linux-kernel - Re: [PATCH 3/6] tracing: Wrap section comparison in tracer_alloc_buffers with COMPARE

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Message-ID: <CAKwvOdktG0vZOZVtNJBk1COhOnLYv3MU5KNQ8Z40L4ph5QcnRg@mail.gmail.com>
Date:   Wed, 19 Feb 2020 13:32:38 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 3/6] tracing: Wrap section comparison in
 tracer_alloc_buffers with COMPARE_SECTIONS

On Wed, Feb 19, 2020 at 11:54 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Wed, Feb 19, 2020 at 11:11:19AM -0800, Nick Desaulniers wrote:
> > > Godbolt says clang is happy if it is written as:
> > >
> > >   if (&__stop___trace_bprintk_fmt[0] != &__start___trace_bprintk_fmt[0])
> > >
> > > Which is probably the best compromise. The type here is const char
> > > *[], so it would be a shame to see it go.
> >
> > If the "address" is never dereferenced, but only used for arithmetic
> > (in a way that the the pointed to type is irrelevant), does the
> > pointed to type matter?
>
> The type is used here:
>
>         if (*pos < start_index)
>                 return __start___trace_bprintk_fmt + *pos;
>
> The return expression should be a const char **
>
> Presumably the caller of find_next derferences it.
>
> Jason

And the callers of find_next just return the return value from
find_next, but suddenly as `void*` (find_next()'s return type is
`char**`).  So it doesn't seem like the pointed to type matters, hence
the recommendation of `void` and then address-of (&) used in
comparison+arithmetic.

-- 
Thanks,
~Nick Desaulniers