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

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Message-ID: <53879B78.5050106@redhat.com>
Date:	Thu, 29 May 2014 22:41:28 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Chema Gonzalez <chema@...gle.com>
CC:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexei Starovoitov <ast@...mgrid.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v1 net-next] net: filter: fix length calculation in BPF
 testsuite

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.
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