linux-kernel - Re: [PATCH v2] coresight: etm4x: Fix PID tracing when perf is run in an init PID namespace

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Message-ID: <33988463-9168-4ac5-9061-8bba6e2eda37@nokia.com>
Date: Fri, 25 Oct 2024 19:56:02 +0200
From: Julien Meunier <julien.meunier@...ia.com>
To: Suzuki K Poulose <suzuki.poulose@....com>,
 Mike Leach <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Leo Yan <leo.yan@...ux.dev>
Cc: stable@...r.kernel.org, coresight@...ts.linaro.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] coresight: etm4x: Fix PID tracing when perf is run in
 an init PID namespace

Hi Suzuki

On 21/10/2024 14:31, Suzuki K Poulose wrote:

> Hi Julien
> 
> 
[...]
> 
> Unfortunately this is not safe. i.e., event->owner is not guaranteed to
> be stable (even NULL or an invalid pointer) (e.g. kernel created events
> or task exit raced event_start on another CPU).

I expected that event->owner was safe, but you are right. During my 
tests, I essentially attached an existing PID executed in an container 
(lxc), so I was not able to detect that race condition. I miss to 
re-validate all the other usage of CoreSight.

> That said, one thing to note is that the ETM4x driver parses the event
> config in each "event_start" call back, instead of doing once during the
> event_init. If we move this to a onetime parsing at the event_init, with
> additional checks in place (e.g, !is_kernel_event()), we may be able to
> solve it.
Thanks for this suggestion, I will try this approach and provide a v3.

Regards,
Julien