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 10:18 PM, Matthias Schiffer wrote:
> 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...)

I've found the issue of this other checksum problem: batman-adv
fragmentation code doesn't handle the checksum on reassembly at all. I
think the best option here is to simply set ip_summed to CHECKSUM_NONE on
reassembly, I will send another patch for that.

The IP fragmentation code does more fancy things when all fragments have
CHECKSUM_COMPLETE, adding up the checksums of the fragments under certain
circumstances. This only works because IP fragments are guaranteed to be
split at even byte offsets (multiples of 8, actually); as far as I can
tell, batman-adv allows odd fragment sizes, making it impossible to add up
the 16bit checksums in the general case.

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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