linux-kernel - Re: [PATCH v8 04/10] drivers: qcom: rpmh: add RPMH helper functions

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Date:   Fri, 11 May 2018 13:17:14 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Lina Iyer <ilina@...eaurora.org>
Cc:     Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        linux-arm-msm@...r.kernel.org,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@...r.kernel.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Evan Green <evgreen@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>, rplsssn@...eaurora.org
Subject: Re: [PATCH v8 04/10] drivers: qcom: rpmh: add RPMH helper functions

Hi,

On Wed, May 9, 2018 at 10:01 AM, Lina Iyer <ilina@...eaurora.org> wrote:
> +int rpmh_write(const struct device *dev, enum rpmh_state state,
> +              const struct tcs_cmd *cmd, u32 n)
> +{
> +       DECLARE_COMPLETION_ONSTACK(compl);
> +       DEFINE_RPMH_MSG_ONSTACK(dev, state, &compl, rpm_msg);
> +       int ret;
> +
> +       if (!cmd || !n || n > MAX_RPMH_PAYLOAD)
> +               return -EINVAL;
> +
> +       memcpy(rpm_msg.cmd, cmd, n * sizeof(*cmd));
> +       rpm_msg.msg.num_cmds = n;
> +
> +       ret = __rpmh_write(dev, state, &rpm_msg);
> +       if (ret)
> +               return ret;
> +
> +       ret = wait_for_completion_timeout(&compl, RPMH_TIMEOUT_MS);

IMO it's almost never a good idea to use wait_for_completion_timeout()
together with a completion that's declared on the stack.  If you
somehow insist that this is a good idea then I need to see incredibly
clear and obvious code/comments that say why it's impossible that the
process might somehow try to signal the completion _after_
RPMH_TIMEOUT_MS has expired.

Specifically if the timeout happens but the process could still signal
a completion later then they will access random data on the stack of a
function that has already returned.  This causes ridiculously
difficult-to-debug crashes.

NOTE: You've got timeout set to 10 seconds here.  Is that really even
useful?  IMO just call wait_for_completion() without a timeout.  It's
much better to have a nice clean hang than a random stack corruption.

-Doug