linux-kernel - [PATCH] Bluetooth: btintel: Replace zero-length array with flexible-array member

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Message-ID: <20200212192807.GA25300@embeddedor>
Date:   Wed, 12 Feb 2020 13:28:07 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>
Cc:     linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] Bluetooth: btintel: Replace zero-length array with
 flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
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 in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertenly introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 drivers/bluetooth/btintel.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/bluetooth/btintel.c b/drivers/bluetooth/btintel.c
index 62e781a18bf0..6a0e2c5a8beb 100644
--- a/drivers/bluetooth/btintel.c
+++ b/drivers/bluetooth/btintel.c
@@ -376,13 +376,13 @@ struct ibt_cp_reg_access {
 	__le32  addr;
 	__u8    mode;
 	__u8    len;
-	__u8    data[0];
+	__u8    data[];
 } __packed;
 
 struct ibt_rp_reg_access {
 	__u8    status;
 	__le32  addr;
-	__u8    data[0];
+	__u8    data[];
 } __packed;
 
 static int regmap_ibt_read(void *context, const void *addr, size_t reg_size,
-- 
2.25.0