linux-kernel - Re: [PATCH][next] zd1211rw/zd_usb.h: Replace zero-length array with flexible-array member

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Date:   Tue, 10 Mar 2020 17:31:35 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jes Sorensen <jes.sorensen@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>
Cc:     Joe Perches <joe@...ches.com>, Daniel Drake <dsd@...too.org>,
        Ulrich Kunitz <kune@...ne-taler.de>,
        "David S. Miller" <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH][next] zd1211rw/zd_usb.h: Replace zero-length array with
 flexible-array member



On 3/10/20 5:20 PM, Jes Sorensen wrote:
> On 3/10/20 6:13 PM, Gustavo A. R. Silva wrote:
>>
>>
>> On 3/10/20 5:07 PM, Jes Sorensen wrote:
>>> As I stated in my previous answer, this seems more code churn than an
>>> actual fix. If this is a real problem, shouldn't the work be put into
>>> fixing the compiler to handle foo[0] instead? It seems that is where the
>>> real value would be.
>>
>> Yeah. But, unfortunately, I'm not a compiler guy, so I'm not able to fix the
>> compiler as you suggest. And I honestly don't see what is so annoying/disturbing
>> about applying a patch that removes the 0 from foo[0] when it brings benefit
>> to the whole codebase.
> 
> My point is that it adds what seems like unnecessary churn, which is not
> a benefit, and it doesn't improve the generated code.
> 

As an example of one of the benefits of this is that the compiler won't trigger
a warning in the following case:

struct boo {
	int stuff;
	struct foo array[0];
	int morestuff;
};

The result of the code above is an undefined behavior.

On the other hand in the case below, the compiles does trigger a warning:

struct boo {
	int stuff;
	struct foo array[];
	int morestuff;
};

See: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=76497732932f15e7323dc805e8ea8dc11bb587cf

Thanks
--
Gustavo