Subject: Re: [PATCH] cfg80211: Fix array-bounds warning in fragment copy

From: Johannes Berg <[email-masked]>
Date: Mon, 27 Mar 2017 12:47:59 +0200

On Fri, 2017-03-24 at 18:06 -0700, Matthias Kaehlcke wrote:
> __ieee80211_amsdu_copy_frag intentionally initializes a pointer to
> array[-1] to increment it later to valid values. clang rightfully
> generates an array-bounds warning on the initialization statement.
> Work around this by initializing the pointer to array[0] and
> decrementing it later, which allows to leave the rest of the
> algorithm untouched.
> 
> Signed-off-by: Matthias Kaehlcke <[email-masked]>
> ---
>  net/wireless/util.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/wireless/util.c b/net/wireless/util.c
> index 68e5f2ecee1a..d3d459e4a070 100644
> --- a/net/wireless/util.c
> +++ b/net/wireless/util.c
> @@ -659,7 +659,7 @@ __ieee80211_amsdu_copy_frag(struct sk_buff *skb,
> struct sk_buff *frame,
>  			    int offset, int len)
>  {
>  	struct skb_shared_info *sh = skb_shinfo(skb);
> -	const skb_frag_t *frag = &sh->frags[-1];
> +	const skb_frag_t *frag = &sh->frags[0];
>  	struct page *frag_page;
>  	void *frag_ptr;
>  	int frag_len, frag_size;
> @@ -669,6 +669,7 @@ __ieee80211_amsdu_copy_frag(struct sk_buff *skb,
> struct sk_buff *frame,
>  	frag_page = virt_to_head_page(skb->head);
>  	frag_ptr = skb->data;
>  	frag_size = head_size;
> +	frag--;

Isn't it just a question of time until the compiler will see through
this trick and warn about it?

johannes