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

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Message-ID: <ae7a3879-3344-b50e-6187-3ee898026ec5@embeddedor.com>
Date:   Mon, 2 Mar 2020 14:05:43 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] sunrpc: Replace zero-length array with
 flexible-array member



On 3/2/20 13:58, J. Bruce Fields wrote:
> On Fri, Feb 28, 2020 at 07:23:23AM -0600, 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]
> 
> I don't understand the quoted sentences at all.  But I assume you're
> telling me that sizeof(struct svc_deferred_req) won't be changed by this
> patch, so, good, applied.  Thanks!
> 

Correct! :)

Thanks
--
Gustavo

> --b.
> 
>>
>> 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>
>> ---
>>  include/linux/sunrpc/svc.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
>> index 1afe38eb33f7..7f0a83451bc0 100644
>> --- a/include/linux/sunrpc/svc.h
>> +++ b/include/linux/sunrpc/svc.h
>> @@ -380,7 +380,7 @@ struct svc_deferred_req {
>>  	struct cache_deferred_req handle;
>>  	size_t			xprt_hlen;
>>  	int			argslen;
>> -	__be32			args[0];
>> +	__be32			args[];
>>  };
>>  
>>  struct svc_process_info {
>> -- 
>> 2.25.0