[Bug]: cachyos-pi crashes with SIGSEGV on QMessageBox close

[Big-Ouden]

Describe the bug

cachyos-pi (CachyOS Package Installer, launched directly or via cachyos-hello's "Install Apps" button) reliably crashes with a segmentation fault when a QMessageBox is shown and then closed — for example the package info dialog opened via the "i" (info) button in the package list.

The crash happens inside Qt6 itself, in QDialogPrivate::setNativeDialogVisible(), not in application code. It matches a known upstream Qt6 bug (see "Additional context" below): Qt fails to reset an internal "native dialog in use" flag when a QMessageBox is hidden, so the next call into the platform dialog helper dereferences a dangling/invalid pointer.

To Reproduce

1. Launch cachyos-pi (directly, or via "Install Apps" in cachyos-hello)
2. Click the "i" (info) button on a package in the list — this opens an info
   dialog (QMessageBox)
3. Close that dialog by clicking the "x" in the top-right corner
4. Application crashes with SIGSEGV

Also reproducible via any other QMessageBox triggered in the app (e.g. clicking an item in the package list), not just the info dialog.

Expected behavior

The dialog closes normally without crashing the application.

Environment

• OS: CachyOS (rolling)
• Kernel: 7.1.3-2-cachyos
• Desktop: KDE Plasma, Wayland
• Qt version: qt6-base 6.11.1-1.1
• cachyos-pi provided by: cachyos-packageinstaller 1.6.2-1
• Full bug report: https://paste.cachyos.org/p/459a17d.log

Logs / stack trace

$ journalctl -f (excerpt)
kernel: cachyos-pi[8185]: segfault at 0 ip 0000000000000000 sp 00007ffde2f4e228 error 14 likely on CPU 9 (core 4, socket 0)
kernel: Code: Unable to access opcode bytes at 0xffffffffffffffd6.
systemd-coredump[8278]: Process 8185 (cachyos-pi) of user 1000 terminated abnormally with signal 11/SEGV, processing...

$ gdb -q --args /usr/bin/cachyos-pi
(gdb) run
(gdb) bt full
Thread 1 "cachyos-pi" received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
#0  0x0000000000000000 in ?? ()
#1  0x00007ffff79ea76d in QDialogPrivate::setNativeDialogVisible(bool) () from /usr/lib/libQt6Widgets.so.6
#2  0x00007ffff7a2c93c in ?? () from /usr/lib/libQt6Widgets.so.6
#3  0x00007ffff7a275c8 in QMessageBox::~QMessageBox() () from /usr/lib/libQt6Widgets.so.6
#4  0x00005555555f6afb in ?? ()
#5  0x00007ffff660fffb in ?? () from /usr/lib/libQt6Core.so.6
#6  0x00007ffff7b2a026 in ?? () from /usr/lib/libQt6Widgets.so.6
#7  0x00007ffff660fffb in ?? () from /usr/lib/libQt6Core.so.6
#8  0x00007ffff7a7bac6 in QAbstractItemView::mouseReleaseEvent(QMouseEvent*) () from /usr/lib/libQt6Widgets.so.6
#9  0x00007ffff77659d5 in QWidget::event(QEvent*) () from /usr/lib/libQt6Widgets.so.6
#10 0x00007ffff77d7e98 in QFrame::event(QEvent*) () from /usr/lib/libQt6Widgets.so.6
#11 0x00007ffff6597ef8 in QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*) () from /usr/lib/libQt6Core.so.6
#12 0x00007ffff7704c0b in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQt6Widgets.so.6
#13 0x00007ffff77066fe in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQt6Widgets.so.6
#14 0x00007ffff6598a48 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /usr/lib/libQt6Core.so.6
#15 0x00007ffff76fbd9e in QApplicationPrivate::sendMouseEvent(...) () from /usr/lib/libQt6Widgets.so.6

Additional context

The crash matches a known Qt6 bug reported upstream on the Qt Forum: https://forum.qt.io/topic/164868/qmessagebox-crash-on-close

In qdialog.cpp, QDialogPrivate::setNativeDialogVisible() sets nativeDialogInUse = helper->show(...) when showing a dialog, but never resets nativeDialogInUse to false when hiding it via helper->hide(). On the next call (typically during QMessageBox destruction), Qt believes a native dialog helper is still active and calls into it, but the underlying helper has already been released — resulting in a call through a dangling/null pointer. This looks like an upstream Qt6 regression rather than a packaging issue, but cachyos-pi triggers it very reliably (every info dialog / message box), which makes it a good reference case.

Ruled out during debugging:

• Not an a11y/AT-SPI bus issue (QT_ACCESSIBILITY=0, NO_AT_BRIDGE=1 — no effect, bus healthy: org.a11y.Bus owned by at-spi-bus-launcher, kwin_wayland as manager)
• Not an xdg-desktop-portal misconfiguration (both xdg-desktop-portal-kde and xdg-desktop-portal-gtk active, QT_QPA_PLATFORMTHEME= empty — no effect)
• Not a native-dialog-type override issue (QT_DIALOG_TYPE=widget : no effect)
• Reproducible from a clean terminal launch of cachyos-pi, not tied to cachyos-hello's "Install Apps" button specifically
• System runs a multi-desktop-session setup (KDE, Hyprland, niri, i3 coexisting via SDDM session selector), but the crash reproduces in a clean KDE/Wayland session with all portals healthy — likely not session-related

Possible workaround for maintainers: avoid triggering native platformdialog helpers for simple in-app QMessageBox usage (e.g. set Qt::AA_DontUseNativeDialogs at application startup) until the upstream Qt6 bug is fixed.

[simiscoool-afk]

unable to replicate.

[kat-sudo-n]

Able to replicate it in two different systems running cachy-os.

For me it crashes only when closing the dialog by clicking the "x" button in the top-right corner (shown red in image). When closing it via the "x Close" button (shown green in image) the crash doesn't trigger.

[Vaiz]

hi, I had this problem as well. It looks like this is a bug in Qt.

TLDR: wait for KDE update

full AI report:

What's happening: the crash is inside Qt itself, in QDialogPrivate::setNativeDialogVisible(), right when a QMessageBox gets destroyed. On a KDE Plasma session, Qt hands off QMessageBox rendering to KDE's own native dialog plugin (KDEPlasmaPlatformTheme6.so, from the plasma-integration package) instead of drawing it itself. That native helper has a bug where it ends up processing the dialog's close twice, and the second time around it's poking at memory that's already gone — hence the segfault.

Found the actual upstream report: KDE Bug 520747 — reported on basically the same setup as ours (Arch, Qt 6.11.1, plasma-integration). It's already fixed in plasma-integration's git history (commits 16fd0841 and 10530d6) — just not released yet. 6.7.2 (currently the newest version in both extra and cachyos-extra-v3) predates the fix, so a regular system update won't help until KDE cuts a new tagged release.

Workaround I've applied: in main.cpp, right before QApplication is constructed:

QCoreApplication::setAttribute(Qt::AA_DontUseNativeDialogs);

This tells Qt to never hand dialogs off to the buggy native helper and just use its own built-in rendering instead. It completely sidesteps the crash regardless of which plasma-integration version is installed, so we don't have to wait on the upstream fix to land. Thought it makes icons ugly, so might be not the best fix.

[Vaiz]

The problem went away after upgrade of plasma-integration to 6.7.4-5.1