linux-kernel - Re: [PATCH][next] mtd: ubi: ubi-media.h: Replace zero-length array with flexible-array member

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Message-ID: <2093889665.60398.1585603814979.JavaMail.zimbra@nod.at>
Date:   Mon, 30 Mar 2020 23:30:14 +0200 (CEST)
From:   Richard Weinberger <richard@....at>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] mtd: ubi: ubi-media.h: Replace zero-length array
 with flexible-array member

----- Ursprüngliche Mail -----
> Von: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
> An: "richard" <richard@....at>, "Miquel Raynal" <miquel.raynal@...tlin.com>, "Vignesh Raghavendra" <vigneshr@...com>
> CC: "linux-mtd" <linux-mtd@...ts.infradead.org>, "linux-kernel" <linux-kernel@...r.kernel.org>, "Gustavo A. R. Silva"
> <gustavo@...eddedor.com>
> Gesendet: Donnerstag, 19. März 2020 23:44:35
> Betreff: [PATCH][next] mtd: ubi: ubi-media.h: 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>

Applied. Thanks!

Thanks,
//richard