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

[Leo Yan <leo.yan@linux.dev>]

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 think we need to check using the software context switch event. With
more clear idea, I will get back at here.

Thanks,
Leo

> Cc: stable@vger.kernel.org
> Fixes: aab473867fed ("coresight: etm4x: Don't trace PID for non-root PID namespace")
> Signed-off-by: Julien Meunier <julien.meunier@nokia.com>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x-core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> 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 */
>  	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 */
> -		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
> 
>