From: Jason Gunthorpe <jgg@ziepe.ca>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Nathan Chancellor <natechancellor@gmail.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Michal Marek <michal.lkml@markovi.net>,
Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@redhat.com>,
Jason Baron <jbaron@akamai.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
Linux Memory Management List <linux-mm@kvack.org>,
clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH 3/6] tracing: Wrap section comparison in tracer_alloc_buffers with COMPARE_SECTIONS
Date: Wed, 19 Feb 2020 14:16:19 -0400 [thread overview]
Message-ID: <20200219181619.GV31668@ziepe.ca> (raw)
In-Reply-To: <CAKwvOdm-N1iX0SMxGDV5Vf=qS5uHPdH3S-TRs-065BuSOdKt1w@mail.gmail.com>
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@goodmis.org> wrote:
> >
> > On Tue, 18 Feb 2020 21:54:20 -0700
> > Nathan Chancellor <natechancellor@gmail.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@gmail.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
next prev parent reply other threads:[~2020-02-19 18:16 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-19 4:54 [PATCH 0/6] Silence some instances of -Wtautological-compare and enable globally Nathan Chancellor
2020-02-19 4:54 ` [PATCH 1/6] asm/sections: Add COMPARE_SECTIONS macro Nathan Chancellor
2020-02-19 4:54 ` [PATCH 2/6] kernel/extable: Wrap section comparison in sort_main_extable with COMPARE_SECTIONS Nathan Chancellor
2020-02-19 4:54 ` [PATCH 3/6] tracing: Wrap section comparison in tracer_alloc_buffers " Nathan Chancellor
2020-02-19 14:34 ` Steven Rostedt
2020-02-19 17:44 ` Nick Desaulniers
2020-02-19 18:16 ` Jason Gunthorpe [this message]
2020-02-19 19:09 ` Steven Rostedt
2020-02-19 19:11 ` Nick Desaulniers
2020-02-19 19:22 ` Nathan Chancellor
2020-02-19 19:43 ` Steven Rostedt
2020-02-19 19:54 ` Jason Gunthorpe
2020-02-19 21:32 ` Nick Desaulniers
2020-02-19 4:54 ` [PATCH 4/6] dynamic_debug: Wrap section comparison in dynamic_debug_init " Nathan Chancellor
2020-02-19 4:54 ` [PATCH 5/6] mm: kmemleak: Wrap section comparison in kmemleak_init " Nathan Chancellor
2020-02-19 4:54 ` [PATCH 6/6] kbuild: Enable -Wtautological-compare Nathan Chancellor
2020-04-21 4:03 ` [PATCH 0/6] Silence some instances of -Wtautological-compare and enable globally Andrew Morton
2020-04-21 4:32 ` Nathan Chancellor
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