netdev - Re: [PATCH v1 net-next] net: filter: fix length calculation in BPF testsuite

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Date:	Fri, 30 May 2014 10:15:11 -0700
From:	Chema Gonzalez <chema@...gle.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v1 net-next] net: filter: fix length calculation in BPF testsuite

On Thu, May 29, 2014 at 1:41 PM, Daniel Borkmann <dborkman@...hat.com> wrote:
> On 05/29/2014 09:29 PM, Chema Gonzalez wrote:
>>
>> The current probe_filter_length() (the function that calculates the
>> length of a test BPF filter) behavior is to declare the end of the
>> filter as soon as it finds {0, *, *, 0}. This is actually a valid
>> insn ("ld #0"), so any filter with includes "BPF_STMT(BPF_LD | BPF_IMM,
>> 0)"
>> fails (its length is cut short).
>>
>> We are changing probe_filter_length() so as to start from the end, and
>> declare the end of the filter as the first instruction which is not
>> {0, *, *, 0}. This solution produces a simpler patch than the
>> alternative of using an explicit end-of-filter mark. It is technically
>> incorrect if your filter ends up with "ld #0", but that should not
>> happen anyway.
>>
>> We also add a new test (LD_IMM_0) that includes ld #0 (does not work
>> without this patch).
>>
>> Signed-off-by: Chema Gonzalez <chema@...gle.com>
>
>
> Looks good to me, thanks a lot Chema!
>
> Acked-by: Daniel Borkmann <dborkman@...hat.com>
>
>
>> +       for (len = MAX_INSNS-1; len > 0; --len)
>> +               if (fp[len].code != 0 || fp[len].k != 0)
>> +                       break;
>>
>> -       return len;
>> +       return len+1;
>
>
> Nit: would be great to have a whitespace between MAX_INSNS-1 and len+1 but
> that shouldn't matter that much, perhaps.
Done.

-Chema
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