', stop reason = signal SIGSEGV: invalid address (fault address: 0x14)
  * frame #0: 0x00007ffff747cf00 libQt5Gui.so.5`QApplicationStateChangeEvent::applicationState(this=0x0000000000000000) const at qevent.cpp:5223:12 [opt]
    frame #1: 0x00000000002abdf8 qview`QVApplication::event(this=0x00007fffffffdef0, event=0x00007fffffffdbc8) at qvapplication.cpp:97:25 [opt]
    frame #2: 0x00007ffff7ba3ad5 libQt5Widgets.so.5`QApplicationPrivate::notify_helper(this=<unavailable>, receiver=0x00007fffffffdef0, e=0x00007fffffffdbc8) at qapplication.cpp:3632:26 [opt]
    frame #3: 0x00007ffff7ba506f libQt5Widgets.so.5`QApplication::notify(this=<unavailable>, receiver=0x00007fffffffdef0, e=0x00007fffffffdbc8) at qapplication.cpp:0 [opt]
    frame #4: 0x00007ffff6f5bbe4 libQt5Core.so.5`QCoreApplication::notifyInternal2(receiver=0x00007fffffffdef0, event=<unavailable>) at qcoreapplication.cpp:1063:18 [opt]
    frame #5: 0x00007ffff7463cb5 libQt5Gui.so.5`QGuiApplicationPrivate::setApplicationState(state=<unavailable>, forcePropagate=<unavailable>) at qguiapplication.cpp:3757:5 [opt]
    frame #6: 0x00007ffff74636f4 libQt5Gui.so.5`QGuiApplicationPrivate::processActivatedEvent(e=0x0000000000548ba0) at qguiapplication.cpp:2498:9 [opt]
    frame #7: 0x00007ffff744645a libQt5Gui.so.5`QWindowSystemInterface::sendWindowSystemEvents(flags=<unavailable>) at qwindowsysteminterface.cpp:1169:13 [opt]
    frame #8: 0x00007ffff4fa9c1a libQt5XcbQpa.so.5`xcbSourceDispatch(source=<unavailable>, (null)=<unavailable>, (null)=<unavailable>)(void*), void*) at qxcbeventdispatcher.cpp:105:5 [opt]
    frame #9: 0x00007ffff5d26566 libglib-2.0.so.0`g_main_context_dispatch + 438
    frame #10: 0x00007ffff5d26ca9 libglib-2.0.so.0`___lldb_unnamed_symbol114$$libglib-2.0.so.0 + 985
    frame #11: 0x00007ffff5d26f7b libglib-2.0.so.0`g_main_context_iteration + 59
    frame #12: 0x00007ffff6fb8218 libQt5Core.so.5`QEventDispatcherGlib::processEvents(this=0x000000000031ac80, flags=<unavailable>) at qeventdispatcher_glib.cpp:423:19 [opt]
    frame #13: 0x00007ffff6f579a7 libQt5Core.so.5`QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) [inlined] QEventLoop::processEvents(this=0x00007fffffffde80, flags=(i = 36)) at qeventloop.cpp:139:55 [opt]
    frame #14: 0x00007ffff6f57988 libQt5Core.so.5`QEventLoop::exec(this=0x00007fffffffde80, flags=<unavailable>) at qeventloop.cpp:232 [opt]
    frame #15: 0x00007ffff6f5c261 libQt5Core.so.5`QCoreApplication::exec() at qcoreapplication.cpp:1371:32 [opt]
    frame #16: 0x0000000000292e0b qview`main(argc=1, argv=<unavailable>) at main.cpp:24:12 [opt]
    frame #17: 0x00007ffff677afba libc.so.6`__libc_start_main + 234
    frame #18: 0x0000000000292a8a qview`_start + 42

[jurplel]

Are you using wayland?

[telans]

No, X11 (i3)

[telans]

Same result when compiling Qt with GCC 10.2.0. Are you able to verify that compiling qview with Clang results in a segfault?

[jurplel]

Just tried with clang and there was no segfault. Maybe it's because of -O3? I am seeing in your backtrace: libQt5Gui.so.5 was compiled with optimization - stepping may behave oddly; variables may not be available.

[telans]

That message is present at all -O levels I beleive. Regardless, no change with -O2. I imagine this is something I'll end up reporting to the Qt folks

[jurplel]

I am on arch linux with clang 11.0.1--maybe it's clang 12?

[jurplel]

Oh, I see you already mentioned that. Nevermind.

[novomesk]

@telans This happens also with media-gfx/qview-4.0::guru package?

[telans]

Yes. No change after multiple emerge -e @world's. I got someone who is also systemwide clang to test and they have no issues. It should probably be mentioned that I have global issues with Clang + LTO and Qt packages, too. I suspect it has something to do with -march=znver2 but I have yet to rebuild my system to test it.

[telans]

@novomesk I've narrowed this down to the flags -rtlib=compiler-rt -libunwind=libunwind which are on by default for me because of sys-devel/clang[default-compiler-rt] and sys-libs/llvm-libunwind. Specifying -rtlib=libgcc -libunwind=libgcc fixes the issue. Do you mind confirming?

[novomesk]

@telans So far I was unable to reproduce problem. I tried clang++ --rtlib=compiler-rt -flto but I don't have default-compiler-rt flag enabled.

I have sys-libs/libunwind installed but not sys-libs/llvm-libunwind

[telans]

@jurplel I filed a bug with Qt and someone there responded with something that may be a fix: https://bugreports.qt.io/browse/QTBUG-91510

I'm not sure why you can't reproduce it novomesk, maybe because I use -stdlib=libc++ too?

Update: In fact it seems to be only libc++ causing it, sorry for the confusion

[telans]

@jurplel using static_cast at

qView/src/qvapplication.cpp

Line 96 in b662db3

auto *stateEvent = dynamic_cast<QApplicationStateChangeEvent*>(event);

does fix the issue but I'm not sure what ramifications (if any) it has.

[jurplel]

I just got schooled, I suppose