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

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Message-ID: <94f38b5e-c850-5fff-1540-fb1a530a5a51@oracle.com>
Date:   Fri, 28 Feb 2020 07:13:31 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jürgen Groß <jgross@...e.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xen: Replace zero-length array with flexible-array member



On 2/27/20 4:31 AM, Jürgen Groß wrote:
> On 26.02.20 22:26, Gustavo A. R. Silva wrote:
>> 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>
>
> Reviewed-by: Juergen Gross <jgross@...e.com>


Applied to for-linus-5.6.

-boris