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

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Message-ID: <20200507192647.GA16710@embeddedor>
Date:   Thu, 7 May 2020 14:26:47 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Jussi Kivilinna <jussi.kivilinna@....fi>
Cc:     linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] rndis_wlan: 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/rndis_wlan.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/wireless/rndis_wlan.c b/drivers/net/wireless/rndis_wlan.c
index c8f8fe5497a8..52375f3e430a 100644
--- a/drivers/net/wireless/rndis_wlan.c
+++ b/drivers/net/wireless/rndis_wlan.c
@@ -201,7 +201,7 @@ struct ndis_80211_pmkid_candidate {
 struct ndis_80211_pmkid_cand_list {
 	__le32 version;
 	__le32 num_candidates;
-	struct ndis_80211_pmkid_candidate candidate_list[0];
+	struct ndis_80211_pmkid_candidate candidate_list[];
 } __packed;

 struct ndis_80211_status_indication {
@@ -246,12 +246,12 @@ struct ndis_80211_bssid_ex {
 	__le32 net_infra;
 	u8 rates[NDIS_802_11_LENGTH_RATES_EX];
 	__le32 ie_length;
-	u8 ies[0];
+	u8 ies[];
 } __packed;

 struct ndis_80211_bssid_list_ex {
 	__le32 num_items;
-	struct ndis_80211_bssid_ex bssid[0];
+	struct ndis_80211_bssid_ex bssid[];
 } __packed;

 struct ndis_80211_fixed_ies {
@@ -333,7 +333,7 @@ struct ndis_80211_bssid_info {
 struct ndis_80211_pmkid {
 	__le32 length;
 	__le32 bssid_info_count;
-	struct ndis_80211_bssid_info bssid_info[0];
+	struct ndis_80211_bssid_info bssid_info[];
 } __packed;

 /*