netdev - Re: [PATCH] Fix up truesize after pskb_expand

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Date:	Sun, 04 Jan 2009 17:05:50 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	linville@...driver.com
Subject: Re: [PATCH] Fix up truesize after pskb_expand_head() in wireless
	stack

On Sun, 2009-01-04 at 16:18 +0100, Andi Kleen wrote:
> Fix up truesize after pskb_expand_head() in wireless stack
>  
> When using a zd1211rw wireless usb stick I regularly got truesize
> warnings in the kernel log.
> 
> This patch fixes those up in the wireless layer. tx.c already
> did that, but not rx.c. I think my messages only came from
> the middle case, but I fixed up all three users in rx.c 
> 
> The underlying problem seems to be that pskb_expand_head() doesn't 
> manipulate truesize. Perhaps it should? I suspect more users of it have 
> the same problem. I didn't change the low level code because
> I was afraid to break some callers, but perhaps it would be
> better to do it this way. Anyways here's a patch that only
> changes it in the wireless layer with minimal risk.
> 
> Patch against 2.6.28, but I think linus git still has the same
> issue.
> 
> I believe this is a 2.6.28 stable candidate. I even saw
> the same problem in 2.6.27.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Thanks, but I'll need to look at this in more detail, we need to make
sure that we orphan the skb before etc. And then, we need to check
whether it makes sense to do this in pskb_expand_head().

> ---
>  net/mac80211/rx.c |   19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> Index: linux-2.6.28-test/net/mac80211/rx.c
> ===================================================================
> --- linux-2.6.28-test.orig/net/mac80211/rx.c	2008-10-24 13:35:11.000000000 +0200
> +++ linux-2.6.28-test/net/mac80211/rx.c	2009-01-01 15:51:34.000000000 +0100
> @@ -263,10 +263,12 @@
>  		 * probably export the length to drivers so that we can have
>  		 * them allocate enough headroom to start with.
>  		 */
> -		if (skb_headroom(skb) < needed_headroom &&
> -		    pskb_expand_head(skb, needed_headroom, 0, GFP_ATOMIC)) {
> -			dev_kfree_skb(skb);
> -			return NULL;
> +		if (skb_headroom(skb) < needed_headroom) {
> +			if (pskb_expand_head(skb, needed_headroom, 0, GFP_ATOMIC)) {
> +				dev_kfree_skb(skb);
> +				return NULL;
> +			}
> +			skb->truesize += needed_headroom;
>  		}
>  	} else {
>  		/*
> @@ -945,6 +947,7 @@
>  			__skb_queue_purge(&entry->skb_list);
>  			return RX_DROP_UNUSABLE;
>  		}
> +		rx->skb->truesize += entry->extra_len;
>  	}
>  	while ((skb = __skb_dequeue(&entry->skb_list))) {
>  		memcpy(skb_put(rx->skb, skb->len), skb->data, skb->len);
> @@ -1691,9 +1694,11 @@
>  	if (rx->flags & IEEE80211_RX_CMNTR_REPORTED)
>  		goto out_free_skb;
>  
> -	if (skb_headroom(skb) < sizeof(*rthdr) &&
> -	    pskb_expand_head(skb, sizeof(*rthdr), 0, GFP_ATOMIC))
> -		goto out_free_skb;
> +	if (skb_headroom(skb) < sizeof(*rthdr)) {
> +		if (pskb_expand_head(skb, sizeof(*rthdr), 0, GFP_ATOMIC))
> +			goto out_free_skb;
> +		skb->truesize += sizeof(*rthdr);
> +	}
>  
>  	rthdr = (void *)skb_push(skb, sizeof(*rthdr));
>  	memset(rthdr, 0, sizeof(*rthdr));
> --
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