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Thu, 27 Nov 2025 00:44:58 -0800 (PST) Date: Thu, 27 Nov 2025 16:44:53 +0800 From: Kuan-Wei Chiu To: James Clark Cc: Mike Leach , suzuki.poulose@arm.com, alexander.shishkin@linux.intel.com, pratikp@codeaurora.org, mathieu.poirier@linaro.org, gregkh@linuxfoundation.org, jserv@ccns.ncku.edu.tw, marscheng@google.com, ericchancf@google.com, milesjiang@google.com, nickpan@google.com, coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] coresight: etm3x: Fix buffer overwrite in cntr_val_show() Message-ID: References: <20251121002350.1166758-1-visitorckw@gmail.com> <172ca2d9-4a6f-4498-bdfd-8aa7428581ce@linaro.org> <05babb6d-a588-49f8-a34a-c82d5f58adf5@linaro.org> <68efd5a2-3b65-4f37-9bf0-40c4e5ade480@linaro.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <68efd5a2-3b65-4f37-9bf0-40c4e5ade480@linaro.org> On Wed, Nov 26, 2025 at 10:57:23AM +0000, James Clark wrote: > > > On 26/11/2025 10:49 am, Mike Leach wrote: > > Hi, > > > > On Mon, 24 Nov 2025 at 16:12, James Clark wrote: > > > > > > > > > > > > On 21/11/2025 5:02 pm, Kuan-Wei Chiu wrote: > > > > Hi James, > > > > > > > > On Fri, Nov 21, 2025 at 09:50:03AM +0000, James Clark wrote: > > > > > > > > > > > > > > > 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 > > > > > > --- > > > > > > 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. > > > > > > > > Thanks for your feedback. > > > > > > > > I agree that if any tool relies on the current behavior, this patch > > > > would break the ABI and violate the hard rule that we must never break > > > > userspace. > > > > > > > > However, I am not sure how to determine if any userspace tools are > > > > actually using this sysfs interface. Is there a recommended way to > > > > verify this, or a standard procedure/convention to follow in this > > > > situation? > > > > > > > > Regards, > > > > Kuan-Wei > > > > > > > > > > There's no way of knowing apart from deducing that there are 0 users of > > > the 'correct' version of the API because it never existed. This isn't > > > even a regression, it was broken from the beginning. > > > > > > I suppose it does work when hardware only has one counter, but I don't > > > know how likely that is? > > > > > > Looking at cntr_val_store() I think I was wrong before when I said it > > > should be a separate file for each counter. Writing to it already writes > > > to the counter currently selected by cntr_idx and splitting it out to > > > separate files would have to break that too. So we can fix the display > > > bug by making show operate on the currently selected counter in the same > > > way as store. Then it makes a bit more sense and we can delete the > > > "counter %d: " prefix. > > > > > > ETM4 already does it this way too. > > > > > I agree with this. > > > > The difficulty in having a file per counter is that different > > implementations have different numbers of counters. Rather than > > complicate the driver by dynamically generating the sysfs files, the > > It wouldn't be that hard because there is a maximum number of counters. > You'd generate them all and then use the is_visible() thing to hide ones > that didn't exist on that device. > > But it looks like we don't want to do it that way for other reasons anyway. > I don't feel it is my place to say whether the etm3x driver should be removed entirely. However, if we decide to keep it, I agree that aligning cntr_val_show with the cntr_val_store behavior (using cntr_idx) makes more sense. Here is my plan for v2: diff --git a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c index 762109307b86..77578885e8f3 100644 --- a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c +++ b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c @@ -717,26 +717,19 @@ static DEVICE_ATTR_RW(cntr_rld_event); static ssize_t cntr_val_show(struct device *dev, struct device_attribute *attr, char *buf) { - int i, ret = 0; u32 val; struct etm_drvdata *drvdata = dev_get_drvdata(dev->parent); struct etm_config *config = &drvdata->config; 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", - i, config->cntr_val[i]); + val = config->cntr_val[config->cntr_idx]; spin_unlock(&drvdata->spinlock); - return ret; - } - - for (i = 0; i < drvdata->nr_cntr; i++) { - val = etm_readl(drvdata, ETMCNTVRn(i)); - ret += sprintf(buf, "counter %d: %x\n", i, val); + return sprintf(buf, "%x\n", val); } - return ret; + val = etm_readl(drvdata, ETMCNTVRn(config->cntr_idx)); + return sprintf(buf, "%x\n", val); } static ssize_t cntr_val_store(struct device *dev, Given the upcoming merge window, I plan to submit this v2 after -rc1 is released. Alternatively, if the consensus is to drop the driver, I am happy to submit a patch for that instead. Regards, Kuan-Wei > > single file + selector paradigm makes things easier. The index is > > validated against the actual number of counters for this instance, and > > read/write occurs for the selected one. > > > > Regards > > > > Mike > > > > > > > > > > > > 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; > > > > > > > > > > > > >