Re: [PATCH 1/2] Compiler Attributes: add support for __fallthrough (gcc >= 7.1)

From: Nick Desaulniers <ndesaulniers@google.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>,
	Greg KH <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	dan.carpenter@oracle.com, adilger.kernel@dilger.ca,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Michal Marek <michal.lkml@markovi.net>,
	rostedt@goodmis.org, mchehab+samsung@kernel.org, olof@lxom.net,
	Konstantin Ryabitsev <konstantin@linuxfoundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Kees Cook <keescook@chromium.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Paul Lawrence <paullawrence@google.com>,
	sandipan@linux.vnet.ibm.com,
	Andrey Konovalov <andreyknvl@google.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	paul.burton@mips.com, David Rientjes <rientjes@google.com>,
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	Jonathan Corbet <corbet@lwn.net>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	joe@perches.com, Arnd Bergmann <arnd@arndb.de>,
	asmadeus@codewreck.org, Stefan Agner <stefan@agner.ch>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-sparse@vger.kernel.org,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Colin Ian King <colin.king@canonical.com>
Subject: Re: [PATCH 1/2] Compiler Attributes: add support for __fallthrough (gcc >= 7.1)
Date: Mon, 22 Oct 2018 10:23:14 -0700	[thread overview]
Message-ID: <CAKwvOdm7Ts1ZpYV2qw1YLSg8ntjPekY8Gu=Q1CbSq_7==ekUMw@mail.gmail.com> (raw)
In-Reply-To: <CANiq72mXV6WTyHXNZJDTfFOpPrXD0zD6cB30bGOuKO44AvRiog@mail.gmail.com>

On Mon, Oct 22, 2018 at 2:26 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Mon, Oct 22, 2018 at 12:27 AM Theodore Y. Ts'o <tytso@mit.edu> wrote:
> >
> > On Sun, Oct 21, 2018 at 07:14:13PM +0200, Miguel Ojeda wrote:
> > > From the GCC manual:
> > >
> > >   fallthrough
> > >
> > >     The fallthrough attribute with a null statement serves as a
> > >     fallthrough statement. It hints to the compiler that a statement
> > >     that falls through to another case label, or user-defined label
> > >     in a switch statement is intentional and thus the -Wimplicit-fallthrough
> > >     warning must not trigger. The fallthrough attribute may appear
> > >     at most once in each attribute list, and may not be mixed with
> > >     other attributes. It can only be used in a switch statement
> > >     (the compiler will issue an error otherwise), after a preceding
> > >     statement and before a logically succeeding case label,
> > >     or user-defined label.
> > >
> > >   https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html
> >
> > Do we know if coverity understands the fallthrough attribute?  One of
> > the reasons why I started using /* fallthrough */ is because it kept
> > Coverity happy.
>
> If Coverity is like gcc, they should be doing both (i.e. I see the
> comment parsing as an "extra" that gcc did, but the "basic stuff" is
> the attribute -- and I would guess it is way easier for them to
> support than the comment parsing).
>
> But I cannot test it myself :-( Someone, please?

+ Colin, who has been running Coverity on the kernel, and sending patches.

>
> However, if I understood Greg correctly in his reply to the cover
> letter, he replied that Coverity knows about it (?).
>
> >
> > If the conversion from /* fallthrough */ to the __fallthrough__
> > attribute means that we start gethting a lot of Coverity warnings,
> > that would be unfortunate.  OTOH, if this is getting standardized,
> > maybe we can get Coverity to understand this attribute?
>
> Indeed! That would be the best for everyone, including Coverity customers.
>
> Cheers,
> Miguel


-- 
Thanks,
~Nick Desaulniers