linux-kernel - Re: [greybus-dev] [PATCH] staging: greybus: Replace zero-length array with flexible-array member

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Date:   Tue, 11 Feb 2020 16:47:06 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Alex Elder <elder@...aro.org>, Johan Hovold <johan@...nel.org>,
        Alex Elder <elder@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, greybus-dev@...ts.linaro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [greybus-dev] [PATCH] staging: greybus: Replace zero-length array
 with flexible-array member



On 2/11/20 16:15, Alex Elder wrote:
> On 2/11/20 3:12 PM, 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
>> inadvertenly introduced[3] to the codebase from now on.
>>
>> 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>
>> ---
>>  drivers/staging/greybus/raw.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/staging/greybus/raw.c b/drivers/staging/greybus/raw.c
>> index 838acbe84ca0..2b301b2aa107 100644
>> --- a/drivers/staging/greybus/raw.c
>> +++ b/drivers/staging/greybus/raw.c
>> @@ -30,7 +30,7 @@ struct gb_raw {
>>  struct raw_data {
>>  	struct list_head entry;
>>  	u32 len;
>> -	u8 data[0];
>> +	u8 data[];
>>  };
>>  
>>  static struct class *raw_class;
>>
> 
> Does the kamlloc() call in receive_data() have any problems
> with the sizeof(*raw_data) passed as its argument?
> 

Not in this case. It'd be different with a one-element array (u8 data[1]),
though.

> I'm not entirely sure what sizeof(struct-with-flexible-array-member)
> produces.
> 

The same as sizeof(struct-with-zero-length-array):

"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]

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

--
Gustavo