On Sun, Nov 22, 2020 at 08:17:03AM -0800, Kees Cook wrote: > 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@xxxxxxxxxxxxxx/ This is a fallthrough to a return and not to a break. That should trigger a warning. The fallthrough to a break should not generate a warning. The bug we're trying to fix is "missing break statement" but if the result of the bug is "we hit a break statement" then now we're just talking about style. GCC should limit itself to warning about potentially buggy code. regards, dan carpenter
