[lttng-dev] Is lttng namespace-ready?

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon Jul 8 12:33:41 EDT 2013

* Thibault, Daniel (Daniel.Thibault at drdc-rddc.gc.ca) wrote:
>    I was reading Michael Kerrisk's "Namespaces in operation"
>    (http://lwn.net/Articles/531114/), which explains most of the
>    namespace functionalities, and this brings to mind a number of
>    questions.
>    Are trace paths namespace-ready?  Per-PID traces are apparently not
>    much at risk since a process can join a PID namespace only upon its
>    creation (that is to say, PIDs are immutable).  However, collisions
>    are still possible, such as if two user-space processes start
>    within the same one-second window in two sibling PID namespaces
>    such that they have the same virtual PID.

yes, this is a possible race indeed for per-PID tracing in UST, since I
think we're using namespaced PIDs to identify programs.

>    But what of per-UID traces?  What happens to a trace in progress if
>    one or more processes switch user namespaces?


>  Presumably new
>    sub-directories would be created under "session-name/ust/uid/"?


>    What about the case where two processes run by two different users
>    (and thus being recorded under different uid subdirectories) switch
>    to private user namespaces which happen to both map to the same
>    numerical virtual uid?  We need to make sure the second process
>    switch does not overwrite the trace files of the first (recall that
>    per-uid trace paths do not include timestamps like per-pid trace
>    paths do).

Disambiguation between different PIDs requires the vpid context if you
use per-UID buffers.

>    Finally, in the case of a remote trace being received by the relay
>    daemon, could a network or UTS namespace switch of the traced
>    processes cause trouble?  I suspect not, since the daemons at
>    either end of the connection are lttng-owned and would presumably
>    not undergo namespace switches themselves.

I don't see it causing an issue. It might be good to try it out though.



> Daniel U. Thibault
> Protection des systèmes et contremesures (PSC) | Systems Protection & Countermeasures (SPC)
> Cyber sécurité pour les missions essentielles (CME) | Mission Critical Cyber Security (MCCS)
> R & D pour la défense Canada - Valcartier (RDDC Valcartier) | Defence R&D Canada - Valcartier (DRDC Valcartier)
> 2459 route de la Bravoure
> Québec QC  G3J 1X5
> Vox : (418) 844-4000 x4245
> Fax : (418) 844-4538
> NAC : 918V QSDJ <http://www.travelgis.com/map.asp?addr=918V%20QSDJ>
> Gouvernement du Canada | Government of Canada
> <http://www.valcartier.drdc-rddc.gc.ca/>
> _______________________________________________
> lttng-dev mailing list
> lttng-dev at lists.lttng.org
> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

Mathieu Desnoyers
EfficiOS Inc.

More information about the lttng-dev mailing list