[lttng-dev] Is lttng namespace-ready?

[Mathieu Desnoyers]

* 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?

nothing.

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

no.

>    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.

Thanks,

Mathieu

> 
> Daniel U. Thibault
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com