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

[Felix Fietkau <nbd@nbd.name>]

On 2017-03-27 12:47, Johannes Berg wrote:
> 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 <mka@chromium.org>
>> ---
>>  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?
Frag is incremented again before being accessed, so there is nothing for
the compiler to see through here.

Acked-by: Felix Fietkau <nbd@nbd.name>

- Felix