netdev - Re: [PATCH net-next] sk-filter: Add ability to get socket filter program

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Date:	Fri, 26 Oct 2012 20:07:13 +0200
From:	Daniel Borkmann <danborkmann@...earbox.net>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] sk-filter: Add ability to get socket filter program

On Fri, Oct 26, 2012 at 7:15 PM, Pavel Emelyanov <xemul@...allels.com> wrote:
> The SO_ATTACH_FILTER option is set only. I propose to add the get
> ability by using SO_ATTACH_FILTER in getsockopt. To be less irritating
> to eyes the SO_GET_FILTER alias to it is declared.
>
> There are two issues with getting filter back.
>
> First, kernel modifies the sock_filter->code on filter load, thus in
> order to return the filter element back to user we have to decode it
> into user-visible constants. Fortunately the modification in question
> is interconvertible.
>
> Second, the BPF_S_ALU_DIV_K code modifies the command argument k to
> speed up the run-time division by doing kernel_k = reciprocal(user_k).
> Bad news is that different user_k may result in same kernel_k, so we
> can't get the original user_k back. Good news is that we don't have
> to do it. What we need to is calculate a user2_k so, that
>
>   reciprocal(user2_k) == reciprocal(user_k) == kernel_k
>
> i.e. if it's re-loaded back the compiled again value will be exactly
> the same as it was. That said, the user2_k can be calculated like this
>
>   user2_k = reciprocal(kernel_k)
>
> with an exception, that if kernel_k == 0, then user2_k == 1.
>
> The optlen argument is treated like this -- when zero, kernel returns
> the amount of sock_fprog elements in filter, otherwise it should be
> large enough for the sock_fprog array.
>
> Signed-off-by: Pavel Emelyanov <xemul@...allels.com>

In your commit message, you address how you implement it, but not
where it could be useful (or ``why one needs it''). For instance, in a
simple case, you have a packet sniffer that uses PF_PACKET, sets up a
socket filter and RX_RING, then you start looking at packets and at
some point you stop and close everything. In which scenario would I
(ever) call this getsockopt()? (Is it just for the sake for having a
missing getter-function?)
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