linux-kernel - Re: [PATCH 3/4] coresight: etm4x: Add support to exclude kernel mode tracing

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Date:   Mon, 22 Feb 2021 12:14:23 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        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@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Leo Yan <leo.yan@...aro.org>, coresight@...ts.linaro.org,
        Stephen Boyd <swboyd@...omium.org>,
        Denis Nikitin <denik@...omium.org>,
        Mattias Nissler <mnissler@...omium.org>,
        Al Grant <al.grant@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/4] coresight: etm4x: Add support to exclude kernel mode tracing

Hi,

On Fri, Jan 29, 2021 at 11:08 AM Sai Prakash Ranjan
<saiprakash.ranjan@...eaurora.org> wrote:
>
> @@ -1202,6 +1207,13 @@ void etm4_config_trace_mode(struct etmv4_config *config)
>         /* excluding kernel AND user space doesn't make sense */
>         WARN_ON_ONCE(mode == (ETM_MODE_EXCL_KERN | ETM_MODE_EXCL_USER));
>
> +       if (!(mode & ETM_MODE_EXCL_KERN) && IS_ENABLED(CONFIG_EXCLUDE_KERNEL_HW_ITRACE)) {
> +               dev_err(&drvdata->csdev->dev,
> +                       "Kernel mode tracing is not allowed, check your kernel config\n");
> +               config->mode |= ETM_MODE_EXCL_KERN;
> +               return;

So I'm not an expert on this code, but the above looks suspicious to
me.  Specifically you are still modifying "config->mode" even though
printing an "error" (dev_err, not dev_warn) and then skipping the rest
of this function.  Since you're skipping the rest of this function
you're not applying the access, right?  Naively I'd have expected one
of these:

1. Maybe the "dev_err" should be a "dev_warn" and then you shouldn't
"return".  In this case you're just implicitly adding
"ETM_MODE_EXCL_KERN" (and shouting) but then making things work.  Of
course, then what happens if the user already specified
"ETM_MODE_EXCL_USER" too?  As per the comment above that "doesn't make
sense".  ...so maybe the code wouldn't behave properly...

2. Maybe you should be modifying this function to return an error code.

3. Maybe you should just be updating the one caller of this function
to error check this right at the beginning of the function and then
fail the sysfs write if the user did the wrong thing.  Then in
etm4_config_trace_mode you could just have a WARN_ON_ONCE if the
kernel wasn't excluded...

-Doug