linux-kernel - [PATCH][next] r8152: Replace zero-length array with flexible-array member

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Message-ID: <20200302122305.GA1200@embeddedor>
Date:   Mon, 2 Mar 2020 06:23:05 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "David S. Miller" <davem@...emloft.net>
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] r8152: 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/net/usb/r8152.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/r8152.c b/drivers/net/usb/r8152.c
index feedc0f964d8..9adfa2bbf593 100644
--- a/drivers/net/usb/r8152.c
+++ b/drivers/net/usb/r8152.c
@@ -891,7 +891,7 @@ struct fw_block {
 struct fw_header {
 	u8 checksum[32];
 	char version[RTL_VER_SIZE];
-	struct fw_block blocks[0];
+	struct fw_block blocks[];
 } __packed;
 
 /**
@@ -930,7 +930,7 @@ struct fw_mac {
 	__le32 reserved;
 	__le16 fw_ver_reg;
 	u8 fw_ver_data;
-	char info[0];
+	char info[];
 } __packed;
 
 /**
@@ -982,7 +982,7 @@ struct fw_phy_nc {
 	__le16 bp_start;
 	__le16 bp_num;
 	__le16 bp[4];
-	char info[0];
+	char info[];
 } __packed;
 
 enum rtl_fw_type {
-- 
2.25.0