Re: [PATCH net] net: sit: fix UBSAN Undefined behaviour in check_6rd

[David Miller]

From: David Miller <davem@xxxxxxxxxxxxx>
Date: Mon, 11 Mar 2019 10:29:37 -0700 (PDT)

> From: linmiaohe <linmiaohe@xxxxxxxxxx>
> Date: Mon, 11 Mar 2019 16:29:32 +0800
> 
>> @@ -778,8 +778,9 @@ static bool check_6rd(struct ip_tunnel *tunnel, const struct in6_addr *v6dst,
>>  		pbw0 = tunnel->ip6rd.prefixlen >> 5;
>>  		pbi0 = tunnel->ip6rd.prefixlen & 0x1f;
>> 
>> -		d = (ntohl(v6dst->s6_addr32[pbw0]) << pbi0) >>
>> -		    tunnel->ip6rd.relay_prefixlen;
>> +		d = tunnel->ip6rd.relay_prefixlen < 32 ?
>> +			(ntohl(v6dst->s6_addr32[pbw0]) << pbi0) >>
>> +		    tunnel->ip6rd.relay_prefixlen : 0;
>> 
> 
> I hate the fact that we have to guard against something which the rest
> of the code makes sure NEVER EVER happens.
> 
> Every assignment of ->relay_prefixlen is guarded by a check against 32.

Sorry, I now understand, it can equal 32.

I'll apply this, thank you.