Bug#1151665: mutter: gnome-shell segfaults with NVIDIA 550 after upgrade to 51.0-3

[Claudio Giordano]

Package: mutter
Version: 51.0-3
Severity: serious

Dear maintainers,

after upgrading mutter and gnome-shell to 51.0-3 (Debian sid, 2026-10-22),
GDM can no longer start a greeter and the GNOME session crashes at startup
on a system using the proprietary NVIDIA driver shipped in testing/sid
(550.163.01-5.1).

Symptoms

- gdm retries until "GdmLocalDisplayFactory: maximum number of display
  failures reached. Giving up."
- gnome-shell[3916]: segfault at ... ip ... sp ... error 4 in
  libmutter-cogl-51.so.0.0.0[5faf4]
- org.gnome.Shell at gdm.service: Main process exited, code=killed,
  status=11/SEGV
- GdmDisplay: Session never registered, failing

Reproduction (deterministic, without GDM)

  $ gnome-shell --headless --mode=user; echo $?
  139
  $ gnome-shell --headless --mode=gdm; echo $?
  139

strace shows the NVIDIA EGL/GBM stack being loaded (renderD129,
nvidia-drm_gbm.so, libEGL_nvidia.so) followed by SIGSEGV. The faulting
offset 0x5faf4 corresponds to cogl_renderer_is_hardware_accelerated(),
which dereferences a NULL renderer. Upstream mutter 51 removed support for
legacy NVIDIA drivers (NEWS 51.alpha, MR !5079); whether 550 counts as
legacy is unclear, but mutter should not segfault when renderer/EGL
initialization fails.

System

- Debian sid/testing (forky), kernel 7.1.13+deb14-amd64
- mutter 51.0-3, gnome-shell 51.0-3, gdm3 51.1-1
- nvidia-driver 550.163.01-5.1, GeForce RTX 3070 Ti Laptop GPU
- Downgrading mutter/gnome-shell to 50.5-1 (testing) makes GNOME work again

Impact

550.163.01 is the newest nvidia-driver in testing and sid; 555.58.02 has
been in experimental since 2024. When mutter 51 migrates to testing, GNOME
will be unusable for users of the packaged NVIDIA driver.

Please consider:

1. keeping mutter 51 out of testing until a compatible driver is
   available, or
2. adding a versioned Breaks/Conflicts against nvidia-driver (<< 555) so
   apt cannot install the broken combination, and/or
3. fixing the NULL dereference so an EGL failure degrades gracefully
   instead of killing gnome-shell.

Thanks,
Claudio Giordano