linux-kernel - Re: [PATCH v2] ftrace: Add FTRACE_MCOUNT_MAX

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Date:   Thu, 26 May 2022 09:14:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, x86@...nel.org
Subject: Re: [PATCH v2] ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding
 weak function

On Thu, 26 May 2022 09:39:49 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > If CONFIG_HAVE_FENTRY is defined for x86, define FTRACE_MCOUNT_MAX_OFFSET
> > to zero, which will have ftrace ignore all locations that are not at the
> > start of the function.  
> 
> ^^^ that paragraph is obsolete by your own changes thing below :-)

Well, it is partially correct (if X86_KERNEL_IBT is disabled).

I'll update the change log to reflect the update.

> 
> > [1] https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/
> > 
> > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> > ---
> > Changes since v1: https://lore.kernel.org/all/20220503150410.2d9e88aa@rorschach.local.home/
> > 
> >  - Changed MAX_OFFSET to 4 on x86 if KERNEL_IBT is enabled
> >    (Reminded by Peter Zijlstra)
> > 
> >  arch/x86/include/asm/ftrace.h | 10 +++++++
> >  kernel/trace/ftrace.c         | 50 +++++++++++++++++++++++++++++++++--
> >  2 files changed, 58 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> > index 024d9797646e..53675fe2d847 100644
> > --- a/arch/x86/include/asm/ftrace.h
> > +++ b/arch/x86/include/asm/ftrace.h
> > @@ -9,6 +9,16 @@
> >  # define MCOUNT_ADDR		((unsigned long)(__fentry__))
> >  #define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */
> >  
> > +/* Ignore unused weak functions which will have non zero offsets */
> > +#ifdef CONFIG_HAVE_FENTRY
> > +# ifdef CONFIG_X86_KERNEL_IBT
> > +/* endbr64 is 4 bytes in front of the fentry */
> > +#  define FTRACE_MCOUNT_MAX_OFFSET	4
> > +# else
> > +#  define FTRACE_MCOUNT_MAX_OFFSET	0
> > +# endif
> > +#endif  
> 
> #define FTRACE_MCOUNT_MAX_OFFSET ENDBR_INSN_SIZE
> 
> Should do the same I think, less lines etc..

Thanks, I figured there was something else I could use besides hard coding
it. But I may add that as a separate patch after this gets merged, because
my branch doesn't have that code in it yet, so it wont compile.

-- Steve