linux-kernel - Re: [PATCH] firmware: arm_scmi: avoid returning uninialized data

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Message-ID: <Zc-ZfgVbAvAZVrPu@pluto>
Date: Fri, 16 Feb 2024 17:21:02 +0000
From: Cristian Marussi <cristian.marussi@....com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Sudeep Holla <sudeep.holla@....com>, Arnd Bergmann <arnd@...db.de>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH] firmware: arm_scmi: avoid returning uninialized data

On Fri, Feb 16, 2024 at 05:32:53PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Clang notices that there is a code path through
> scmi_powercap_notify_supported() that returns an
> undefined value:
> 

Hi Arnd,

> drivers/firmware/arm_scmi/powercap.c:821:11: error: variable 'supported' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>   821 |         else if (evt_id == SCMI_EVENT_POWERCAP_MEASUREMENTS_CHANGED)
>       |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/firmware/arm_scmi/powercap.c:824:9: note: uninitialized use occurs here
>   824 |         return supported;
>       |                ^~~~~~~~~
> drivers/firmware/arm_scmi/powercap.c:821:7: note: remove the 'if' if its condition is always true
>   821 |         else if (evt_id == SCMI_EVENT_POWERCAP_MEASUREMENTS_CHANGED)
>       |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   822 |                 supported = dom_info->notify_powercap_measurement_change;
> drivers/firmware/arm_scmi/powercap.c:811:16: note: initialize the variable 'supported' to silence this warning
>   811 |         bool supported;
>       |                       ^
> 
> Return 'false' here, which is probably what was intended.
> 
> Fixes: c92a75fe84ce ("firmware: arm_scmi: Implement Powercap .is_notify_supported callback")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

thanks for looking at this, this series that I've just posted is still
to be reviewd at all, so I would expect issues :D...BUT in this case I
dont think that the clang report is valid since, inside the culprit
function scmi_powercap_notify_supported(), a few lines before the
reported usage of unitialized data there is a check (@line 816) on the
'bounds' of evt_id itself

	if (evt_id >= ARRAY_SIZE(evt_2_cmd) || src_id >= pi->num_domains)
		return false;

so basically the mentioned if/else WILL be evaluated in some of its
branches for sure and supported wont be uninitialized.

Indeed, I removed from here (and from all the series) the explicit
initialization at definition time right before posting the series.

Having saidm that...maybe it is just brain-dead this approach of mine
since it is able to fool clang & friends...I would add bACK an explicit
initialization of supported all across this series in V2, if this
sounds good to you.

Thanks,
Cristian


> ---
>  drivers/firmware/arm_scmi/powercap.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/firmware/arm_scmi/powercap.c b/drivers/firmware/arm_scmi/powercap.c
> index aae91f47303e..8ee3be8776b0 100644
> --- a/drivers/firmware/arm_scmi/powercap.c
> +++ b/drivers/firmware/arm_scmi/powercap.c
> @@ -820,6 +820,8 @@ scmi_powercap_notify_supported(const struct scmi_protocol_handle *ph,
>  		supported = dom_info->notify_powercap_cap_change;
>  	else if (evt_id == SCMI_EVENT_POWERCAP_MEASUREMENTS_CHANGED)
>  		supported = dom_info->notify_powercap_measurement_change;
> +	else
> +		supported = false;
>  
>  	return supported;
>  }
> -- 
> 2.39.2
>