linux-kernel - [PATCH 4.9 108/128] cxgb3/l2t: Fix undefined behaviour

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Message-Id: <20190530030453.994763760@linuxfoundation.org>
Date:   Wed, 29 May 2019 20:07:20 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.9 108/128] cxgb3/l2t: Fix undefined behaviour

[ Upstream commit 76497732932f15e7323dc805e8ea8dc11bb587cf ]

The use of zero-sized array causes undefined behaviour when it is not
the last member in a structure. As it happens to be in this case.

Also, the current code makes use of a language extension to the C90
standard, but the preferred mechanism to declare variable-length
types such as this one is a flexible array member, introduced in
C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last. Which is beneficial
to cultivate a high-quality code.

Fixes: e48f129c2f20 ("[SCSI] cxgb3i: convert cdev->l2opt to use rcu to prevent NULL dereference")
Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/net/ethernet/chelsio/cxgb3/l2t.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/chelsio/cxgb3/l2t.h b/drivers/net/ethernet/chelsio/cxgb3/l2t.h
index 8cffcdfd56782..38b5858c335a9 100644
--- a/drivers/net/ethernet/chelsio/cxgb3/l2t.h
+++ b/drivers/net/ethernet/chelsio/cxgb3/l2t.h
@@ -75,8 +75,8 @@ struct l2t_data {
 	struct l2t_entry *rover;	/* starting point for next allocation */
 	atomic_t nfree;		/* number of free entries */
 	rwlock_t lock;
-	struct l2t_entry l2tab[0];
 	struct rcu_head rcu_head;	/* to handle rcu cleanup */
+	struct l2t_entry l2tab[];
 };
 
 typedef void (*arp_failure_handler_func)(struct t3cdev * dev,
-- 
2.20.1