Re: [PATCH] coresight: etm4x: Fix PID tracing when perf is run in an init PID namespace

[Suzuki K Poulose]

On 08/10/2024 07:52, Leo Yan wrote:
> On 10/7/2024 9:05 PM, Leo Yan wrote:
> > Hi Julien,
> >
> > On Wed, Sep 25, 2024 at 03:13:56PM +0200, Julien Meunier wrote:
> > > The previous implementation limited the tracing capabilities when perf
> > > was run in the init PID namespace, making it impossible to trace
> > > applications in non-init PID namespaces.
> > >
> > > This update improves the tracing process by verifying the event owner.
> > > This allows us to determine whether the user has the necessary
> > > permissions to trace the application.
> >
> > The original commit aab473867fed is not for constraint permission. It is
> > about PID namespace mismatching issue.
> >
> > E.g. Perf runs in non-root namespace, thus it records process info in the
> > non-root PID namespace. On the other hand, Arm CoreSight traces PID for
> > root namespace, as a result, it will lead mess when decoding.
> >
> > With this change, I am not convinced that Arm CoreSight can trace PID for
> > non-root PID namespace. Seems to me, the concerned issue is still existed
> > - it might cause PID mismatching issue between hardware trace data and
> > Perf's process info.
>
> I thought again and found I was wrong with above conclusion. This patch is a
> good fixing for the perf running in root namespace to profile programs in
> non-root namespace. Sorry for noise.
>
> Maybe it is good to improve a bit comments to avoid confusion. See below.
>
> [...]
>
> > > diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > > index bf01f01964cf..8365307b1aec 100644
> > > --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > > +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > > @@ -695,7 +695,7 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
> > >
> > >        /* Only trace contextID when runs in root PID namespace */
>
> We can claim the requirement for the *tool* running in root PID namespae.
>
>    /* Only trace contextID when the tool runs in root PID namespace */

minor nit: I wouldn't call "tool". Let keep it "event owner".

	/* Only trace contextID when the event owner is in root PID namespace */


Julien,

Please could you respin the patch with the comments addressed.

Kind regards
Suzuki


> > >        if ((attr->config & BIT(ETM_OPT_CTXTID)) &&
> > > -         task_is_in_init_pid_ns(current))
> > > +         task_is_in_init_pid_ns(event->owner))
> > >                /* bit[6], Context ID tracing bit */
> > >                config->cfg |= TRCCONFIGR_CID;
> > >
> > > @@ -710,7 +710,7 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
> > >                        goto out;
> > >                }
> > >                /* Only trace virtual contextID when runs in root PID namespace */
>
> Ditto.
>
>    /* Only trace virtual contextID when the tool runs in root PID namespace */
>
> With above change:
>
> Reviewed-by: Leo Yan <leo.yan@xxxxxxx>
>
> > > -             if (task_is_in_init_pid_ns(current))
> > > +             if (task_is_in_init_pid_ns(event->owner))
> > >                        config->cfg |= TRCCONFIGR_VMID | TRCCONFIGR_VMIDOPT;
> > >        }
> > >
> > > --
> > > 2.34.1