linux-kernel - Re: [PATCH] hwmon: (w83l786ng) Convert macros to functions to avoid TOCTOU

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Message-ID: <20251129101736.000fac82@pumpkin>
Date: Sat, 29 Nov 2025 10:17:36 +0000
From: david laight <david.laight@...box.com>
To: Gui-Dong Han <hanguidong02@...il.com>
Cc: linux@...ck-us.net, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org, baijiaju1990@...il.com,
 stable@...r.kernel.org
Subject: Re: [PATCH] hwmon: (w83l786ng) Convert macros to functions to avoid
 TOCTOU

On Sat, 29 Nov 2025 08:33:42 +0800
Gui-Dong Han <hanguidong02@...il.com> wrote:

> On Sat, Nov 29, 2025 at 3:37 AM david laight <david.laight@...box.com> wrote:
> >
> > On Fri, 28 Nov 2025 20:38:16 +0800
> > Gui-Dong Han <hanguidong02@...il.com> wrote:
> >  
> > > The macros FAN_FROM_REG and TEMP_FROM_REG evaluate their arguments
> > > multiple times. When used in lockless contexts involving shared driver
> > > data, this causes Time-of-Check to Time-of-Use (TOCTOU) race
> > > conditions.
> > >
> > > Convert the macros to static functions. This guarantees that arguments
> > > are evaluated only once (pass-by-value), preventing the race
> > > conditions.
> > >
> > > Adhere to the principle of minimal changes by only converting macros
> > > that evaluate arguments multiple times and are used in lockless
> > > contexts.
> > >
> > > Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@mail.gmail.com/
> > > Fixes: 85f03bccd6e0 ("hwmon: Add support for Winbond W83L786NG/NR")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Gui-Dong Han <hanguidong02@...il.com>
> > > ---
> > > Based on the discussion in the link, I will submit a series of patches to
> > > address TOCTOU issues in the hwmon subsystem by converting macros to
> > > functions or adjusting locking where appropriate.
> > > ---
> > >  drivers/hwmon/w83l786ng.c | 26 ++++++++++++++++++--------
> > >  1 file changed, 18 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/hwmon/w83l786ng.c b/drivers/hwmon/w83l786ng.c
> > > index 9b81bd406e05..1d9109ca1585 100644
> > > --- a/drivers/hwmon/w83l786ng.c
> > > +++ b/drivers/hwmon/w83l786ng.c
> > > @@ -76,15 +76,25 @@ FAN_TO_REG(long rpm, int div)
> > >       return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
> > >  }
> > >
> > > -#define FAN_FROM_REG(val, div)       ((val) == 0   ? -1 : \
> > > -                             ((val) == 255 ? 0 : \
> > > -                             1350000 / ((val) * (div))))
> > > +static int fan_from_reg(int val, int div)
> > > +{
> > > +     if (val == 0)
> > > +             return -1;
> > > +     if (val == 255)
> > > +             return 0;
> > > +     return 1350000 / (val * div);
> > > +}
> > >
> > >  /* for temp */
> > >  #define TEMP_TO_REG(val)     (clamp_val(((val) < 0 ? (val) + 0x100 * 1000 \
> > >                                                     : (val)) / 1000, 0, 0xff))  
> >
> > Can you change TEMP_TO_REG() as well.
> > And just use plain clamp() while you are at it.
> > Both these temperature conversion functions have to work with negative temperatures.
> > But the signed-ness gets passed through from the parameter - which may not be right.
> > IIRC some come from FIELD_GET() and will be 'unsigned long' unless cast somewhere.
> > The function parameter 'corrects' the type to a signed one.
> >
> > So you are fixing potential bugs as well.  
> 
> Hi David,
> 
> Thanks for your feedback on TEMP_TO_REG and the detailed explanation
> regarding macro risks.
> 
> Guenter has already applied this patch.

Patches are supposed to be posted for review and applied a few days later.

> Since the primary scope here
> was strictly addressing TOCTOU race conditions (and TEMP_TO_REG is not
> used in lockless contexts), it wasn't included.
> 
> However, I appreciate your point regarding type safety. I will look
> into addressing that in a future separate patch.

It's not just type safety, and #define that evaluates an argument more
than one is just a bug waiting to happen.
We've been removing (or trying not to write) those since the 1980s.

You also just didn't read the code:

-#define TEMP_FROM_REG(val)	(((val) & 0x80 ? \
-				  (val) - 0x100 : (val)) * 1000)
+
+static int temp_from_reg(int val)
+{
+	if (val & 0x80)
+		return (val - 0x100) * 1000;
+	return val * 1000;
+}

Both those only work if 'val' is 8 bits.
They are just ((s8)(val) * 1000) and generate a milli-centigrade
value from the 8-bit signed centigrade value the hardware provides.

TEMP_TO_REG() is the opposite, so is:
	(u8)clamp((val) / 1000, -128, 127)
That is subtly different since it truncates negative values towards
zero rather than -infinity.
The more complicated:
		(u8)(clamp(((val) + 128 * 1000)/1000, 0, 0xff) - 128)
would exactly match the original (and generate less code) but I suspect
it doesn't matter here.

	David

> 
> Best regards,
> Gui-Dong Han
>