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

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Message-ID: <8e9e1b59-a7a8-782e-990b-8a7023c2292d@embeddedor.com>
Date:   Thu, 20 Feb 2020 13:14:27 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Adham.Abozaeid@...rochip.com, gregkh@...uxfoundation.org,
        vaibhav.sr@...il.com, mgreer@...malcreek.com, johan@...nel.org,
        elder@...nel.org, m.tretter@...gutronix.de, kernel@...gutronix.de,
        mchehab@...nel.org, Larry.Finger@...inger.net,
        florian.c.schilhabel@...glemail.com, Ajay.Kathat@...rochip.com
Cc:     devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        greybus-dev@...ts.linaro.org, linux-media@...r.kernel.org,
        linux-wireless@...r.kernel.org
Subject: Re: [PATCH] staging: Replace zero-length array with flexible-array
 member

Hi,

On 2/20/20 13:04, Adham.Abozaeid@...rochip.com wrote:
> Hi Gustavo
> 
> On 2/20/20 6:29 AM, 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>
>>
>>  static void cfg_scan_result(enum scan_event scan_event,
>> diff --git a/drivers/staging/wilc1000/spi.c b/drivers/staging/wilc1000/spi.c
>> index 44f7d48851b5..11653ac118cd 100644
>> --- a/drivers/staging/wilc1000/spi.c
>> +++ b/drivers/staging/wilc1000/spi.c
>> @@ -139,7 +139,7 @@ struct wilc_spi_read_rsp_data {
>>         u8 status;
>>         u8 resp_header;
>>         u8 resp_data[4];
>> -       u8 crc[0];
>> +       u8 crc[];
>>  } __packed;
> more zero-length arrays in wilc1000, spi.c, struct wilc_spi_cmd, and in fw.h
> 

Oh wow, I hadn't thought about cases like this:

struct wilc_spi_cmd {
        u8 cmd_type;
        union {
                struct {
                        u8 addr[3];
                        u8 crc[0];
                } __packed simple_cmd;
                struct {
                        u8 addr[3];
                        u8 size[2];
                        u8 crc[0];
                } __packed dma_cmd;
                struct {
                        u8 addr[3];
                        u8 size[3];
                        u8 crc[0];
                } __packed dma_cmd_ext;
                struct {
                        u8 addr[2];
                        __be32 data;
                        u8 crc[0];
                } __packed internal_w_cmd;
                struct {
                        u8 addr[3];
                        __be32 data;
                        u8 crc[0];
                } __packed w_cmd;
        } u;
} __packed;

Thanks for the feedback.
--
Gustavo