dconf sometimes causes signal 7 (bus error) when homedir is NFS

[Margarita Manterola]

Hi,

We are using Precise, with dconf version 0.12.0. Last weekend we had two users who have their homes in NFS report that their screensavers had died. When investigating the issue, it turns out that they died with the funny "left the bus" error:

Mar 17 03:51:01 zork gnome-session[4387]: WARNING: Detected that screensaver has left the bus
Mar 17 03:51:02 zork gnome-session[4387]: WARNING: Application 'gnome-settings-daemon.desktop' killed by signal

Mar 18 08:54:07 jan gnome-session[3217]: WARNING: Detected that screensaver has left the bus
Mar 18 08:54:14 jan gnome-session[3217]: WARNING: Application 'gnome-panel.desktop' killed by signal
Mar 18 08:54:23 jan gnome-session[3217]: WARNING: Application 'gnome-settings-daemon.desktop' killed by signal

I'm attaching the backtrace of the first of these crashes, which shows that the crash happened when calling gvdb_table_lookup, with "/org/gnome/desktop/interface/clock-format" as key.

When discussing this with Ryan Lortie, he told me:

<desrt> sigbus in dconf code is almost always (maybe _always_) caused by the user not having a fully-functional XDG_RUNTIME_DIR
<desrt> this issue has been known to periodically occur with NFS or ecryptfs for a while
<desrt> basically it happens when a file on the NFS server gets replaced and the clients are left looking at a stale copy
<desrt> with read() or write() you would get ESTALE
<desrt> but with mmap() you get SIGBUS

We don't seem to have XDG_RUNTIME_DIR variable set in precise. These are the XDG variables that I have:

XDG_SESSION_COOKIE=<removed>
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
XDG_CONFIG_DIRS=/etc/xdg/xdg-cinnamon:/etc/xdg
XDG_DATA_DIRS=/usr/share/cinnamon:/usr/share/gnome:/usr/local/share/:/usr/share/
XDG_CURRENT_DESKTOP=GNOME

BTW, this has happened both running cinnamon and gnome-classic.

According to what Ryan tells me this issue has been known for a while, and has even been fixed in the latest versions, but precise is stuck with a broken version. Given the fact that an unlocked screen is a security issue, it would be nice if the fix could be backported to precise.

--
Regards,
Marga

[Margarita Manterola (marga-9)]

In our case it happens because of expired credentials, until this credentials are renewed, the homedir IS NOT there. This means that the new version that fixes this issue wouldn't really make a difference for us. We need to somehow find a way of catching the event that causes the SIGBUS, and keep dconf from dying in this case.