linux-kernel - RE: [PATCH] coresight: etm3x: Fix buffer overwrite in cntr_val

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Message-ID:
 <GV1PR08MB1093215C7FBF6603DBA2CD8A886DCA@GV1PR08MB10932.eurprd08.prod.outlook.com>
Date: Fri, 28 Nov 2025 15:14:08 +0000
From: Al Grant <Al.Grant@....com>
To: James Clark <james.clark@...aro.org>, Leo Yan <Leo.Yan@....com>, Mike
 Leach <mike.leach@...aro.org>
CC: Kuan-Wei Chiu <visitorckw@...il.com>, "alexander.shishkin@...ux.intel.com"
	<alexander.shishkin@...ux.intel.com>, "pratikp@...eaurora.org"
	<pratikp@...eaurora.org>, "mathieu.poirier@...aro.org"
	<mathieu.poirier@...aro.org>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "jserv@...s.ncku.edu.tw"
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	<milesjiang@...gle.com>, "nickpan@...gle.com" <nickpan@...gle.com>,
	"coresight@...ts.linaro.org" <coresight@...ts.linaro.org>,
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	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] coresight: etm3x: Fix buffer overwrite in cntr_val_show()

> Based on the the build system I don't think what the hardware supports matters
> that much. We've always only built the ETM4 driver on Arm64 and only build the
> ETM3 driver on Arm32.

ETMv3/PTM don't support Arm64, so a trace driver in an Arm64 kernel
will only need to drive  ETMv4 (or ETE). There's no need to build
ETMv3/PTM in 64-bit kernels.

But a 32-bit kernel could be running on a CPU with ETMv4 - this could
be a legacy 32-bit kernel on a 64-bit-capable CPU, but there are pure
32-bit CPUs with ETMv4: Cortex-A32 is a 32-bit-only Armv8-A CPU with
ETMv4 and all kernels on this CPU will be 32-bit. If the 32-bit kernel only
builds the ETMv3/PTM driver, then it won't support trace on this CPU.

Al


> 
> I just found another bug which I think makes sense to log here. We've pretty
> much always turned on timestamps automatically in Perf, but for some reason the
> option validator in Perf thinks they're not supported in ETM3. The driver has
> always accepted it as an option so I'm not sure why. The end result is that the
> command will always fail, and you couldn't even undo it with "timestamp=0" until
> more recently when I fixed that.
> 
> Sysfs or per-thread Perf mode would be working, but it's one more bit of evidence
> to take in.
> 
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