netdev - Re: [PATCH net] net: phy: fix a signedness bug in genphy

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Date: Tue, 30 May 2023 22:04:52 +0200
From: Andrew Lunn <andrew@...n.ch>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Jakub Kicinski <kuba@...nel.org>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Oleksij Rempel <linux@...pel-privat.de>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] net: phy: fix a signedness bug in genphy_loopback()

> > This is what I meant FWIW:
> > 
> > diff --git a/include/linux/phy.h b/include/linux/phy.h
> > index 7addde5d14c0..829bd57b8794 100644
> > --- a/include/linux/phy.h
> > +++ b/include/linux/phy.h
> > @@ -1206,10 +1206,13 @@ static inline int phy_read(struct phy_device *phydev, u32 regnum)
> >  #define phy_read_poll_timeout(phydev, regnum, val, cond, sleep_us, \
> >  				timeout_us, sleep_before_read) \
> >  ({ \
> > -	int __ret = read_poll_timeout(phy_read, val, val < 0 || (cond), \
> > +	int __ret, __val;						\
> > +									\
> > +	__ret = read_poll_timeout(phy_read, __val, __val < 0 || (cond),	\
> >  		sleep_us, timeout_us, sleep_before_read, phydev, regnum); \
> > -	if (val < 0) \
> > -		__ret = val; \
> > +	val = __val;

This results in the sign being discarded if val is unsigned. Yes, the
test is remove, which i assume will stop Smatch complaining, but it is
still broken.

> > +	if (__val < 0) \
> > +		__ret = __val; \
> >  	if (__ret) \
> >  		phydev_err(phydev, "%s failed: %d\n", __func__, __ret); \
> >  	__ret; \

> > I tried enabling -Wtype-limits but it's _very_ noisy :(

This is a no go until GENMASK gets fixed :-(

However, if that is fixed, we might be able to turn it on. But it will
then trigger with this fix.

So i still think a BUILD_BUG_ON is a better fix. Help developers get
the code correct, rather than work around them getting it wrong.

I also wonder if this needs to go down a level. Dan, how often do you
see similar problems with the lower level read_poll_timeout() and
friends?

    Andrew