linux-kernel - [PATCH] w1_netlink.h: Replace zero-length array with flexible-array member

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Message-ID: <20200227202332.GA15501@embeddedor>
Date:   Thu, 27 Feb 2020 14:23:32 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Evgeniy Polyakov <zbr@...emap.net>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] w1_netlink.h: 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
inadvertently 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/w1/w1_netlink.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/w1/w1_netlink.h b/drivers/w1/w1_netlink.h
index 3041092e84b3..449680a61569 100644
--- a/drivers/w1/w1_netlink.h
+++ b/drivers/w1/w1_netlink.h
@@ -73,7 +73,7 @@ struct w1_netlink_msg
 			__u32		res;
 		} mst;
 	} id;
-	__u8				data[0];
+	__u8				data[];
 };
 
 /**
@@ -122,7 +122,7 @@ struct w1_netlink_cmd
 	__u8				cmd;
 	__u8				res;
 	__u16				len;
-	__u8				data[0];
+	__u8				data[];
 };
 
 #ifdef __KERNEL__
-- 
2.25.0