Unexpected gdbus behaviour during PAM authentication

[David Llewellyn-Jones]

On 15/01/17 07:46, Linus Lewandowski wrote:
> If you won't find a solution, you might want to try pam_exec.

Thank you for the suggestion. I suppose the approach in this case would
be to pass the details needed to the external command using environment
variables. The external command could then perform the dbus calls?

> On Sun, Jan 15, 2017, 02:27 David Llewellyn-Jones <david at flypig.co.uk
> <mailto:david at flypig.co.uk>> wrote:
> 
>     On 13/01/17 15:47, Bastien Nocera wrote:
>     > On Fri, 2017-01-13 at 14:37 +0000, David Llewellyn-Jones wrote:
>     >>
>     >> I've been experiencing strange behaviour connecting to the system bus
>     >> from a pam module. This is on Ubuntu 16.04 using gdbus and gdbus-
>     >> codegen
>     >> for both the caller and listening service.
>     >>
> 
>     [snip - gdbus details]
> 
>     >> Does anyone have any suggestion for why this might be happening?
>     >
>     > GDBus's threads and PAM really don't mix. You'll need to either use
>     > something like dbus-glib, which is single-threaded and sync, move your
>     > D-Bus code to a small helper you'd talk to over Unix sockets (yeah, I
>     > know...), or wait until PAM dies.
>     >
>     > FWIW, this problem is one of the reasons why fprintd (and pam_fprintd)
>     > is written using dbus-glib rather than GDBus.
> 
>     I've attempted to convert the pam module to use dbus-glib, using your
>     pam_fprintd.c code as a template (which has been very helpful, thank
>     you!). Unfortunately I'm still running in to problems.
> 
>     After removing all of the previous gdbus code and replacing it with the
>     following dbus-glib code, I now get segfaults occurring.
> 
>     -------
>     GError * error = NULL;
>     DBusGConnection * connection;
> 
>     connection = dbus_g_bus_get (DBUS_BUS_SYSTEM, &error);
>     if (connection == NULL) {
>       syslog(LOG_ERR, "pam_test: connection error: %s\n", error->message);
>       g_clear_error (&error);
>     }
>     else {
>       dbus_g_connection_unref (connection);
>     }
>     -------
> 
>     Originally I had a more complete set of code following this, but I found
>     the above was enough to trigger the problem.
> 
>     Here's the code on github:
>     https://github.com/llewelld/dbus-pam-test/blob/5940e11d939f587a7c1b42c00a02d0a5cc7dc2c3/src/pam_test.c#L131
> 
>     and here's the segfault entry from syslog:
> 
>     Jan 14 23:12:17 Owen kernel: [ 4683.886530] compiz[11249]: segfault at
>     7f34cd0ac8a0 ip 00007f34f8692908 sp 00007ffe6e955970 error 4 in
>     libglib-2.0.so.0.4800.1[7f34f8649000+10f000]
> 
>     Am I doing something wrong? In case it's important, the segfault doesn't
>     occur the first time the module is run, but consistently when it's
>     called for a second time. Again, this is all using the Unity lockscreen
>     on Ubuntu 16.04.


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