linux-kernel - Re: [PATCH] perf test: Add test case for event group throttling with inactive events

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Message-ID: <a40f008e-d552-4ce2-8279-5a25183b7726@kzalloc.com>
Date: Fri, 15 Aug 2025 05:23:44 +0900
From: Yunseong Kim <ysk@...lloc.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Liang Kan <kan.liang@...ux.intel.com>, James Clark <james.clark@...aro.org>,
 Collin Funk <collin.funk1@...il.com>, Ravi Bangoria <ravi.bangoria@....com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf test: Add test case for event group throttling with
 inactive events

Hi Namhyung,

Thanks for your reply.

On 8/15/25 5:06 AM, Namhyung Kim wrote:
> Hi Yunseong,
> 
> On Wed, Aug 13, 2025 at 04:32:52PM +0000, Yunseong Kim wrote:
>> A recent UBSAN shift-out-of-bounds report was identified when throttling
>> event groups that included inactive (PERF_EVENT_STATE_OFF) events.
>> This occurred because pmu->start()/stop() could be called on these events,
>> potentially leaving event->hw.idx at -1. This leads to undefined behavior
>> when PMU code later uses this negative index as a shift exponent.
>>
>> The issue need to ensuring perf_event_throttle() and
>> perf_event_unthrottle() skip inactive events entirely.
>>
>> Introduce a new perf test suite, "event group throttle", to verify this
>> fix and prevent regressions.
> 
> Thanks for your work!
> 
> I'm curious what happens if it runs on unfixed kernels.  I suspect this
> should be in the selftest so that it can tie with the kernel fix.

I agree with your point. I believe it's more appropriate for this test to
be included in "kself test" alongside the kernel fix. I'll modify the
test case based on Liang's suggestion.

> By having this in the perf tools test suite, you cannot guarantee if the
> kernel has the fix.  And it may turn the kernel into an unstable state
> easily.

Would it be possible to add this test if it were changed to focus on
a different aspect?

> Thanks,
> Namhyung

Thank you,
Yunseong Kim