netdev - Re: [PATCH v2 1/7] bpf: Track alignment of register values in the verifier.

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Date:   Thu, 11 May 2017 21:14:06 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     ast@...com
Cc:     daniel@...earbox.net, alexei.starovoitov@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 1/7] bpf: Track alignment of register values in the
 verifier.

From: Alexei Starovoitov <ast@...com>
Date: Thu, 11 May 2017 15:53:06 -0700

> On 5/11/17 9:05 AM, David Miller wrote:
>> +		had_id = (dst_reg->id != 0);
>> +
>>  		/* dst_reg stays as pkt_ptr type and since some positive
>>  		 * integer value was added to the pointer, increment its 'id'
>>  		 */
>>  		dst_reg->id = ++env->id_gen;
>>
>> -		/* something was added to pkt_ptr, set range and off to zero */
>> +		/* something was added to pkt_ptr, set range to zero */
>> +		dst_reg->aux_off = dst_reg->off;
> 
> what about 2nd addition of a variable to pkt_ptr ?
> aux_off sort-of remembers already accumulated offset in pkt_ptr, but
> above line will hard assign it which doesn't seem right for the 2nd
> addition.
> Ex:
> before first add, reg->off == 14
> after first add, aux_off = 14, off = 0
> then imm4 added, now we have reg->off=4, aux_off=14
> now we do 2nd add of variable and
> reg->aux_off becomes 4
> and if we later do u64 load from the packet it will be rejected
> due to (net_ip_align + 4) whereas it should have been ok
> due to (net_ip_align + 14 + 4).

Indeed, we have to accumulate.  I was just thinking about this earlier
today.

Thanks for pointing this out, I'll work on a fix and write some test
cases.