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

From: Gustavo A. R. Silva
Date: Mon Mar 02 2020 - 17:01:06 EST




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@xxxxxxxxxxxxxx>
>> ---
>> 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