Re: [PATCH net-next RFC v2] net: dsa: mv88e6xxx: Correct check for empty list

[Simon Horman <horms@kernel.org>]

On Sat, Apr 27, 2024 at 09:52:03AM +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, use list_first_entry() which does return NULL if the list is
> empty.
> 
> Flagged by Smatch.
> Compile tested only.
> 
> Signed-off-by: Simon Horman <horms@kernel.org>

Sorry, I forgot to drop the RFC tag.
I'll send a v3 after waiting for review.

> ---
> Changes in v2:
> - Use list_first_entry_or_null() instead of open-coding
>   a condition on list_empty().
>   Suggested by Dan Carpenter.
> - Update commit message.
> - Link to v1: https://lore.kernel.org/r/20240419-mv88e6xx-list_empty-v1-1-64fd6d1059a8@kernel.org
> ---
> As discussed in v1, this is not being considered a fix
> as it has been like this for a long time without any
> reported problems.
> ---
>  drivers/net/dsa/mv88e6xxx/chip.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
> index f29ef72a2f1d..fc6e2e3ab0f0 100644
> --- a/drivers/net/dsa/mv88e6xxx/chip.c
> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> @@ -131,8 +131,8 @@ struct mii_bus *mv88e6xxx_default_mdio_bus(struct mv88e6xxx_chip *chip)
>  {
>  	struct mv88e6xxx_mdio_bus *mdio_bus;
>  
> -	mdio_bus = list_first_entry(&chip->mdios, struct mv88e6xxx_mdio_bus,
> -				    list);
> +	mdio_bus = list_first_entry_or_null(&chip->mdios,
> +					    struct mv88e6xxx_mdio_bus, list);
>  	if (!mdio_bus)
>  		return NULL;
>  
> 
>