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

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Message-ID: <alpine.DEB.2.20.1711281911280.2222@nanos>
Date:   Tue, 28 Nov 2017 19:17:37 +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, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

On Tue, 28 Nov 2017, Thomas Gleixner wrote:
> On Tue, 28 Nov 2017, Gustavo A. R. Silva wrote:
> > Quoting Thomas Gleixner <tglx@...utronix.de>:
> > 
> > > On Mon, 27 Nov 2017, Gustavo A. R. Silva wrote:
> > > 
> > > > In preparation to enabling -Wimplicit-fallthrough, mark switch cases
> > > > where we are expecting to fall through.
> > > 
> > > > 		case 0:
> > > > 			if (!n--) break;
> > > > 			*args++ = regs->bx;
> > > > +			/* fall through */
> > > 
> > > And these gazillions of pointless comments help enabling of
> > > -Wimplicit-fallthrough in which way?
> > > 
> > 
> > The -Wimplicit-fallthrough option was added to GCC 7. We want to add that
> > option to the top-level Makefile so we can have the compiler help us not make
> > mistakes as missing "break"s or "continue"s. This also documents the intention
> > for humans and provides a way for analyzers to report issues or ignore False
> > Positives.
> > 
> > So prior to adding such option to the Makefile, we have to properly add a code
> > comment wherever the code is intended to fall through.
> > 
> > During the process of placing these comments I have identified actual bugs
> > (missing "break"s/"continue"s) in a variety of components in the kernel, so I
> > think this effort is valuable. Lastly, such a simple comment in the code can
> > save a person plenty of time during a code review.
> 
> 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.

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.

Sure, but slapping a comment everywhere is just simpler than reading the
documentation and make something useful and understandable.

Thanks,

	tglx