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Date:   Fri, 20 Nov 2020 11:30:40 -0800
From:   Kees Cook <>
To:     Jakub Kicinski <>
Cc:     "Gustavo A. R. Silva" <>,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
        Nick Desaulniers <>,
        Nathan Chancellor <>,
        Miguel Ojeda <>, Joe Perches <>
Subject: Re: [PATCH 000/141] Fix fall-through warnings for Clang

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

Kees Cook

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