/usr/sbin/lightdm-gtk-greeter:11:cairo_surface_get_type:set_surface_as_root:greeter_background_save_xroot:start_session:user_combo_key_press_cb

[errors.ubuntu.com bug bridge]

The Ubuntu Error Tracker has been receiving reports about a problem regarding lightdm-gtk-greeter. This problem was most recently seen with package version 2.0.1-2ubuntu4, the problem page at https://errors.ubuntu.com/problem/186a7f61dba62b6bcb526a43eb037ea4c310f596 contains more details, including versions of packages affected, stacktrace or traceback, and individual crash reports.
If you do not have access to the Ubuntu Error Tracker you can request it at http://forms.canonical.com/reports/.

[Stefan Dösinger (stefandoesinger-b)]

I see this with lightdm-gtk-greeter 2.0.3 on Gentoo. It appears that this crash happens when the hostname changes between lightdm start and login.

My system's hostname defaults to "localhost", but I set proper hostnames via dhcp (the Linux installation boots on multiple computers). Network setup is handled by networkmanager. It seems that the IP setup happens somewhen between lightdm starting and the greeter screen appearing.

Not connecting the network (so the hostname stays as "localhost") or setting the hostname earlier via the kernel's ip=dhcp boot argument works around the bug.

Here is an excerpt from strace:

connect(11, {sa_family=AF_UNIX, sun_path=@"/tmp/.X11-unix/X0"}, 20) = 0
getpeername(11, {sa_family=AF_UNIX, sun_path=@"/tmp/.X11-unix/X0"}, [124->20]) = 0
uname({sysname="Linux", nodename="ripper", ...}) = 0
access("/root/.Xauthority", R_OK) = 0
open("/root/.Xauthority", O_RDONLY) = 12
fstat(12, {st_mode=S_IFREG|0600, st_size=54, ...}) = 0
read(12, "\1\0\0\tlocalhost\0\0010\0\22MIT-MAGIC-COOK"..., 4096) = 54
read(12, "", 4096) = 0
close(12) = 0
fcntl(11, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(11, F_SETFL, O_RDWR|O_NONBLOCK) = 0
fcntl(11, F_SETFD, FD_CLOEXEC) = 0
poll([{fd=11, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=11, revents=POLLOUT}])
writev(11, [{iov_base="l\0\v\0\0\0\0\0\0\0\0\0", iov_len=12}, {iov_base="", iov_len=0}], 2) = 12
recvfrom(11, 0x1047e80, 8, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=11, events=POLLIN}], 1, -1) = 1 ([{fd=11, revents=POLLIN|POLLHUP}])
recvfrom(11, "\0\26\v\0\0\0\6\0", 8, 0, NULL, NULL) = 8
recvfrom(11, "No protocol specified\n\0\0", 24, 0, NULL, NULL) = 24
write(2, "No protocol specified\n", 22) = 22
shutdown(11, SHUT_RDWR) = 0
close(11) = 0
ioctl(2, TCGETS, 0x7ffe4bc60930) = -1 ENOTTY (Inappropriate ioctl for device)
getpid() = 3418
write(2, "\n** (lightdm-gtk-greeter:3418): "..., 86) = 86
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x10} ---

I have attached the greeter log. I do not wish to attach the full strace log because it will probably contain my password in one way or another.

[Stefan Dösinger (stefandoesinger-b)]

I hit "post" a bit too soon.

Here is the backtrace from the core dump:

(gdb) bt
#0 0x00007fc94980f330 in cairo_surface_get_type () from /usr/lib64/libcairo.so.2
#1 0x0000000000413893 in greeter_background_save_xroot ()
#2 0x000000000040e07a in ?? ()
#3 0x00007fc948fa20d4 in ?? () from /usr/lib64/libgobject-2.0.so.0
#4 0x00007fc948fbc5a7 in g_signal_emit_valist () from /usr/lib64/libgobject-2.0.so.0
#5 0x00007fc948fbcf1f in g_signal_emit () from /usr/lib64/libgobject-2.0.so.0
#6 0x00007fc948a6ab31 in ?? () from /usr/lib64/liblightdm-gobject-1.so.0
#7 0x00007fc948a6aeed in ?? () from /usr/lib64/liblightdm-gobject-1.so.0
#8 0x00007fc948cc84dd in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#9 0x00007fc948cc88b0 in ?? () from /usr/lib64/libglib-2.0.so.0
#10 0x00007fc948cc8bd2 in g_main_loop_run () from /usr/lib64/libglib-2.0.so.0
#11 0x00007fc949fb4645 in gtk_main () from /usr/lib64/libgtk-3.so.0
#12 0x000000000040c463 in main ()
(gdb)

I cannot compare this to the error report, but "greeter_background_save_xroot" and "cairo_surface_get_type" from the title of this bug ring a bell.

Here are some other similar bugs I found while searching for existing reports:
https://bugzilla.redhat.com/show_bug.cgi?id=1478720
https://bugzilla.redhat.com/show_bug.cgi?id=1468864
https://bugs.launchpad.net/ubuntu/+source/lightdm-gtk-greeter/+bug/1422794

[Sean Davis (bluesabre)]

This error hasn't been seen since 18.10. Closing out.