Re: [PATCH] coresight: etm3x: Fix buffer overwrite in cntr_val_show()

[James Clark <james.clark@linaro.org>]

On 21/11/2025 12:23 am, Kuan-Wei Chiu wrote:
> The cntr_val_show() function is meant to display the values of all
> available counters. However, the sprintf() call inside the loop was
> always writing to the beginning of the buffer, causing the output of
> previous iterations to be overwritten. As a result, only the value of
> the last counter was actually returned to the user.
> 
> Fix this by using the return value of sprintf() to calculate the
> correct offset into the buffer for the next write, ensuring that all
> counter values are appended sequentially.
> 
> Fixes: a939fc5a71ad ("coresight-etm: add CoreSight ETM/PTM driver")
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
> Build tested only. I do not have the hardware to run the etm3x driver,
> so I would be grateful if someone could verify this on actual hardware.
> 
> I noticed this issue while browsing the coresight code after attending
> a technical talk on the subject. This code dates back to the initial
> driver submission over 10 years ago, so I was surprised it hadn't been
> caught earlier. Although I cannot perform runtime testing, the logic
> error seems obvious to me, so I still decided to submit this patch.

Nice find. I think the point that it wasn't caught changes how we fix 
it. Either nobody used it ever - so we can just delete it. Or someone 
was using it and they expect it to always return a single entry with the 
value of the last counter and this is a potentially breaking change. So 
maybe instead of fixing this we should add a new cntr_vals_show() which 
works correctly. But then again if nobody is using it we shouldn't do 
that either.

The interface isn't even that great, it should be a separate file per 
counter. You don't want to be parsing strings and colons to try to read 
a single value, especially in C. Separate files allows you to read it 
directly without any hassle.

> 
>   drivers/hwtracing/coresight/coresight-etm3x-sysfs.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> index 762109307b86..312033e74b7a 100644
> --- a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> +++ b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
> @@ -725,7 +725,7 @@ static ssize_t cntr_val_show(struct device *dev,
>   	if (!coresight_get_mode(drvdata->csdev)) {
>   		spin_lock(&drvdata->spinlock);
>   		for (i = 0; i < drvdata->nr_cntr; i++)
> -			ret += sprintf(buf, "counter %d: %x\n",
> +			ret += sprintf(buf + ret, "counter %d: %x\n",
>   				       i, config->cntr_val[i]);
>   		spin_unlock(&drvdata->spinlock);
>   		return ret;
> @@ -733,7 +733,7 @@ static ssize_t cntr_val_show(struct device *dev,
>   
>   	for (i = 0; i < drvdata->nr_cntr; i++) {
>   		val = etm_readl(drvdata, ETMCNTVRn(i));
> -		ret += sprintf(buf, "counter %d: %x\n", i, val);
> +		ret += sprintf(buf + ret, "counter %d: %x\n", i, val);
>   	}
>   
>   	return ret;