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

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Message-ID: <20200302210437.GA30285@embeddedor>
Date:   Mon, 2 Mar 2020 15:04:37 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "mlxsw@...lanox.com David S. Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] net: mlxfw: 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/ethernet/mellanox/mlxfw/mlxfw_mfa2.c     | 2 +-
 drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.c b/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.c
index 79057af4fe99..5d9ddf36fb4e 100644
--- a/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.c
+++ b/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2.c
@@ -496,7 +496,7 @@ mlxfw_mfa2_file_component_tlv_get(const struct mlxfw_mfa2_file *mfa2_file,
 
 struct mlxfw_mfa2_comp_data {
 	struct mlxfw_mfa2_component comp;
-	u8 buff[0];
+	u8 buff[];
 };
 
 static const struct mlxfw_mfa2_tlv_component_descriptor *
diff --git a/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv.h b/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv.h
index 33c971190bba..2014a5de5a01 100644
--- a/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv.h
+++ b/drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv.h
@@ -11,7 +11,7 @@ struct mlxfw_mfa2_tlv {
 	u8 version;
 	u8 type;
 	__be16 len;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 static inline const struct mlxfw_mfa2_tlv *
-- 
2.25.0