linux-kernel - Re: [PATCH] media: camss: csiphy-3ph: avoid undefined behavior

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Message-ID: <Y5xVHU5FBr5qzAOs@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Dec 2022 12:23:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Robert Foss <robert.foss@...aro.org>,
        Todor Tomov <todor.too@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Jonathan Marek <jonathan@...ek.ca>,
        Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: camss: csiphy-3ph: avoid undefined behavior

On Thu, Dec 15, 2022 at 05:28:46PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Marking a case of the switch statement as unreachable means the
> compiler treats it as undefined behavior, which is then caught by
> an objtool warning:
> 
> drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.o: warning: objtool: csiphy_lanes_enable() falls through to next function csiphy_lanes_disable()
> 
> Instead of simply continuing execution at a random place of the
> driver, print a warning and return from to the caller, which
> makes it possible to understand what happens and avoids the
> warning.
> 
> Fixes: 53655d2a0ff2 ("media: camss: csiphy-3ph: add support for SM8250 CSI DPHY")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> index 451a4c9b3d30..04baa80494c6 100644
> --- a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> +++ b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> @@ -429,7 +429,8 @@ static void csiphy_gen2_config_lanes(struct csiphy_device *csiphy,
>  		array_size = ARRAY_SIZE(lane_regs_sm8250[0]);
>  		break;
>  	default:
> -		unreachable();
> +		WARN(1, "unknown cspi version\n");
> +		return;
>  	}

So no real objection here; but unreachable() does have an objtool
annotation inside, so clearly the compiler managed to defeat that --
perhaps we should look at that too.