Bug#756068: gdm3 blocks SIGUSR1 for all applications

[Mathieu Desnoyers]

Package: gdm3
Version: 3.12.2-2
Severity: important
Tags: upstream

Dear Maintainer,

We've been puzzled to see that when using recent gdm3 and
gnome3, our application stopped receiving SIGUSR1 signals.

Digging into the issue, we found this gdm commit that seems
to block the SIGUSR1 signal in a process-wide fashion:

commit 956d7d1c7a0cfbf2beacdb9e88e645e15ad32047
Author: Jasper St. Pierre <jstpierre at mecheye.net>
Date:   Fri Feb 14 14:32:50 2014 -0500

    server: Process SIGUSR1 more carefully
    
    If the slave is removed as a separate process, it means that we need
    to be more careful with our handling of SIGUSR1. If multiple X servers
    are launched at once, we need to use siginfo_t to get the PID of the
    thing that sent the user signal, and make sure we signal the correct
    GdmServer.
    
    glib doesn't have native siginfo support, so do it ourselves by using
    a worker thread that spins around waiting for sigwaitinfo.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=724382

Our issue is that this signal mask blocking of SIGUSR1 trickles
down to every application launched by the window manager. This
breaks POSIX guarantees about signal masks (see signal(7)). We
would expect SIGUSR1 to be delivered to the application by default.

Moreover, reading the commit, we notice that the intent using
sigprocmask in sigusr1_thread_main() is to handle SIGUSR1, but
the code actually blocks SIGUSR1. This is exactly the opposite of
what is expected. Therefore, we can expect that the signal is
never handled by sigusr1_thread_main, so there are probably other
issues hidden under this. Also, we would recommend using
pthread_sigmask rather than sigprocmask so the effect of the
signal masking only applies to the thread running
sigusr1_thread_main.

Thanks,

Mathieu

-- System Information:
Debian Release: jessie/sid
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.14-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_CA.UTF-8, LC_CTYPE=en_CA.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages gdm3 depends on:
ii  accountsservice                              0.6.37-2
ii  adduser                                      3.113+nmu3
ii  dconf-cli                                    0.20.0-2
ii  dconf-gsettings-backend                      0.20.0-2
ii  debconf [debconf-2.0]                        1.5.53
ii  gir1.2-gdm3                                  3.12.2-2
ii  gnome-session [x-session-manager]            3.12.1-3
ii  gnome-session-bin                            3.12.1-3
ii  gnome-session-flashback [x-session-manager]  3.8.0-3
ii  gnome-settings-daemon                        3.12.2-1
ii  gnome-shell                                  3.12.2-3
ii  gnome-terminal [x-terminal-emulator]         3.12.3-1
ii  gsettings-desktop-schemas                    3.12.2-1
ii  libaccountsservice0                          0.6.37-2
ii  libatk1.0-0                                  2.12.0-1
ii  libaudit1                                    1:2.3.7-1
ii  libc6                                        2.19-7
ii  libcairo-gobject2                            1.12.16-2
ii  libcairo2                                    1.12.16-2
ii  libcanberra-gtk3-0                           0.30-2
ii  libcanberra0                                 0.30-2
ii  libgdk-pixbuf2.0-0                           2.30.7-1
ii  libgdm1                                      3.12.2-2
ii  libglib2.0-0                                 2.40.0-3
ii  libglib2.0-bin                               2.40.0-3
ii  libgtk-3-0                                   3.12.2-1+b1
ii  libpam-modules                               1.1.8-3
ii  libpam-runtime                               1.1.8-3
ii  libpam-systemd                               208-6
ii  libpam0g                                     1.1.8-3
ii  libpango-1.0-0                               1.36.3-1
ii  libpangocairo-1.0-0                          1.36.3-1
ii  librsvg2-common                              2.40.2-1
ii  libselinux1                                  2.3-1
ii  libsystemd-daemon0                           208-6
ii  libsystemd-id128-0                           208-6
ii  libsystemd-journal0                          208-6
ii  libsystemd-login0                            208-6
ii  libwrap0                                     7.6.q-25
ii  libx11-6                                     2:1.6.2-2
ii  libxau6                                      1:1.0.8-1
ii  libxdmcp6                                    1:1.1.1-1
ii  libxrandr2                                   2:1.4.2-1
ii  lsb-base                                     4.1+Debian13
ii  metacity [x-window-manager]                  1:2.34.13-1
ii  policykit-1                                  0.105-6
ii  twm [x-window-manager]                       1:1.0.8-1
ii  ucf                                          3.0030
ii  x11-common                                   1:7.7+7
ii  x11-xserver-utils                            7.7+3
ii  xfce4-session [x-session-manager]            4.10.1-6
ii  xfwm4 [x-window-manager]                     4.10.1-2
ii  xterm [x-terminal-emulator]                  308-1

Versions of packages gdm3 recommends:
ii  at-spi2-core               2.12.0-2
ii  desktop-base               7.0.3
ii  gnome-icon-theme           3.12.0-1
ii  gnome-icon-theme-symbolic  3.12.0-1
ii  x11-xkb-utils              7.7+1
ii  xserver-xephyr             2:1.16.0-1
ii  xserver-xorg               1:7.7+7
ii  zenity                     3.12.1-1

Versions of packages gdm3 suggests:
pn  gnome-orca            <none>
ii  libpam-gnome-keyring  3.8.2-2+b1

-- debconf information:
  gdm3/daemon_name: /usr/sbin/gdm3
* shared/default-x-display-manager: gdm3