linux-kernel - [PATCH][next] mtd: spi-nor: controllers: aspeed-smc: Replace zero-length array with flexible-array member

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Date:   Thu, 19 Mar 2020 17:43:26 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Joel Stanley <joel@....id.au>, Andrew Jeffery <andrew@...id.au>
Cc:     linux-mtd@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] mtd: spi-nor: controllers: aspeed-smc: 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/mtd/spi-nor/controllers/aspeed-smc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/spi-nor/controllers/aspeed-smc.c b/drivers/mtd/spi-nor/controllers/aspeed-smc.c
index 395127349aa8..e26a1897db0e 100644
--- a/drivers/mtd/spi-nor/controllers/aspeed-smc.c
+++ b/drivers/mtd/spi-nor/controllers/aspeed-smc.c
@@ -109,7 +109,7 @@ struct aspeed_smc_controller {
 	void __iomem *ahb_base;			/* per-chip windows resource */
 	u32 ahb_window_size;			/* full mapping window size */
 
-	struct aspeed_smc_chip *chips[0];	/* pointers to attached chips */
+	struct aspeed_smc_chip *chips[];	/* pointers to attached chips */
 };
 
 /*
-- 
2.23.0