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From: Sven Eckelmann <sven@narfation.org>
Date: Mon, 19 Mar 2018 14:14:04 +0100
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Subject: Re: [B.A.T.M.A.N.] [PATCH] batman-adv: Fix skbuff rcsum on packet
 reroute
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To: Matthias Schiffer <mschiffer@universe-factory.net>
Cc: b.a.t.m.a.n@lists.open-mesh.org

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On Montag, 19. M=E4rz 2018 14:02:43 CET Matthias Schiffer wrote:
> Using skb_postpull_rcsum here is incorrect: As the name indicates, it is
> supposed to be used after pulling, and will thus subtract the checksum of
> the header *before* skb->data, while we are interested in the
> sizeof(*unicast_packet) bytes *after* skb->data.

Can you please explain this a little bit further. You give skb_postpull_rcs=
um=20
the start and length via parameter and it is writing the resulting csum in =
the
skb. It is not checking or accessing the position of skb->data [1]

    /**
     *	skb_postpull_rcsum - update checksum for received skb after pull
     *	@skb: buffer to update
     *	@start: start of data before pull
     *	@len: length of data pulled
     *
     *	After doing a pull on a received packet, you need to call this to
     *	update the CHECKSUM_COMPLETE checksum, or set ip_summed to
     *	CHECKSUM_NONE so that it can be recomputed from scratch.
     */

See also __skb_postpull_rcsum [2]:

		skb->csum =3D csum_block_sub(skb->csum,
					   csum_partial(start, len, 0), off);

What about the examples I gave in my patch - for example the easy-to-read=20
set_eth_addr [3]?

Kind regards,
	Sven

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree=
/include/linux/skbuff.h?id=3Dc698ca5278934c0ae32297a8725ced2e27585d7f#n3070
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree=
/include/linux/skbuff.h?id=3Dc698ca5278934c0ae32297a8725ced2e27585d7f#n3058
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree=
/net/openvswitch/actions.c?id=3Dc698ca5278934c0ae32297a8725ced2e27585d7f#n3=
19
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