net: dsa: mv88e6xxx: Correct check for empty list

stable inclusion
from stable-v4.19.318
commit 47d28dde172696031c880c5778633cdca30394ee
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/IAGSOT
CVE: CVE-2024-42224

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=47d28dde172696031c880c5778633cdca30394ee



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[ Upstream commit 4c7f3950a9fd53a62b156c0fe7c3a2c43b0ba19b ]

Since commit a3c53be5 ("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@lunn.ch>
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240430-mv88e6xx-list_empty-v3-1-c35c69d88d2e@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Liu Jian <liujian56@huawei.com>