Re: [B.A.T.M.A.N.] [PATCH maint] batman-adv: fix packet checksum in receive path

[Matthias Schiffer <mschiffer@universe-factory.net>]

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On 01/22/2018 09:52 PM, Sven Eckelmann wrote:
> On Montag, 22. Januar 2018 20:24:50 CET Matthias Schiffer wrote:
>> skb_postpull_rcsum() is necessary after eth_type_trans() to adjust the
>> skb checksum, otherwise log spam of the form "bat0: hw csum failure" will
>> result when packets with CHECKSUM_COMPLETE are received (at least in some
>> setups, e.g. when stacking batman-adv on top of VXLAN).
> 
> Would be nice to have a better explanation here.
> 
> The comment previously assumed that skb_pull_rcsum would be enough. But the 
> problem here is that the skb_pull_rcsum only pulls the batman-adv headers. The 
> actual pull of the ethernet header (with skb_pull_inline) happens inside 
> eth_type_trans. Or did I miss anything?

This is correct, eth_type_trans() contains a simple skb_pull(), so the csum
must be adjusted afterwards (grepping the kernel for eth_type_trans will
find a lot of this). I can send a v2 with a better commit message later.

> 
> [...]
>> I don't know what the exact circumstances are that trigger the log spam,
>> but it seems this was broken forever (I could also reproduce the issue with
>> our compat-14 legacy branch)... so please ask David to queue this up for
>> stable :)
> 
> Yes, this is broken since earliest commits. The most relevant commit in 
> batman-adv is:
> 
> Fixes: fe28a94c01e1 ("batman-adv: receive packets directly using skbs")
> 
> But I would propose to use following in the kernel tree:
> 
> Fixes: c6c8fea29769 ("net: Add batman-adv meshing protocol")
> 
> The 4.15 release will be soon(tm) and Simon is currently on vacation. So we 
> will most likely postpone the submission to David until Simon found a way out 
> of the snow and after 4.15 is released...
> 
> But it would be nice when some people could test the patch [1] (together with 
> vxlan?) on batman-adv or batman-adv-legacy. And please provide a 
> "Tested-by: Full Name <email@example.org>" [2] reply when it works.
> 
> Thanks,> 	Sven

I've tested this on Kernel 4.14.14 (everything working correctly now) and
4.4.110 (here, there are still checksum errors; it seems on older kernels,
the checksum handling in VXLAN is broken too? Still debugging this...)

Matthias



> 
> [1] https://patchwork.open-mesh.org/patch/17250/
> [2] https://www.kernel.org/doc/html/v4.12/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes
> 



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