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

From: Daniel Borkmann <[email-masked]>
Date: Thu, 29 May 2014 22:41:28 +0200

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 <[email-masked]>

Looks good to me, thanks a lot Chema!

Acked-by: Daniel Borkmann <[email-masked]>

> +	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.