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Date:   Fri, 14 Feb 2020 11:19:07 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] Input: gpio_keys_polled - 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/input/keyboard/gpio_keys_polled.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/input/keyboard/gpio_keys_polled.c b/drivers/input/keyboard/gpio_keys_polled.c
index 6eb0a2f3f9de..c3937d2fc744 100644
--- a/drivers/input/keyboard/gpio_keys_polled.c
+++ b/drivers/input/keyboard/gpio_keys_polled.c
@@ -38,7 +38,7 @@ struct gpio_keys_polled_dev {
 	const struct gpio_keys_platform_data *pdata;
 	unsigned long rel_axis_seen[BITS_TO_LONGS(REL_CNT)];
 	unsigned long abs_axis_seen[BITS_TO_LONGS(ABS_CNT)];
-	struct gpio_keys_button_data data[0];
+	struct gpio_keys_button_data data[];
 };
 
 static void gpio_keys_button_event(struct input_dev *input,
-- 
2.25.0

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