[lttng-dev] Segmentation fault in lttng-sessiond which is fixed in master

[David Goulet]

Hi David,

Yes, it was fixed and cleaned up not long ago in 2.1.

I'll make sure to port the fix here.

Comments below:

David OShea:
> Hi all,
> 
> I'm using lttng-tools-2.0.4.  I got a segmentation fault in lttng-sessiond with this stack trace:
> 
> """
> #0  delete_ust_app_session (sock=-1, ua_sess=0xffffffffffffffff) at ust-app.c:147
> #1  0x000000000040cab5 in create_ust_app_session (usess=0x148adc10, app=0x148c0ef0) at ust-app.c:914
> #2  0x000000000040db14 in ust_app_global_update (usess=0x148adc10, sock=187) at ust-app.c:2309
> #3  0x0000000000412c96 in update_ust_app (app_sock=<optimized out>) at main.c:855
> #4  thread_manage_apps (data=<optimized out>) at main.c:1257
> #5  0x00002b872bffb617 in start_thread () from /lib64/libpthread.so.0
> #6  0x00002b872c2e3c2d in clone () from /lib64/libc.so.6
> #7  0x0000000000000000 in ?? ()
> """
> 
> It looks like in create_ust_app_session() this code must have executed:
> 
> """
> 	if (ua_sess->handle == -1) {
> 		ret = ustctl_create_session(app->sock);
> 		if (ret < 0) {
> 			ERR("Creating session for app pid %d", app->pid);
> 			/* This means that the tracer is gone... */
> 			ua_sess = (void*) -1UL;
> 			goto error;
> """
> 
> as this is the location of the call to delete_ust_app_session():
> 
> """
> error:
> 	delete_ust_app_session(-1, ua_sess);
> """
> 
> I see that in git master, this seems to have been fixed by:
> 
> """
> commit 0f83395d5e0e927fdab025eae023e68e9021af26
> Author: David Goulet <dgoulet at efficios.com>
> Date:   Mon Oct 29 14:49:50 2012 -0400
> 
>     Fix: Cleanup UST app session on ustctl create session error
> """
> 
> which ensures that ua_sess is not set to -1 until after delete_ust_app_session() is called.
> 
> I have two queries about this:
> 
> 1. Why is this error handling code being reached - what does the comment /* This means that the tracer is gone... */ mean - does it mean lttng-ust in a user process connected to the session daemon but the process went away or something like that?
> 

Yes this is basically it. Tracer malfunction or the process is gone.

> 2. Should the above commit be merged into the stable branch?  I'm not concerned either way as I'll probably just apply the patch to 2.0.4 when I get a chance.

It will be.

Thanks!
David

> 
> Thanks!
> David
> 
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