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

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Message-ID: <20200221153713.GA23760@embeddedor>
Date:   Fri, 21 Feb 2020 09:37:13 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Khalid Aziz <khalid@...ehiking.org>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] pcdp: 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/firmware/pcdp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/pcdp.h b/drivers/firmware/pcdp.h
index ce75d1da9e84..e02540571c52 100644
--- a/drivers/firmware/pcdp.h
+++ b/drivers/firmware/pcdp.h
@@ -103,6 +103,6 @@ struct pcdp {
 	u8			creator_id[4];
 	u32			creator_rev;
 	u32			num_uarts;
-	struct pcdp_uart	uart[0];	/* actual size is num_uarts */
+	struct pcdp_uart	uart[];	/* actual size is num_uarts */
 	/* remainder of table is pcdp_device structures */
 } __attribute__((packed));
-- 
2.25.0