linux-kernel - Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

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Date:   Tue, 28 Nov 2017 17:07:36 -0800
From:   Joe Perches <joe@...ches.com>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

On Tue, 2017-11-28 at 14:37 -0600, Gustavo A. R. Silva wrote:
> Quoting Linus Torvalds <torvalds@...ux-foundation.org>:
> 
> > On Tue, Nov 28, 2017 at 11:00 AM, Alan Cox  
> > <gnomes@...rguk.ukuu.org.uk> wrote:
> > > 
> > > The notation in question has been standard in tools like lint since the
> > > end of the 1970s
> > 
> > Yes.
> > 
> > That said, maybe one option would be to annotate the "case:" and
> > "default:" statements if that makes people happier.
> > 
> > IOW, we could do something like
> > 
> >     #define fallthrough __atttibute__((fallthrough))
> > 
> > and then write
> > 
> >     fallthrough case 1:
> >         ...
> > 
> > which while absolutely not traditional, might look and read a bit more
> > logical to people. I mean, it literally _is_ a "fallthrough case", so
> > it makes semantic sense.
> > 
> 
> This is elegant. The thing is that this makes it appear as if there is  
> an unconditional fall through.
> 
> It is not uncommon to have multiple break statements in the same case  
> block and to fall through also.

My preferred syntax would be to use __fallthrough or fallthrough
in the same manner as break;

	switch (foo) {
	case bar:
		bar();
		fallthrough;
	case baz:
		baz();
		break;
	default;
		qux();
		exit(1);
	}