Thanks to Doug's addition info, I think I managed to track down this
problem after creating a new user to test. When clicking the username in
lightdm-example-gtk-greeter, start_authentication() is executed in
lightdm-example-gtk-greeter.c. If the user is anything but "Other" or
"Guest", the greeter attempts to load the user's ~/.dmrc file to look up
the default session. Since this user has never logged in, there is no
~/.dmrc file, so the session name is NULL. This NULL session is then
passed to set_session() in lightdm-example-gtk-greeter.c, which attempts
to compare the name with all the available session using strcmp, which
crashes because session is NULL. Could probably use g_strcmp0, but I
think the better option is just to add a g_return_if_fail (session !=
NULL) to the start of the file. I rebuilt with this patch, and haven't
been able to crash it in the same way since.

** Patch added: "check_for_null_session.patch"
   
https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/809890/+attachment/2212010/+files/check_for_null_session.patch

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https://bugs.launchpad.net/bugs/809890

Title:
  lightdm-example-gtk-greeter crashed with SIGSEGV in __strcmp_ssse3()

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