netdev - Re: [Bug #16626] Machine hangs with EIP at skb_copy_and_csum

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Date:	Fri, 3 Sep 2010 16:30:28 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jarek Poplawski <jarkao2@...il.com>,
	Plamen Petrov <pvp-lsts@...uni-ruse.bg>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: [Bug #16626] Machine hangs with EIP at skb_copy_and_csum_dev

On Wed, Sep 01, 2010 at 12:50:51PM +0200, Eric Dumazet wrote:
> Plamen, could you test following patch ?
> 
> I reproduced problem on a dev machine and following patch cured it.
> 
> Thanks
> 
> [PATCH] gro: fix different skb headrooms
> 
> packets entering GRO might have different headrooms, even for a given
> flow (because of implementation details in drivers, like copybreak).
> We cant force drivers to deliver packets with a fixed headroom.
> 
> 1) fix skb_segment()
> 
> skb_segment() makes the false assumption headrooms of fragments are same
> than the head. When CHECKSUM_PARTIAL is used, this can give csum_start
> errors, and crash later in skb_copy_and_csum_dev()
> 
> 2) allocate a minimal skb for head of frag_list
> 
> skb_gro_receive() uses netdev_alloc_skb(headroom + skb_gro_offset(p)) to
> allocate a fresh skb. This adds NET_SKB_PAD to a padding already
> provided by netdevice, depending on various things, like copybreak.
> 
> Use alloc_skb() to allocate an exact padding, to reduce cache line
> needs:
> NET_SKB_PAD + NET_IP_ALIGN
> 
> bugzilla : https://bugzilla.kernel.org/show_bug.cgi?id=16626
> 
> Many thanks to Plamen Petrov, testing many debugging patches !
> With help of Jarek Poplawski.
> 
> Reported-by: Plamen Petrov <pvp-lsts@...uni-ruse.bg>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> CC: Jarek Poplawski <jarkao2@...il.com>

Thanks for diagnosing and fixing this!

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 3a2513f..26396ff 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2573,6 +2573,10 @@ struct sk_buff *skb_segment(struct sk_buff *skb, int features)
>  		__copy_skb_header(nskb, skb);
>  		nskb->mac_len = skb->mac_len;
>  
> +		/* nskb and skb might have different headroom */
> +		if (nskb->ip_summed == CHECKSUM_PARTIAL)
> +			nskb->csum_start += skb_headroom(nskb) - headroom;

This test is redundant since we require CHECKSUM_PARTIAL for
GSO packets.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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