Host endianness dependent DHT lookup

Host endianness dependent DHT lookup

[Sven Eckelmann]

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Hi,

I just saw following in batadv_hash_dat():

	key = (const unsigned char *)&dat->vid;
	for (i = 0; i < sizeof(dat->vid); i++) {
		hash += key[i];
		hash += (hash << 10);
		hash ^= (hash >> 6);
	}

But the vid is in host order - not big endian like the IP part. So the 
batadv_dat_select_candidates will select different candidates depending on 
whether it is a little or big endian system, right?

If this is a correct assumption, then we would have this problem since 
3e26722bc9f2 ("batman-adv: make the Distributed ARP Table vlan aware")

Kind regards,
	Sven

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Re: Host endianness dependent DHT lookup

[Antonio Quartulli]

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Hi,

On 28/11/2019 11:01, Sven Eckelmann wrote:
> Hi,
> 
> I just saw following in batadv_hash_dat():
> 
> 	key = (const unsigned char *)&dat->vid;
> 	for (i = 0; i < sizeof(dat->vid); i++) {
> 		hash += key[i];
> 		hash += (hash << 10);
> 		hash ^= (hash >> 6);
> 	}
> 
> But the vid is in host order - not big endian like the IP part. So the 
> batadv_dat_select_candidates will select different candidates depending on 
> whether it is a little or big endian system, right?
> 

That sounds like a correct statement.

We access the VID byte by byte, therefore different endianness will lead
to different hash values (and thus different candidate selection).

I imagine I haven't observed this issue so far because I always worked
with networks made up by very similar hardware.

> If this is a correct assumption, then we would have this problem since 
> 3e26722bc9f2 ("batman-adv: make the Distributed ARP Table vlan aware")
> 

That's it. Very nice catch!

Will you send a patch?
I guess converting the VID into network order before accessing it should
be enough, no?


Thanks!

Regards,

-- 
Antonio Quartulli


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Re: Host endianness dependent DHT lookup

[Sven Eckelmann]

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On Thursday, 28 November 2019 11:13:21 CET Antonio Quartulli wrote:
[...]
> Will you send a patch?
> I guess converting the VID into network order before accessing it should
> be enough, no?

Yes, I will prepare a patch. Just want to finish debugging another problem 
before doing this.

Kind regards,
	Sven

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