linux-kernel - Re: [PATCH] firmware: arm_scpi: prevent ternary sign expansion bug

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Message-ID: <20210422101709.GF43717@e120937-lin>
Date:   Thu, 22 Apr 2021 11:17:09 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        "Jon Medhurst (Tixy)" <tixy@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] firmware: arm_scpi: prevent ternary sign expansion bug

Hi,

On Thu, Apr 22, 2021 at 12:02:29PM +0300, Dan Carpenter wrote:
> How type promotion works in ternary expressions is a bit tricky.
> The problem is that scpi_clk_get_val() returns longs, "ret" is a int
> which holds a negative error code, and le32_to_cpu() is an unsigned int.
> We want the negative error code to be cast to a negative long.  But
> because le32_to_cpu() is an u32 then "ret" is type promoted to u32 and
> becomes a high positive and then it is promoted to long and it is still
> a high positive value.
> 
> Fix this by getting rid of the ternary.

I wonder how/if the callers up in the stack check/expect ever effectively for a
2-complement negative value inside the returned unsigned long...given that this
plugs finally into CLK framework struct clk_ops.recalc_rate via clk-scpi.c which
also expects unsigned long....but that's another story.

FWIW regarding this patch:

Reviewed-by: Cristian Marussi <cristian.marussi@....com>

Thanks

Cristian

> 
> Fixes: 8cb7cf56c9fe ("firmware: add support for ARM System Control and Power Interface(SCPI) protocol")
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>  drivers/firmware/arm_scpi.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/arm_scpi.c b/drivers/firmware/arm_scpi.c
> index d0dee37ad522..3bf61854121d 100644
> --- a/drivers/firmware/arm_scpi.c
> +++ b/drivers/firmware/arm_scpi.c
> @@ -552,8 +552,10 @@ static unsigned long scpi_clk_get_val(u16 clk_id)
>  
>  	ret = scpi_send_message(CMD_GET_CLOCK_VALUE, &le_clk_id,
>  				sizeof(le_clk_id), &rate, sizeof(rate));
> +	if (ret)
> +		return ret;
>  
> -	return ret ? ret : le32_to_cpu(rate);
> +	return le32_to_cpu(rate);
>  }
>  
>  static int scpi_clk_set_val(u16 clk_id, unsigned long rate)
> -- 
> 2.30.2
>