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

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Message-ID: <d667834a-476b-4f33-9c94-10b3672b6edb@moroto.mountain>
Date: Mon, 22 Apr 2024 13:40:01 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Simon Horman <horms@...nel.org>
Cc: Andrew Lunn <andrew@...n.ch>, Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next RFC] net: dsa: mv88e6xxx: Correct check for
 empty list

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 <horms@...nel.org>
> ---
> 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().

regards,
dan carpenter