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

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Date:   Tue, 28 Nov 2017 19:45:23 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

On Tue, 28 Nov 2017, Thomas Gleixner wrote:

+CC Linus.

> On Tue, 28 Nov 2017, Thomas Gleixner wrote:
> 
> > On Tue, 28 Nov 2017, Gustavo A. R. Silva wrote:
> > > Quoting Thomas Gleixner <tglx@...utronix.de>:
> > > > > To be honest, such comments annoy me during a code review especially when
> > > > > the fallthrough is so obvious as in this case. There might be cases where
> > > > > its worth to document because it's non obvious, but documenting the
> > > > > obvious
> > > > > just for the sake of documenting it is just wrong.
> > > > 
> > > 
> > > I understand that and I agree that in this particular case it is just obvious.
> > > The thing is that if we want to benefit from having the compiler help us to
> > > spot these kind of issues before committing our code, we have to address every
> > > place in the whole code-base.
> > > 
> > > > And _IF_ at all then you want a fixed macro for this and not a comment
> > > > which will be formatted as people see it fit.
> > > > 
> > > > GCC supports: __attribute__ ((fallthrough)) which we can wrap into a macro,
> > > > e.g. falltrough()
> > > > 
> > > > That'd be useful, but adding all these comments and then having to chase a
> > > > gazillion of warning instances to figure out whether there is a comment or
> > > > not is just backwards.
> > > > 
> > > 
> > > I have run into this before and people find what you suggest even uglier.
> > 
> > It's not about ugly. It's about _USEFULL_.
> > 
> > The comments are ugly AND completely useless for the compiler and they are
> > going to be malformatted so checker tools can't differentiate the false
> > positives.
> > 
> > The macro, in which more or less ugly form written, is both documentation
> > and helps the compiler NOT to emit the same crap over and over.
> 
> Just checked and GCC really supports analyzing the comment to some extent.
> 
> But just look at
> 
>     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817
> 
>  " It is not really possible.  __attribute__((fallthrough)) has precise
>    rules on where it can appear, while /* FALLTHRU */ comments, being
>    comments, can appear anywhere.  Especially with -Wimplicit-fallthrough=1
>    when all comments are considered fallthru comments... "
> 
> I have no idea who came up with that brilliant idea of parsing comments in
> the code. It's so simple to make this parser completely fail that it's not
> even funny anymore.
> 
> I don't care what other people prefer. The code base I'm responsible for
> gets either proper annotations or nothing.

And in fact we want ONE solution for the whole kernel. And comments are
obviously the wrong one.

Thanks,

	tglx