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

[Leo Yan <leo.yan@arm.com>]

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 */


>>       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@arm.com>

>> -             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
>>
>>