linux-kernel - [PATCH] carl9170: Replace zero-length array with flexible-array

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Message-ID: <20200507185132.GA14046@embeddedor>
Date:   Thu, 7 May 2020 13:51:32 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Christian Lamparter <chunkeey@...glemail.com>
Cc:     linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] carl9170: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 <gustavoars@...nel.org>
---
 drivers/net/wireless/ath/carl9170/fwcmd.h |    2 +-
 drivers/net/wireless/ath/carl9170/hw.h    |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/ath/carl9170/fwcmd.h b/drivers/net/wireless/ath/carl9170/fwcmd.h
index ea1d80f9a50e..56999a3b9d3b 100644
--- a/drivers/net/wireless/ath/carl9170/fwcmd.h
+++ b/drivers/net/wireless/ath/carl9170/fwcmd.h
@@ -127,7 +127,7 @@ struct carl9170_write_reg {
 struct carl9170_write_reg_byte {
 	__le32	addr;
 	__le32  count;
-	u8	val[0];
+	u8	val[];
 } __packed;

 #define	CARL9170FW_PHY_HT_ENABLE		0x4
diff --git a/drivers/net/wireless/ath/carl9170/hw.h b/drivers/net/wireless/ath/carl9170/hw.h
index 08e0ae9c5836..555ad4975970 100644
--- a/drivers/net/wireless/ath/carl9170/hw.h
+++ b/drivers/net/wireless/ath/carl9170/hw.h
@@ -851,7 +851,7 @@ struct ar9170_stream {
 	__le16 length;
 	__le16 tag;

-	u8 payload[0];
+	u8 payload[];
 } __packed __aligned(4);
 #define AR9170_STREAM_LEN				4