linux-kernel - RE: [PATCH][next] bpf, sockmap: Replace zero-length array with flexible-array member

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Date:   Tue, 18 Feb 2020 22:31:46 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: RE: [PATCH][next] bpf, sockmap: Replace zero-length array with
 flexible-array member

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 <gustavo@...eddedor.com>
> ---
>  net/core/sock_map.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 085cef5857bb..3a7a96ab088a 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -518,7 +518,7 @@ struct bpf_htab_elem {
>  	u32 hash;
>  	struct sock *sk;
>  	struct hlist_node node;
> -	u8 key[0];
> +	u8 key[];
>  };
>  
>  struct bpf_htab_bucket {
> -- 
> 2.25.0
> 

The same pattern is in ./kernel/bpf/hashtab.c can you also change
it here then if this is the case so sockmap is aligned with bpf
coding style.

.John