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

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Message-ID: <20200212194602.GA31712@embeddedor>
Date:   Wed, 12 Feb 2020 13:46:02 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Joshua Morris <josh.h.morris@...ibm.com>,
        Philip Kelleher <pjk1939@...ux.ibm.com>,
        Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] rsxx: 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
inadvertenly 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/block/rsxx/dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/rsxx/dma.c b/drivers/block/rsxx/dma.c
index 111eb659e66d..1914f5488b22 100644
--- a/drivers/block/rsxx/dma.c
+++ b/drivers/block/rsxx/dma.c
@@ -80,7 +80,7 @@ struct dma_tracker {
 struct dma_tracker_list {
 	spinlock_t		lock;
 	int			head;
-	struct dma_tracker	list[0];
+	struct dma_tracker	list[];
 };
 
 
-- 
2.25.0