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 <[email protected]>
---
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
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()?
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
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 <[email protected]>
> ---
> 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 <[email protected]>
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
[email protected] +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
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