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

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Message-ID: <c519631b-e02f-438c-a61f-68fa97583668@arm.com>
Date: Tue, 8 Oct 2024 10:59:33 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Leo Yan <leo.yan@....com>, Leo Yan <leo.yan@...ux.dev>,
 Julien Meunier <julien.meunier@...ia.com>
Cc: Mike Leach <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 stable@...r.kernel.org, coresight@...ts.linaro.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coresight: etm4x: Fix PID tracing when perf is run in an
 init PID namespace

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@....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
>>>
>>>