From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dominique Martinet Subject: Re: [PATCH v2] kcm: remove any offset before parsing messages Date: Tue, 18 Sep 2018 04:58:37 +0200 Message-ID: <20180918025837.GA3548@nautica> References: <20180918015723.GA26300@nautica> <20180917.194059.1970452340378032090.davem@davemloft.net> <20180918024536.GA2061@nautica> <20180917.195150.315319338322641005.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: doronrk@fb.com, tom@quantonium.net, davejwatson@fb.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: David Miller Return-path: Content-Disposition: inline In-Reply-To: <20180917.195150.315319338322641005.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org David Miller wrote on Mon, Sep 17, 2018: > > No, they can see it, so it's possible to make a KCM program that works > > right now if you are careful (I'm not sure why the offset within bpf is > > different from the offset in the kernel though, it looks like the bpf > > program skips the qos part of the control buffer) > > What helper is used in the BPF program to get this offset value? > > (also good info to add to the commit message) Dave defined one himself ; for a simple protocol where the offset is in the first four bytes of the message. The whole bpf program could look like this: ------ struct kcm_rx_msg { int full_len; int offset; }; static inline struct kcm_rx_msg *kcm_rx_msg(struct __sk_buff *skb) { return (struct kcm_rx_msg *)skb->cb; } int decode_framing(struct __sk_buff *skb) { return load_word(skb, kcm_rx_msg(skb)->offset); } ------ If we go towards documenting it, adding a helper would be useful yes; buf if we pull that becomes unnecessary. (I'll add the example program in the commit message anyway at your suggestion) -- Dominique Martinet