linux-kernel - Re: [PATCH] clk: samsung: s3c2443: Mark expected switch fall-through

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Message-ID: <155078541292.77512.12594007340364187685@swboyd.mtv.corp.google.com>
Date:   Thu, 21 Feb 2019 13:43:32 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Krzysztof Kozlowski <krzk@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kukjin Kim <kgene@...nel.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Tomasz Figa <tomasz.figa@...il.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Michael Turquette <mturquette@...libre.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>, linux-clk@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: samsung: s3c2443: Mark expected switch fall-through

Quoting Kees Cook (2019-02-20 14:26:06)
> On Fri, Feb 15, 2019 at 4:34 PM Stephen Boyd <sboyd@...nel.org> wrote:
> > Quoting Kees Cook (2019-02-12 10:57:05)
> > > On Mon, Feb 11, 2019 at 11:41 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
> > > >
> > > > It is just the GCC which has to be fixed not the code. You want to
> > > > adjust the code for specific version of GCC and what if GCC changes
> > > > its warning? For example GCC might require "fall through: "... or any
> > > > other syntax. Another point - what about clang's syntax?
> > >
> > > -Wimplicit-fallthrough=3 is stricter and maps to -Wextra, hence its
> > > choice. GCC's levels were chosen based on the existing linters, static
> > > analyzers, etc. The patterns are unlikely to change (see the gcc
> > > man-page).
> > >
> > > Clang doesn't recognize anything in C mode (hopefully this will be
> > > fixed in the future[1]).
> > >
> > > As long as one of the compilers is able to check this, we'll avoid the
> > > bugs associated with this mis-pattern. Gustavo's efforts here have
> > > found kind of a lot of bugs, so I think it's worth a little churn to
> > > add these (and make minor adjustments to existing comments).
> >
> > Just curious, what compilation phase does this check run in? Could we
> > gain a macro like FALLTHRU or even lowercase 'fallthru' that expanded to
> > whatever the compiler wants to see and then there would only be "one
> > way" to do this?  It would alleviate the above concerns, but maybe I'm
> > rehashing something that's already been proposed and rejected.
> 
> When this got discussed a while back, the thinking was that since
> we're also dealing with static analyzers (e.g. Coverity) and IDEs that
> literally parse comments in the code, it was most sensible (at least
> for now, prior to there being a formal C "fall through" statement --
> there is for C++ but not yet for C), we'd stick to explicit comments.
> In theory, we will be able to do a tree-wide change to add the C
> statement once it exists.

Ok, thanks for the background. Looks like the perf tool already
introduced the #define __fallthrough that they use for this purpose.
Maybe they're hoping that it will be formalized.