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

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Date:   Wed, 19 Feb 2020 14:16:19 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        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 09:44:31AM -0800, Nick Desaulniers wrote:
> On Wed, Feb 19, 2020 at 6:34 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Tue, 18 Feb 2020 21:54:20 -0700
> > Nathan Chancellor <natechancellor@...il.com> wrote:
> >
> > > Clang warns:
> > >
> > > ../kernel/trace/trace.c:9335:33: warning: array comparison always
> > > evaluates to true [-Wtautological-compare]
> > >         if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
> > >                                        ^
> > > 1 warning generated.
> > >
> > > These are not true arrays, they are linker defined symbols, which are
> > > just addresses so there is not a real issue here. Use the
> > > COMPARE_SECTIONS macro to silence this warning by casting the linker
> > > defined symbols to unsigned long, which keeps the logic the same.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/765
> > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > >  kernel/trace/trace.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > index c797a15a1fc7..e1f3b16e457b 100644
> > > +++ b/kernel/trace/trace.c
> > > @@ -9332,7 +9332,7 @@ __init static int tracer_alloc_buffers(void)
> > >               goto out_free_buffer_mask;
> > >
> > >       /* Only allocate trace_printk buffers if a trace_printk exists */
> > > -     if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
> > > +     if (COMPARE_SECTIONS(__stop___trace_bprintk_fmt, !=, __start___trace_bprintk_fmt))
> >
> > Sorry, but this is really ugly and unreadable. Please find some other
> > solution to fix this.
> >
> > NAK-by: Steven Rostedt
> >
> 
> Hey Nathan,
> Thanks for the series; enabling the warning will help us find more
> bugs.  Revisiting what the warning is about, I checked on this
> "referring to symbols defined in linker scripts from C" pattern.  This
> document [0] (by no means definitive, but I think it has a good idea)
> says:
> 
> Linker symbols that represent a data address: In C code, declare the
> variable as an extern variable. Then, refer to the value of the linker
> symbol using the & operator. Because the variable is at a valid data
> address, we know that a data pointer can represent the value.
> Linker symbols for an arbitrary address: In C code, declare this as an
> extern symbol. The type does not matter. If you are using GCC
> extensions, declare it as "extern void".
> 
> Indeed, it seems that Clang is happier with that pattern:
> https://godbolt.org/z/sW3t5W
> 
> Looking at __stop___trace_bprintk_fmt in particular:
> 
> kernel/trace/trace.h
> 1923:extern const char *__stop___trace_bprintk_fmt[];

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.

I think this warning is specific to arrays and is designed to detect
programmer errors like:
  int a[1];
  int b[1];
  return a < b;

Where the author intended to use memcmp()

Jason