linux-kernel - Re: [PATCH] hwmon: max31827: Fix uninitialized variable lsb_idx in max31827_init

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Message-ID: <f0ad5113-eab9-4e71-8363-1b7a005189c4@roeck-us.net>
Date: Thu, 10 Apr 2025 13:01:08 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Purva Yeshi <purvayeshi550@...il.com>
Cc: Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: max31827: Fix uninitialized variable lsb_idx in
 max31827_init_client

On Fri, Apr 11, 2025 at 01:18:33AM +0530, Purva Yeshi wrote:
> Fix Smatch-detected issue:
> drivers/hwmon/max31827.c:564 max31827_init_client() error:
> uninitialized symbol 'lsb_idx'.
> 
> In the max31827_init_client() function, the variable lsb_idx is assigned
> a value only when data has exactly one bit set (hweight32(data) == 1).
> If this condition isn't met, lsb_idx remains uninitialized, leading to
> undefined behavior when it's subsequently used.

That is not correct.

> 
> Ensure that data is non-zero and has exactly one bit set before
> calling __ffs(data) to determine lsb_idx. Additionally, verify that
> lsb_idx does not exceed 4. This approach prevents the use of an
> uninitialized lsb_idx and resolves the Smatch warning.
> 
> Signed-off-by: Purva Yeshi <purvayeshi550@...il.com>
> ---
>  drivers/hwmon/max31827.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwmon/max31827.c b/drivers/hwmon/max31827.c
> index 48e8f8ba4d05..c62eaf186d9d 100644
> --- a/drivers/hwmon/max31827.c
> +++ b/drivers/hwmon/max31827.c
> @@ -558,10 +558,13 @@ static int max31827_init_client(struct max31827_state *st,
>  		/*
>  		 * Convert the desired fault queue into register bits.
>  		 */
> -		if (data != 0)
> -			lsb_idx = __ffs(data);

lsb_idx is assigned if data != 0, not if hweight32(data) == 1 ...

> +		if (data == 0 || hweight32(data) != 1) {
> +			dev_err(dev, "Invalid data in adi,fault-q\n");
> +			return -EINVAL;
> +		}
>  
> -		if (hweight32(data) != 1 || lsb_idx > 4) {

... and if hweight32(data) != 1, it bails out here before using lsb_idx.
The problem you describe does not exist.

Guenter

> +		lsb_idx = __ffs(data);
> +		if (lsb_idx > 4) {
>  			dev_err(dev, "Invalid data in adi,fault-q\n");
>  			return -EINVAL;
>  		}
> -- 
> 2.34.1
>