Bug#1114853: xserver-xorg-core: xset -dpms cause Xorg segfault

[Claude Heiland-Allen <claude@mathr.co.uk>]

Thanks Bernhard for the information.


I found a fix (originally dating back to 2014 afaict) that I could 
backport to the package in Debian trixie:


```
git clone https://gitlab.freedesktop.org/rherilier/libpciaccess.git
cd libpciaccess
git show 2aed459b0b8bd5f81bd793fff70396728eed75af > 
../libpciaccess-fix-pci-sys-null-deref.patch
cd ..
sudo apt-get build-dep libpciaccess0
sudo apt-get build-dep --host-architecture i386 libpciaccess0
apt source libpciaccess0
cd libpciaccess-0.17
patch -p1 < ../libpciaccess-fix-pci-sys-null-deref.patch
debuild -i -us -uc -b
debuild -a i386 -i -us -uc -b
cd ..
sudo dpkg -i libpciaccess*.deb
# reboot
xset -dpms # no longer crashes Xorg
```


the commit adds reference counting so that


```

open

open

close

use

close

```


doesn't explode at "use".


I also tried a testing (forky) live build with xfce (2025-11-10) and 
couldn't reproduce the bug (xset -dpms didn't crash Xorg), so maybe it's 
fixed in that version already (or maybe it's some other environmental 
difference in live boot).


Thanks again,


Claude


On 26/09/2025 13:57, Bernhard Übelacker wrote:
> On Wed, 10 Sep 2025 20:18:17 +0200 Claude Heiland-Allen 
> <claude@mathr.co.uk> wrote:
> > Package: xserver-xorg-core
> > Version: 2:21.1.16-1.3
>
> > Xorg keeps crashing when I leave my session idle.
> >
> > To try to turn off the screen saver, I ran from a fresh XFCE4 session:
>
> Hello,
> with the help of the dbgsym packages the backtrace would look like in 
> [1].
>
> The backtrace leads to upstream reports in [2].
>
> There is also a patch to leave the crashing function
> in case pci_sys is a NULL pointer.
>
> Kind regards,
> Bernhard
>
>
> [1]
> 0x0000556c30a41c68 in OsSigHandler at ../../../../os/osinit.c:135
> 0x00007fe3180b1df0 <__restore_rt>
> 0x00007fe3185b19b3 in pci_device_vgaarb_set_target at 
> ../../src/common_vgaarb.c:235
> 0x0000556c30921a7b in xf86VGAarbiterLock at 
> ../../../../../../hw/xfree86/common/xf86VGAarbiter.c:92
> 0x0000556c30904bd3 in xf86DPMS at 
> ../../../../../../hw/xfree86/common/xf86DPMS.c:56
> 0x0000556c3097a614 in DPMSSet at ../../../../Xext/dpms.c:106
> 0x0000556c3097a76d in ProcDPMSDisable at ../../../../Xext/dpms.c:228
> 0x0000556c308c4c08 in Dispatch at ../../../../dix/dispatch.c:551
> 0x0000556c308c8ccf in dix_main at ../../../../dix/main.c:274
>
>
> [2]
> https://sources.debian.org/src/libpciaccess/0.17-3/src/common_vgaarb.c#L235 
>
> https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/blob/master/src/common_vgaarb.c?ref_type=heads#L235 
>
>     235         dev = pci_sys->vga_default_dev;
> https://sources.debian.org/src/xorg-server/2%3A21.1.16-1.3/hw/xfree86/common/xf86VGAarbiter.c#L92 
>
> https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/hw/xfree86/common/xf86VGAarbiter.c?ref_type=heads#L92 
>
>     92     pci_device_vgaarb_set_target(pScrn->vgaDev);
>
>
> [3]
> https://gitlab.freedesktop.org/xorg/xserver/-/issues/1831
> https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/issues/25