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From: Jean Delvare <khali@linux-fr.org>
To: kernel-janitors@vger.kernel.org
Subject: Re: [patch] hwmon: prevent some divide by zeros in FAN_TO_REG()
Date: Thu, 05 Dec 2013 12:06:13 +0000	[thread overview]
Message-ID: <20131205130613.292abd75@endymion.delvare> (raw)
In-Reply-To: <20131205105845.GA23161@elgon.mountain>

Hi Dan,

On Thu, 5 Dec 2013 13:58:45 +0300, Dan Carpenter wrote:
> It's not enough to just test if "rpm" is zero, the "rpm * div" operation
> could overflow and that could also lead to a divide by zero.

If you believe an overflow can happen (and indeed it can) then this
isn't the way to handle it. Avoiding a divide by zero is certainly nice
but properly handling the other overflow cases too would be better.

In practice, this means for the vt8231 driver:

	if (rpm = 0 || rpm > 1310720)
		return 0;

and for the lm78 and sis5595 drivers:

	if (rpm <= 0)
		return 255;
	if (rpm > 1350000)
		return 0;

That way you're certain to never overflow (the maximum value for div is
8), and insanely large values are handled properly instead of resulting
in random register values.

Thanks,
Jean

> 
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> 
> diff --git a/drivers/hwmon/vt8231.c b/drivers/hwmon/vt8231.c
> index 0e7017841f7d..923c18034a5f 100644
> --- a/drivers/hwmon/vt8231.c
> +++ b/drivers/hwmon/vt8231.c
> @@ -145,7 +145,7 @@ static const u8 regtempmin[] = { 0x3a, 0x3e, 0x2c, 0x2e, 0x30, 0x32 };
>   */
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm = 0)
> +	if (rpm * div = 0)
>  		return 0;
>  	return clamp_val(1310720 / (rpm * div), 1, 255);
>  }
> diff --git a/drivers/hwmon/lm78.c b/drivers/hwmon/lm78.c
> index 6cf6bff79003..fc4578195674 100644
> --- a/drivers/hwmon/lm78.c
> +++ b/drivers/hwmon/lm78.c
> @@ -92,7 +92,7 @@ static inline u8 IN_TO_REG(unsigned long val)
>  
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm <= 0)
> +	if (rpm <= 0 || rpm * div = 0)
>  		return 255;
>  	return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
>  }
> diff --git a/drivers/hwmon/sis5595.c b/drivers/hwmon/sis5595.c
> index 1404e6319deb..811620fe63b4 100644
> --- a/drivers/hwmon/sis5595.c
> +++ b/drivers/hwmon/sis5595.c
> @@ -139,7 +139,7 @@ static inline u8 IN_TO_REG(unsigned long val)
>  
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm <= 0)
> +	if (rpm <= 0 || rpm * div = 0)
>  		return 255;
>  	return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
>  }

WARNING: multiple messages have this Message-ID (diff)
From: Jean Delvare <khali@linux-fr.org>
To: kernel-janitors@vger.kernel.org
Subject: Re: [lm-sensors] [patch] hwmon: prevent some divide by zeros in FAN_TO_REG()
Date: Thu, 05 Dec 2013 12:06:13 +0000	[thread overview]
Message-ID: <20131205130613.292abd75@endymion.delvare> (raw)
In-Reply-To: <20131205105845.GA23161@elgon.mountain>

Hi Dan,

On Thu, 5 Dec 2013 13:58:45 +0300, Dan Carpenter wrote:
> It's not enough to just test if "rpm" is zero, the "rpm * div" operation
> could overflow and that could also lead to a divide by zero.

If you believe an overflow can happen (and indeed it can) then this
isn't the way to handle it. Avoiding a divide by zero is certainly nice
but properly handling the other overflow cases too would be better.

In practice, this means for the vt8231 driver:

	if (rpm = 0 || rpm > 1310720)
		return 0;

and for the lm78 and sis5595 drivers:

	if (rpm <= 0)
		return 255;
	if (rpm > 1350000)
		return 0;

That way you're certain to never overflow (the maximum value for div is
8), and insanely large values are handled properly instead of resulting
in random register values.

Thanks,
Jean

> 
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> 
> diff --git a/drivers/hwmon/vt8231.c b/drivers/hwmon/vt8231.c
> index 0e7017841f7d..923c18034a5f 100644
> --- a/drivers/hwmon/vt8231.c
> +++ b/drivers/hwmon/vt8231.c
> @@ -145,7 +145,7 @@ static const u8 regtempmin[] = { 0x3a, 0x3e, 0x2c, 0x2e, 0x30, 0x32 };
>   */
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm = 0)
> +	if (rpm * div = 0)
>  		return 0;
>  	return clamp_val(1310720 / (rpm * div), 1, 255);
>  }
> diff --git a/drivers/hwmon/lm78.c b/drivers/hwmon/lm78.c
> index 6cf6bff79003..fc4578195674 100644
> --- a/drivers/hwmon/lm78.c
> +++ b/drivers/hwmon/lm78.c
> @@ -92,7 +92,7 @@ static inline u8 IN_TO_REG(unsigned long val)
>  
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm <= 0)
> +	if (rpm <= 0 || rpm * div = 0)
>  		return 255;
>  	return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
>  }
> diff --git a/drivers/hwmon/sis5595.c b/drivers/hwmon/sis5595.c
> index 1404e6319deb..811620fe63b4 100644
> --- a/drivers/hwmon/sis5595.c
> +++ b/drivers/hwmon/sis5595.c
> @@ -139,7 +139,7 @@ static inline u8 IN_TO_REG(unsigned long val)
>  
>  static inline u8 FAN_TO_REG(long rpm, int div)
>  {
> -	if (rpm <= 0)
> +	if (rpm <= 0 || rpm * div = 0)
>  		return 255;
>  	return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
>  }

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  reply	other threads:[~2013-12-05 12:06 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-12-05 10:58 [patch] hwmon: prevent some divide by zeros in FAN_TO_REG() Dan Carpenter
2013-12-05 10:58 ` [lm-sensors] " Dan Carpenter
2013-12-05 12:06 ` Jean Delvare [this message]
2013-12-05 12:06   ` Jean Delvare
2013-12-05 12:59 ` Dan Carpenter
2013-12-05 12:59   ` [lm-sensors] " Dan Carpenter
2013-12-05 13:13 ` Dan Carpenter
2013-12-05 13:13   ` [lm-sensors] " Dan Carpenter
2013-12-05 14:29 ` Jean Delvare
2013-12-05 14:29   ` [lm-sensors] " Jean Delvare
  -- strict thread matches above, loose matches on Subject: below --
2013-12-05 11:07 roger

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