From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Date: Fri, 20 Nov 2020 12:48:01 -0800 Subject: [Cluster-devel] [PATCH 000/141] Fix fall-through warnings for Clang In-Reply-To: <20201120115142.292999b2@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> References: <20201120105344.4345c14e@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> <202011201129.B13FDB3C@keescook> <20201120115142.292999b2@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> Message-ID: <202011201244.78E002D5@keescook> List-Id: To: cluster-devel.redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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. I'd like to avoid splitting common -W options between default and W=2 just based on the compiler. Getting -Wimplicit-fallthrough enabled found plenty of bugs, so making sure it works correctly for both compilers feels justified to me. (This is just a subset of the same C language short-coming.) > I think clang is just being annoying here, but if I'm the only one who > feels this way chances are I'm wrong :) It's being pretty pedantic, but I don't think it's unreasonable to explicitly state how every case ends. GCC's silence for the case of "fall through to a break" doesn't really seem justified. -- Kees Cook