linux-kernel - [PATCH] IB/mthca: Replace zero-length array with flexible-array member

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Message-ID: <20200213010851.GA13977@embeddedor.com>
Date:   Wed, 12 Feb 2020 19:08:51 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Doug Ledford <dledford@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] IB/mthca: 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/infiniband/hw/mthca/mthca_memfree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/mthca/mthca_memfree.c b/drivers/infiniband/hw/mthca/mthca_memfree.c
index 78a48aea3faf..dc10376f09cb 100644
--- a/drivers/infiniband/hw/mthca/mthca_memfree.c
+++ b/drivers/infiniband/hw/mthca/mthca_memfree.c
@@ -58,7 +58,7 @@ struct mthca_user_db_table {
 		u64                uvirt;
 		struct scatterlist mem;
 		int                refcount;
-	}                page[0];
+	} page[];
 };
 
 static void mthca_free_icm_pages(struct mthca_dev *dev, struct mthca_icm_chunk *chunk)
-- 
2.23.0