Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751934AbdFZVDV (ORCPT ); Mon, 26 Jun 2017 17:03:21 -0400 Received: from mail3-relais-sop.national.inria.fr ([192.134.164.104]:62085 "EHLO mail3-relais-sop.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751403AbdFZVDP (ORCPT ); Mon, 26 Jun 2017 17:03:15 -0400 X-IronPort-AV: E=Sophos;i="5.39,397,1493676000"; d="scan'208";a="229645016" Date: Mon, 26 Jun 2017 17:03:09 -0400 (EDT) From: Julia Lawall X-X-Sender: jll@hadrien To: Frans Klaver cc: Joe Perches , Greg Kroah-Hartman , kernel-janitors , Guenter Roeck , Yueyao Zhu , Rui Miguel Silva , Guru Das Srinagesh , Javier Martinez Canillas , devel@driverdev.osuosl.org, "linux-kernel@vger.kernel.org" Subject: Re: endian bitshift defects [ was: staging: fusb302: don't bitshift __le16 type ] In-Reply-To: <20170626205748.GA1899@bugger> Message-ID: References: <20170616174556.2358-1-fransklaver@gmail.com> <1497653077.10546.23.camel@perches.com> <20170626205748.GA1899@bugger> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2671 Lines: 78 On Mon, 26 Jun 2017, Frans Klaver wrote: > On Fri, Jun 23, 2017 at 07:37:28PM -0400, Julia Lawall wrote: > > > > > > On Sat, 24 Jun 2017, Frans Klaver wrote: > > > > > Hm. For some reason the great mail filtering scheme decided to push > > > this past my inbox :-/ > > > > > > On Sat, Jun 17, 2017 at 12:44 AM, Joe Perches wrote: > > > > On Fri, 2017-06-16 at 19:45 +0200, Frans Klaver wrote: > > > >> The header field in struct pd_message is declared as an __le16 type. The > > > >> data in the message is supposed to be little endian. This means we don't > > > >> have to go and shift the individual bytes into position when we're > > > >> filling the buffer, we can just copy the contents right away. As an > > > >> added benefit we don't get fishy results on big endian systems anymore. > > > > > > > > Thanks for pointing this out. > > > > > > > > There are several instances of this class of error. > > > > > > There are other smells around __(le|be) types that show up in staging > > > that might be worth checking in the rest of the kernel as well. e.g. > > > converting to cpu and storing it back into itself (possibly with its > > > bytes reversed), direct assignments without conversion and what else > > > you might have. sparse obviously already flags anything fishy going on > > > with these types, but cannot distinguish between the classes of > > > errors. I'll need to acquaint myself with spatch a bit more to be able > > > to track that down. > > > > If you have concrete code examples, even fake ones, illustrating a class > > of problem, then that would be great. > > Alright, I'll describe two fairly simple cases for starters. > > One class of issue that I have on top of mind is simply > > __le16 val; > > val = le16_to_cpu(val); > > The problem there obviously being that val is supposed to be guaranteed > little endian. Sparse will throw a warning at this. It may also appear > as (or be 'fixed' as) > > __le16 val; > > le16_to_cpus(val); > > Sparse doesn't flag this second version as an issue, while it causes the > same problem. It is especially a potential problem when the value is > stored in driver data. > > Another smell that is prevalent, at least in staging, is > > u16 in; > u16 out; > > out = cpu_to_le16(in); > > or in one instance (drivers/staging/fbtft/fbtft-io.c) I saw > > u64 tmp; > > *(u64*)dst = cpu_to_be64(tmp); > > Now these aren't necessarily problematic. Usually this typo of code is > preparing the data to be sent out in a specific byte ordering, but again > issues may arise if this specifically ordered data is stored somewhere. > > I'll leave it at that for now. OK, thanks! julia