* [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member
@ 2020-02-28 13:47 Gustavo A. R. Silva
2020-02-28 13:52 ` Marcel Holtmann
0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-28 13:47 UTC (permalink / raw)
To: Alexander Aring, Jukka Rissanen, David S. Miller, Jakub Kicinski
Cc: linux-bluetooth, linux-wpan, netdev, linux-kernel, 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>
---
include/net/6lowpan.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/net/6lowpan.h b/include/net/6lowpan.h
index a71378007e61..c80539be1542 100644
--- a/include/net/6lowpan.h
+++ b/include/net/6lowpan.h
@@ -138,7 +138,7 @@ struct lowpan_dev {
struct lowpan_iphc_ctx_table ctx;
/* must be last */
- u8 priv[0] __aligned(sizeof(void *));
+ u8 priv[] __aligned(sizeof(void *));
};
struct lowpan_802154_neigh {
--
2.25.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member
2020-02-28 13:47 [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-02-28 13:52 ` Marcel Holtmann
2020-02-28 14:02 ` Gustavo A. R. Silva
0 siblings, 1 reply; 3+ messages in thread
From: Marcel Holtmann @ 2020-02-28 13:52 UTC (permalink / raw)
To: Gustavo A. R. Silva
Cc: Alexander Aring, Jukka Rissanen, David S. Miller, Jakub Kicinski,
linux-bluetooth, linux-wpan, netdev, linux-kernel
Hi 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:
>
> 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>
> ---
> include/net/6lowpan.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
patch has been applied to bluetooth-next tree.
Regards
Marcel
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member
2020-02-28 13:52 ` Marcel Holtmann
@ 2020-02-28 14:02 ` Gustavo A. R. Silva
0 siblings, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-28 14:02 UTC (permalink / raw)
To: Marcel Holtmann
Cc: Alexander Aring, Jukka Rissanen, David S. Miller, Jakub Kicinski,
linux-bluetooth, linux-wpan, netdev, linux-kernel
On 2/28/20 07:52, Marcel Holtmann wrote:
>>
>> [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>
>> ---
>> include/net/6lowpan.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> patch has been applied to bluetooth-next tree.
>
Thanks, Marcel.
--
Gustavo
^ permalink raw reply [flat|nested] 3+ messages in thread
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