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From: Kees Cook <keescook@chromium.org>
To: Jakub Kicinski <kuba@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>,
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	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Miguel Ojeda <ojeda@kernel.org>, Joe Perches <joe@perches.com>
Subject: Re: [PATCH 000/141] Fix fall-through warnings for Clang
Date: Fri, 20 Nov 2020 12:48:01 -0800	[thread overview]
Message-ID: <202011201244.78E002D5@keescook> (raw)
In-Reply-To: <20201120115142.292999b2@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>

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

  reply	other threads:[~2020-11-20 20:49 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-20 18:21 [PATCH 000/141] Fix fall-through warnings for Clang Gustavo A. R. Silva
2020-11-20 18:28 ` Joe Perches
2020-11-20 19:02   ` Gustavo A. R. Silva
2020-11-20 18:39 ` [PATCH 125/141] sctp: " Gustavo A. R. Silva
2020-11-20 18:53 ` [PATCH 000/141] " Jakub Kicinski
2020-11-20 19:04   ` Gustavo A. R. Silva
2020-11-20 19:30   ` Kees Cook
2020-11-20 19:51     ` Jakub Kicinski
2020-11-20 20:48       ` Kees Cook [this message]
2020-11-22 16:17       ` Kees Cook
2020-11-22 18:21         ` James Bottomley
2020-11-22 18:25           ` Joe Perches
2020-11-22 19:12             ` James Bottomley
2020-11-22 19:22               ` Joe Perches
2020-11-22 19:53                 ` James Bottomley
2020-11-23 13:03                   ` [Intel-wired-lan] " Gustavo A. R. Silva
2020-11-23 16:31                     ` James Bottomley
2020-11-24 21:32                       ` Kees Cook
2020-11-24 22:24                         ` Finn Thain
2020-11-24 23:15                           ` Miguel Ojeda
2020-11-24 23:53                             ` Finn Thain
2020-11-25  1:05                               ` Miguel Ojeda
2020-11-25  7:05                         ` James Bottomley
2020-11-25 12:24                           ` Nick Desaulniers
2020-11-25 21:33                             ` Finn Thain
2020-11-25 22:09                               ` Nick Desaulniers
2020-11-25 23:21                                 ` Finn Thain
2020-11-26  0:30                                 ` Finn Thain
     [not found]                             ` <20201125082405.1d8c23dc@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
2020-11-25 17:04                               ` Miguel Ojeda
2020-11-25 22:09                               ` Nick Desaulniers
2020-11-25 21:10                           ` Kees Cook
2020-11-22 20:35           ` Miguel Ojeda
2020-11-22 22:36             ` James Bottomley
2020-11-23 14:19               ` Miguel Ojeda
2020-11-23 15:58                 ` James Bottomley
2020-11-23 16:24                   ` Rafael J. Wysocki
2020-11-23 16:32                   ` Joe Perches
2020-11-23 18:56                   ` Miguel Ojeda
2020-11-23 20:37                     ` James Bottomley
2020-11-25  0:32                       ` Miguel Ojeda
2020-11-25 22:44                         ` Edward Cree
2020-11-26 14:53                           ` Miguel Ojeda
2020-11-26 15:28                             ` Geert Uytterhoeven
2020-11-26 16:18                               ` Karol Herbst
2020-11-26 17:05                               ` Miguel Ojeda
2020-11-25 10:38                       ` Andy Shevchenko
2020-11-25  9:01                   ` Sean Young
2020-11-22 22:54             ` Finn Thain
2020-11-22 23:04               ` James Bottomley
2020-11-23 14:05               ` Miguel Ojeda
2020-11-24  0:58                 ` Finn Thain
2020-11-24  1:05                   ` Joe Perches
2020-11-24  2:48                     ` Finn Thain
2020-11-24 23:46                   ` Miguel Ojeda
2020-11-22 22:10           ` Sam Ravnborg
2020-11-24  1:32         ` Nick Desaulniers
2020-11-24 21:25           ` Kees Cook
2020-11-25 23:02             ` Edward Cree
2020-12-01 14:08           ` Dan Carpenter
2020-12-01 14:04         ` Dan Carpenter
2020-11-20 22:21 ` Miguel Ojeda
2020-11-23 20:03 ` Jason Gunthorpe
2020-11-24 14:47   ` Gustavo A. R. Silva
     [not found] ` <160616392671.21180.16517492185091399884.b4-ty@kernel.org>
2020-11-24 14:47   ` Gustavo A. R. Silva
2020-12-01  5:52 ` Martin K. Petersen
2020-12-01  8:20   ` Gustavo A. R. Silva
2020-12-08  4:52 ` (subset) " Martin K. Petersen

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