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References: <20200929194318.548707-1-ndesaulniers@google.com> <20200929200801.GA2668747@rani.riverdale.lan>
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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 29 Sep 2020 13:25:47 -0700
Message-ID: <CAKwvOdm=H3GDOPo-dbgsqH7UXzC1twz1h2Rdcidh8OXtFtCY4Q@mail.gmail.com>
Subject: Re: [PATCH] compiler.h: avoid escaped section names
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        linux-sparse@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Joe Perches <joe@perches.com>
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On Tue, Sep 29, 2020 at 1:08 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Sep 29, 2020 at 12:43:18PM -0700, Nick Desaulniers wrote:
> > The stringification operator, `#`, in the preprocessor escapes strings.
> > For example, `# "foo"` becomes `"\"foo\""`.  GCC and Clang differ in how
> > they treat section names that contain \".
> >
> > The portable solution is to not use a string literal with the
> > preprocessor stringification operator.
> >
> > In this case, since __section unconditionally uses the stringification
> > operator, we actually want the more verbose
> > __attribute__((__section__())).
> >
> > Link: https://bugs.llvm.org/show_bug.cgi?id=42950
> > Fixes: commit e04462fb82f8 ("Compiler Attributes: remove uses of __attribute__ from compiler.h")
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> >  include/linux/compiler.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 92ef163a7479..ac45f6d40d39 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -155,7 +155,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >       extern typeof(sym) sym;                                 \
> >       static const unsigned long __kentry_##sym               \
> >       __used                                                  \
> > -     __section("___kentry" "+" #sym )                        \
> > +     __attribute__((__section__("___kentry+" #sym)))         \
> >       = (unsigned long)&sym;
> >  #endif
> >
> > --
> > 2.28.0.709.gb0816b6eb0-goog
> >
>
> There was this previous mini-thread:
> https://lore.kernel.org/lkml/20200629205448.GA1474367@rani.riverdale.lan/
> and this older one:
> https://lore.kernel.org/lkml/20190904181740.GA19688@gmail.com/
>
> Just for my own curiosity: how does KENTRY actually get used? grep
> doesn't show any hits, and the thread from 2019 was actually going to
> drop it if I read it right, and also just remove stringification from
> the __section macro.

Oh, sorry I didn't respond on that thread; I could have sworn I ran a
grep for KENTRY back then.

$ git log -S KENTRY

Doesn't seem to get any hits, so I'm not sure what I should use for a
proper Fixes tag in the event we just remove it.  Let me grab lunch,
then I'll send a v2 that just removes the KENTRY block.  Thanks for
the reminder!

And I don't remember what ever happened to Joe's script for treewide
conversion of __section.
-- 
Thanks,
~Nick Desaulniers