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* [PATCH][next] net: inet_sock: Replace zero-length array with flexible-array member
@ 2020-03-02 12:07 Gustavo A. R. Silva
  2020-03-02 19:27 ` David Miller
  0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-02 12:07 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/inet_sock.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 34c4436fd18f..a7ce00af6c44 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -52,7 +52,7 @@ struct ip_options {
 	unsigned char	router_alert;
 	unsigned char	cipso;
 	unsigned char	__pad2;
-	unsigned char	__data[0];
+	unsigned char	__data[];
 };
 
 struct ip_options_rcu {
-- 
2.25.0


^ permalink raw reply related	[flat|nested] 2+ messages in thread

* Re: [PATCH][next] net: inet_sock: Replace zero-length array with flexible-array member
  2020-03-02 12:07 [PATCH][next] net: inet_sock: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-02 19:27 ` David Miller
  0 siblings, 0 replies; 2+ messages in thread
From: David Miller @ 2020-03-02 19:27 UTC (permalink / raw)
  To: gustavo; +Cc: kuba, netdev, linux-kernel

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Date: Mon, 2 Mar 2020 06:07:42 -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.

^ permalink raw reply	[flat|nested] 2+ messages in thread

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