linux-kernel - [PATCH][next] chrome: wilco_ec: event: Replace zero-length array with flexible-array member

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Message-ID: <20200320231634.GA20040@embeddedor.com>
Date:   Fri, 20 Mar 2020 18:16:34 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] chrome: wilco_ec: event: 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/platform/chrome/wilco_ec/event.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/platform/chrome/wilco_ec/event.c b/drivers/platform/chrome/wilco_ec/event.c
index dba3d445623f..814518509739 100644
--- a/drivers/platform/chrome/wilco_ec/event.c
+++ b/drivers/platform/chrome/wilco_ec/event.c
@@ -79,7 +79,7 @@ static DEFINE_IDA(event_ida);
 struct ec_event {
 	u16 size;
 	u16 type;
-	u16 event[0];
+	u16 event[];
 } __packed;
 
 #define ec_event_num_words(ev) (ev->size - 1)
@@ -96,7 +96,7 @@ struct ec_event_queue {
 	int capacity;
 	int head;
 	int tail;
-	struct ec_event *entries[0];
+	struct ec_event *entries[];
 };
 
 /* Maximum number of events to store in ec_event_queue */
-- 
2.23.0