* [PATCH][next] net: lwtunnel: Replace zero-length array with flexible-array member
@ 2020-02-29 0:50 Gustavo A. R. Silva
2020-03-01 5:53 ` David Miller
0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-29 0:50 UTC (permalink / raw)
To: David S. Miller, Jakub Kicinski; +Cc: 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/lwtunnel.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/net/lwtunnel.h b/include/net/lwtunnel.h
index 5d6c5b1fc695..b5e6edf74b70 100644
--- a/include/net/lwtunnel.h
+++ b/include/net/lwtunnel.h
@@ -30,7 +30,7 @@ struct lwtunnel_state {
int (*orig_output)(struct net *net, struct sock *sk, struct sk_buff *skb);
int (*orig_input)(struct sk_buff *);
struct rcu_head rcu;
- __u8 data[0];
+ __u8 data[];
};
struct lwtunnel_encap_ops {
--
2.25.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH][next] net: lwtunnel: Replace zero-length array with flexible-array member
2020-02-29 0:50 [PATCH][next] net: lwtunnel: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-01 5:53 ` David Miller
0 siblings, 0 replies; 2+ messages in thread
From: David Miller @ 2020-03-01 5:53 UTC (permalink / raw)
To: gustavo; +Cc: kuba, netdev, linux-kernel
From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Date: Fri, 28 Feb 2020 18:50:23 -0600
> 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>
Applied.
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