netdev - [PATCH net-next v3] net: dsa: mv88e6xxx: Correct check for empty list

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Message-Id: <20240430-mv88e6xx-list_empty-v3-1-c35c69d88d2e@kernel.org>
Date: Tue, 30 Apr 2024 18:46:45 +0100
From: Simon Horman <horms@...nel.org>
To: Andrew Lunn <andrew@...n.ch>, Florian Fainelli <f.fainelli@...il.com>, 
 Vladimir Oltean <olteanv@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Dan Carpenter <dan.carpenter@...aro.org>, 
 netdev@...r.kernel.org
Subject: [PATCH net-next v3] net: dsa: mv88e6xxx: Correct check for empty
 list

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_or_null() which does return NULL if the
list is empty.

Flagged by Smatch.
Compile tested only.

Reviewed-by: Andrew Lunn <andrew@...n.ch>
Signed-off-by: Simon Horman <horms@...nel.org>
---
Changes in v3:
- Drop RFC designation, which should have been done for v2
- Refer to fix being to use list_first_entry_or_null().
  Thanks to Dan Carpenter.
- Link to v2: https://lore.kernel.org/r/20240427-mv88e6xx-list_empty-v2-1-b7ce47c77bc7@kernel.org

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;