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Message-ID: <202011220816.8B6591A@keescook> Date: Sun, 22 Nov 2020 08:17:03 -0800 From: Kees Cook <keescook@...omium.org> To: Jakub Kicinski <kuba@...nel.org> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-kernel@...r.kernel.org, alsa-devel@...a-project.org, amd-gfx@...ts.freedesktop.org, bridge@...ts.linux-foundation.org, ceph-devel@...r.kernel.org, cluster-devel@...hat.com, coreteam@...filter.org, devel@...verdev.osuosl.org, dm-devel@...hat.com, drbd-dev@...ts.linbit.com, dri-devel@...ts.freedesktop.org, GR-everest-linux-l2@...vell.com, GR-Linux-NIC-Dev@...vell.com, intel-gfx@...ts.freedesktop.org, intel-wired-lan@...ts.osuosl.org, keyrings@...r.kernel.org, linux1394-devel@...ts.sourceforge.net, linux-acpi@...r.kernel.org, linux-afs@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, linux-arm-msm@...r.kernel.org, linux-atm-general@...ts.sourceforge.net, linux-block@...r.kernel.org, linux-can@...r.kernel.org, linux-cifs@...r.kernel.org, linux-crypto@...r.kernel.org, linux-decnet-user@...ts.sourceforge.net, linux-ext4@...r.kernel.org, linux-fbdev@...r.kernel.org, linux-geode@...ts.infradead.org, linux-gpio@...r.kernel.org, linux-hams@...r.kernel.org, linux-hwmon@...r.kernel.org, linux-i3c@...ts.infradead.org, linux-ide@...r.kernel.org, linux-iio@...r.kernel.org, linux-input@...r.kernel.org, linux-integrity@...r.kernel.org, linux-mediatek@...ts.infradead.org, linux-media@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org, linux-mtd@...ts.infradead.org, linux-nfs@...r.kernel.org, linux-rdma@...r.kernel.org, linux-renesas-soc@...r.kernel.org, linux-scsi@...r.kernel.org, linux-sctp@...r.kernel.org, linux-security-module@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, linux-usb@...r.kernel.org, linux-watchdog@...r.kernel.org, linux-wireless@...r.kernel.org, netdev@...r.kernel.org, netfilter-devel@...r.kernel.org, nouveau@...ts.freedesktop.org, op-tee@...ts.trustedfirmware.org, oss-drivers@...ronome.com, patches@...nsource.cirrus.com, rds-devel@....oracle.com, reiserfs-devel@...r.kernel.org, samba-technical@...ts.samba.org, selinux@...r.kernel.org, target-devel@...r.kernel.org, tipc-discussion@...ts.sourceforge.net, usb-storage@...ts.one-eyed-alien.net, virtualization@...ts.linux-foundation.org, wcn36xx@...ts.infradead.org, x86@...nel.org, xen-devel@...ts.xenproject.org, linux-hardening@...r.kernel.org, Nick Desaulniers <ndesaulniers@...gle.com>, Nathan Chancellor <natechancellor@...il.com>, Miguel Ojeda <ojeda@...nel.org>, Joe Perches <joe@...ches.com> Subject: Re: [PATCH 000/141] Fix fall-through warnings for Clang On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote: > On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote: > > On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote: > > > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote: > > > > This series aims to fix almost all remaining fall-through warnings in > > > > order to enable -Wimplicit-fallthrough for Clang. > > > > > > > > In preparation to enable -Wimplicit-fallthrough for Clang, explicitly > > > > add multiple break/goto/return/fallthrough statements instead of just > > > > letting the code fall through to the next case. > > > > > > > > Notice that in order to enable -Wimplicit-fallthrough for Clang, this > > > > change[1] is meant to be reverted at some point. So, this patch helps > > > > to move in that direction. > > > > > > > > Something important to mention is that there is currently a discrepancy > > > > between GCC and Clang when dealing with switch fall-through to empty case > > > > statements or to cases that only contain a break/continue/return > > > > statement[2][3][4]. > > > > > > Are we sure we want to make this change? Was it discussed before? > > > > > > Are there any bugs Clangs puritanical definition of fallthrough helped > > > find? > > > > > > IMVHO compiler warnings are supposed to warn about issues that could > > > be bugs. Falling through to default: break; can hardly be a bug?! > > > > It's certainly a place where the intent is not always clear. I think > > this makes all the cases unambiguous, and doesn't impact the machine > > code, since the compiler will happily optimize away any behavioral > > redundancy. > > If none of the 140 patches here fix a real bug, and there is no change > to machine code then it sounds to me like a W=2 kind of a warning. FWIW, this series has found at least one bug so far: https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/ -- Kees Cook
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