Bug#816596: gtk3 programs segfault in init_randr15 when odd monitors are configured

[chrysn]

Package: libgtk-3-0
Version: 3.18.8-1
Severity: important

when odd monitors are configured in xrandr, no gtk3 program can start up
any more, as it segfaults in init_randr15.

i can't pinpoint what "odd monitor" exactly means, but it appears that
it's about xrandr 1.5 monitors (more of an abstraction of what one means
when saying "fullscreen" than actual graphics cards outputs which are
called outputs) that are not 1:1 related to an output.

to reproduce the behavior, i'm executing

$ xrandr --setmonitor foobar 1000/1000x1000/1000+1920+0 none

and then try running any gtk3 application. the offending monitor can be
removed again by running

$ xrandr --delmonitor foobar

. the segfault occurs in gdkscreen-x11.c line 659, where output_info
should be checked for being NULL before accessing the *struct's members.
i didn't find the documentation on XRRGetOutputInfo on whether it can
return NULL without overthrowing the complete protocol, but here it
does, and the output should be checked, but [1] indicated it does.


this issue might now only surface when playing around low-level with
xrandr, but might easily become an issue once the capabilities of xrandr
1.5 are leveraged outside of developer boxes.

best regards
chrysn

[1] https://bitbucket.org/portix/srandrd/commits/8b4627f80bd18a247222705da8d1a01dc57e9f35

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.5.0-rc5 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages libgtk-3-0 depends on:
ii  libatk-bridge2.0-0                      2.18.1-2
ii  libatk1.0-0                             2.18.0-1
ii  libc6                                   2.21-9
ii  libcairo-gobject2                       1.14.6-1
ii  libcairo2                               1.14.6-1
ii  libcolord2                              1.2.12-1
ii  libcups2                                2.1.3-3
ii  libepoxy0                               1.3.1-1
ii  libfontconfig1                          2.11.0-6.3
ii  libfreetype6                            2.6.3-3
ii  libgdk-pixbuf2.0-0                      2.32.3-1.2
ii  libglib2.0-0                            2.46.2-3
ii  libgtk-3-common                         3.18.8-1
ii  libjson-glib-1.0-0                      1.0.4-2
ii  libpango-1.0-0                          1.38.1-1
ii  libpangocairo-1.0-0                     1.38.1-1
ii  libpangoft2-1.0-0                       1.38.1-1
ii  librest-0.7-0                           0.7.93-1
ii  libsoup2.4-1                            2.52.2-1
ii  libwayland-client0                      1.9.0-1
ii  libwayland-cursor0                      1.9.0-1
ii  libwayland-egl1-mesa [libwayland-egl1]  11.1.2-1
ii  libx11-6                                2:1.6.3-1
ii  libxcomposite1                          1:0.4.4-1
ii  libxcursor1                             1:1.1.14-1+b1
ii  libxdamage1                             1:1.1.4-2+b1
ii  libxext6                                2:1.3.3-1
ii  libxfixes3                              1:5.0.1-2+b2
ii  libxi6                                  2:1.7.6-1
ii  libxinerama1                            2:1.1.3-1+b1
ii  libxkbcommon0                           0.5.0-1
ii  libxml2                                 2.9.3+dfsg1-1
ii  libxrandr2                              2:1.5.0-1
ii  shared-mime-info                        1.5-2

Versions of packages libgtk-3-0 recommends:
ii  libgtk-3-bin  3.18.8-1

Versions of packages libgtk-3-0 suggests:
ii  gvfs             1.26.2-1+b1
ii  librsvg2-common  2.40.13-3

-- no debconf information

-- 
To use raw power is to make yourself infinitely vulnerable to greater powers.
  -- Bene Gesserit axiom
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