Subject: Re: [PATCH net-next RFC] net: dsa: mv88e6xxx: Correct check for empty list

From: Simon Horman <[email-masked]>
Date: Tue, 23 Apr 2024 20:52:54 +0100

On Mon, Apr 22, 2024 at 01:40:01PM +0300, Dan Carpenter wrote:
> On Fri, Apr 19, 2024 at 01:17:48PM +0100, Simon Horman wrote:
> > Since commit a3c53be55c95 ("net: dsa: mv88e6xxx: Support multiple MDIO
> > busses") mv88e6xxx_default_mdio_bus() has checked that the
> > return value of list_first_entry() is non-NULL. This appears to be
> > intended to guard against the list chip->mdios being empty.
> > However, it is not the correct check as the implementation of
> > list_first_entry is not designed to return NULL for empty lists.
> > 
> > Instead check directly if the list is empty.
> > 
> > Flagged by Smatch
> > 
> > Signed-off-by: Simon Horman <[email-masked]>
> > ---
> > I'm unsure if this should be considered a fix: it's been around since
> > v4.11 and the patch is dated January 2017. Perhaps an empty list simply
> > cannot occur. If so, the function could be simplified by not checking
> > for an empty list. And, if mdio_bus->bus, then perhaps caller may be
> > simplified not to check for an error condition.
> > 
> > It is because I am unsure that I have marked this as an RFC.
> > 
> > diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
> > index e950a634a3c7..a236c9fe6a74 100644
> > --- a/drivers/net/dsa/mv88e6xxx/chip.c
> > +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> > @@ -131,10 +131,11 @@ struct mii_bus *mv88e6xxx_default_mdio_bus(struct mv88e6xxx_chip *chip)
> >  {
> >  	struct mv88e6xxx_mdio_bus *mdio_bus;
> > 
> > +	if (list_empty(&chip->mdios))
> > +		return NULL;
> > +
> >  	mdio_bus = list_first_entry(&chip->mdios, struct mv88e6xxx_mdio_bus,
> >  				    list);
> > -	if (!mdio_bus)
> > -		return NULL;
> 
> The other option here would have been to use list_first_entry_or_null().

Thanks, I guess that is nicer than the open-coded approach I suggested.