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

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Message-ID: <20200211222941.GA7657@embeddedor>
Date:   Tue, 11 Feb 2020 16:29:41 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Amit Shah <amit@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] char: virtio: 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.

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/char/virtio_console.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index fbeb71953526..a136239e4c14 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -125,7 +125,7 @@ struct port_buffer {
 	unsigned int sgpages;
 
 	/* sg is used if spages > 0. sg must be the last in is struct */
-	struct scatterlist sg[0];
+	struct scatterlist sg[];
 };
 
 /*
-- 
2.25.0