[PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

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@embeddedor.com>
---
 drivers/scsi/advansys.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/scsi/advansys.c b/drivers/scsi/advansys.c
index a242a62caaa1..c2c7850ff7b4 100644
--- a/drivers/scsi/advansys.c
+++ b/drivers/scsi/advansys.c
@@ -316,7 +316,7 @@ typedef struct asc_sg_head {
 	ushort queue_cnt;
 	ushort entry_to_copy;
 	ushort res;
-	ASC_SG_LIST sg_list[0];
+	ASC_SG_LIST sg_list[];
 } ASC_SG_HEAD;
 
 typedef struct asc_scsi_q {
-- 
2.23.0

Re: [PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[Matthew Wilcox]

On Wed, Feb 12, 2020 at 06:02:11PM -0600, Gustavo A. R. Silva wrote:
> Also, notice that, dynamic memory allocations won't be affected by
> this change:

Shouldn't you also convert this:
                asc_sg_head = kzalloc(sizeof(asc_scsi_q->sg_head) +
                        use_sg * sizeof(struct asc_sg_list), GFP_ATOMIC);
to use struct_size()?

Re: [PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[Gustavo A. R. Silva]

On 2/12/20 19:14, Matthew Wilcox wrote:
> On Wed, Feb 12, 2020 at 06:02:11PM -0600, Gustavo A. R. Silva wrote:
>> Also, notice that, dynamic memory allocations won't be affected by
>> this change:
> 
> Shouldn't you also convert this:
>                 asc_sg_head = kzalloc(sizeof(asc_scsi_q->sg_head) +
>                         use_sg * sizeof(struct asc_sg_list), GFP_ATOMIC);
> to use struct_size()?
> 

It's not a requirement to do that prior to applying the change proposed by
this patch.

Thanks
--
Gustavo

Re: [PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[Hannes Reinecke]

On 2/13/20 1:02 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@embeddedor.com>
> ---
>   drivers/scsi/advansys.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/advansys.c b/drivers/scsi/advansys.c
> index a242a62caaa1..c2c7850ff7b4 100644
> --- a/drivers/scsi/advansys.c
> +++ b/drivers/scsi/advansys.c
> @@ -316,7 +316,7 @@ typedef struct asc_sg_head {
>   	ushort queue_cnt;
>   	ushort entry_to_copy;
>   	ushort res;
> -	ASC_SG_LIST sg_list[0];
> +	ASC_SG_LIST sg_list[];
>   } ASC_SG_HEAD;
>   
>   typedef struct asc_scsi_q {
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH] scsi: advansys: Replace zero-length array with flexible-array member

[Martin K. Petersen]

Gustavo,

> 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:

Applied to 5.7/scsi-queue, thanks!

-- 
Martin K. Petersen	Oracle Linux Engineering