From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Berg Subject: Re: __sk_buff.data_end Date: Thu, 20 Apr 2017 08:01:05 +0200 Message-ID: <1492668065.3109.1.camel@sipsolutions.net> References: <1492637460.22185.6.camel@sipsolutions.net> (sfid-20170419_233114_060429_CAFE85B8) <1492640459.22185.7.camel@sipsolutions.net> <58F7FA6D.5030000@iogearbox.net> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Cc: netdev To: Daniel Borkmann , Alexei Starovoitov Return-path: Received: from s3.sipsolutions.net ([5.9.151.49]:51074 "EHLO sipsolutions.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1765656AbdDTGBM (ORCPT ); Thu, 20 Apr 2017 02:01:12 -0400 In-Reply-To: <58F7FA6D.5030000@iogearbox.net> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, 2017-04-20 at 02:01 +0200, Daniel Borkmann wrote: > > Yeah, should work as well for the 32 bit archs, on 64 bit we > have this effectively already: Right. [...] > Can you elaborate on why this works for mac80211? It uses cb > only up to that point from where you invoke the prog? No, it works because then I can move a u64 field to the same offset, and save/restore it across the BPF call :) But I don't have a *pointer* field to move there, and no space for the alignment anyway (already using all 48 bytes). Come to think of it - somebody had proposed extensions to this by passing an on-stack pointer in addition to the data in the cb. Perhaps we can extend BPF to have an optional second argument, and track a second context around the verifier, if applicable? Then we can solve all of this really easily, because it means we don't always have to go from the SKB context but could go from the other one (which could be that on-stack buffer). Alternatively I can clear another pointer (u64) in the CB, store a pointer there, and always emit code following that pointer - should be possible right? johannes