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Re: [gdm-list] Help with GDM 2.20.2: gdm ignoring custom server config


Martin:

Thanks for catching this bug.  The fix is now upstream in the 2.20
branch.

Brian



That patch fixed the problems!  Everything appears to be working fine
now.

In particular, as you probably guessed, I use the gdmflexiserver to
start a new X server for dual head or Xinerama as necessary.  That
feature is really great for my needs.

Thanks,

--Martin
On Fri, Jan 04, 2008 at 03:22:39PM -0600, Brian Cameron wrote: 
Martin:

Looking at the code a bit more closely, I noticed the code was
allocating xserver_group once outside the loop, assigning it
to srv_entry->group, then it gets freed in the gdm_config_entry_free().
Then next time around the loop, it is already freed!

I think the attached patch (which applies on top of the previous patch)
should fix the problem.  Now the code makes sure to call stdup each
time it sets srv_entry->group.  Can you verify this avoids the crash?
If so, then I'll commit this upstream.

Thanks,

Brian



However, the patch gdm dies with a seg fault in parsing the config in
gdm-daemon-config.c at line 1211 in load_xservers_group().  I compiled
with debug to see if there was something obvious to me.  Does this
help?  I don't think this is on my end (e.g. in the patching).  The
loop looks like:

I don't see any crashing problems here, but sometimes crashing problems
can be OS-speicifc.

You don't specify which line in the loop is line 1211.  In my copy of
the file it is the gdm_config_entry_free line, but I suspect it is a
different line in your file (since that would be an odd line for it
to crash on just by looking at the code).

Yes it really is in gdm_config_entry_free().  I'm attaching three
files:
	script.out	a "script" capture of the gdb session
	/etc/gdm.conf
	/usr/share/gdm/defaults.conf

The latter two from the results of:

gdmflexiserver --command=GET_CONFIG_FILE
gdmflexiserver --command=GET_CUSTOM_CONFIG_FILE


>from the unpatched version.

Also, could you try running gdm in the debugger and see if it gives any
further info about what is causing the crash?  Since the crash is
happening early it is probably a situation where you can actually run
GDM in a debugger and get some useful information.  This isn't always
the case, since GDM forks into slave daemons - so it is harder to use
the debugger to find issues after such forks happen.

So, what I'd recommend doing is:

1) Run gdm-binary (not the gdm script) in the debugger like this:
  gdb /usr/sbin/gdm-binary
  run --nodaemon

  The --nodaemon argument tells gdm-binary not to fork immediately
  which is what you want when debugging.

2) Then it should crash and tell you that the SEGV happened and drop
  you into a prompt.  Try printing out various values like run
  "print srv_entry" and see if the group variable is NULL or something
  causing a crash when we try to access srv_entry->group.
for (j = 0; j < G_N_ELEMENTS (gdm_daemon_server_config_entries); j++) { 
				GdmConfigEntry *srv_entry;
if (gdm_daemon_server_config_entries[j].key == NULL) { 
					continue;
				}
srv_entry = gdm_config_entry_copy (&gdm_daemon_server_config_entries[j]); 
				g_free (srv_entry->group);
				srv_entry->group = xserver_group;
gdm_config_process_entry (config, srv_entry, NULL); 
				gdm_config_entry_free (srv_entry);
			}


BTW, the [servers] section in my gdm.conf is default, that is to say,
empty.

It would be helpful if you could run these commands:

gdmflexiserver --command=GET_CONFIG_FILE
gdmflexiserver --command=GET_CUSTOM_CONFIG_FILE

And send me these two files as attachments.  Perhaps the crash is
triggered by how your [servers] and [server-foo] sections are set up.
If you can send me your files, I can see if I can trigger the crash
on my system using your style of configuration.

Brian

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Index: gdm-daemon-config.c
===================================================================
--- gdm-daemon-config.c	(revision 5599)
+++ gdm-daemon-config.c	(working copy)
@@ -1184,21 +1184,18 @@
 {
 	GPtrArray  *server_groups;
 	char      **vname_array;
-	char       *xserver_value;
+	char       *xserver_group;
 	int         i, j;
server_groups = gdm_config_get_server_groups (config); for (i=0; i < server_groups->len; i++) { 
-		xserver_value = g_ptr_array_index (server_groups, i);
-		gdm_debug ("Processing server group <%s>", xserver_value);
+		xserver_group = g_ptr_array_index (server_groups, i);
+		gdm_debug ("Processing server group <%s>", xserver_group);
- if (g_str_has_prefix (xserver_value, "server-")) { 
-			char * xserver_group;
+		if (g_str_has_prefix (xserver_group, "server-")) {
 			char * xserver_name;
- xserver_group = g_strdup (xserver_value); 
-			
 			for (j = 0; j < G_N_ELEMENTS (gdm_daemon_server_config_entries); j++) {
 				GdmConfigEntry *srv_entry;
 				if (gdm_daemon_server_config_entries[j].key == NULL) {
@@ -1206,7 +1203,7 @@
 				}
 				srv_entry = gdm_config_entry_copy (&gdm_daemon_server_config_entries[j]);
 				g_free (srv_entry->group);
-				srv_entry->group = xserver_group;
+				srv_entry->group = g_strdup (xserver_group);
 				gdm_config_process_entry (config, srv_entry, NULL);
 				gdm_config_entry_free (srv_entry);
 			}
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