[next] ocfs2: cluster: Replace zero-length array with flexible-array member
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Message ID 20200309201907.GA8005@embeddedor
State New
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  • [next] ocfs2: cluster: Replace zero-length array with flexible-array member
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Commit Message

Gustavo A. R. Silva March 9, 2020, 8:19 p.m. UTC
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://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!NzMr-YRl2zy-K3lwLVVatz7x0uD2z7-ykQag4GrGigxmfWU8TWzDy6xrkTiW3hYl00czlw$ 
[2] https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!NzMr-YRl2zy-K3lwLVVatz7x0uD2z7-ykQag4GrGigxmfWU8TWzDy6xrkTiW3hYHG1nAnw$ 
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 fs/ocfs2/cluster/tcp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Joseph Qi March 10, 2020, 1:06 a.m. UTC | #1
On 2020/3/10 04:19, 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://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!INdYafacLMiUgRrK-HDy3FhbqtTkRakuBQtEmPbN6cZAHCDBAvGOTj194lmB8udIjmmjqw$ 
> [2] https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!INdYafacLMiUgRrK-HDy3FhbqtTkRakuBQtEmPbN6cZAHCDBAvGOTj194lmB8ufEoscJWg$ 
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> ---
>  fs/ocfs2/cluster/tcp.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ocfs2/cluster/tcp.h b/fs/ocfs2/cluster/tcp.h
> index de87cbffd175..736338f45c59 100644
> --- a/fs/ocfs2/cluster/tcp.h
> +++ b/fs/ocfs2/cluster/tcp.h
> @@ -32,7 +32,7 @@ struct o2net_msg
>  	__be32 status;
>  	__be32 key;
>  	__be32 msg_num;
> -	__u8  buf[0];
> +	__u8  buf[];
>  };
>  
>  typedef int (o2net_msg_handler_func)(struct o2net_msg *msg, u32 len, void *data,
>

Patch
diff mbox series

diff --git a/fs/ocfs2/cluster/tcp.h b/fs/ocfs2/cluster/tcp.h
index de87cbffd175..736338f45c59 100644
--- a/fs/ocfs2/cluster/tcp.h
+++ b/fs/ocfs2/cluster/tcp.h
@@ -32,7 +32,7 @@  struct o2net_msg
 	__be32 status;
 	__be32 key;
 	__be32 msg_num;
-	__u8  buf[0];
+	__u8  buf[];
 };
 
 typedef int (o2net_msg_handler_func)(struct o2net_msg *msg, u32 len, void *data,