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

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Date:   Thu, 26 May 2022 09:11:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.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>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Subject: Re: [PATCH v2] ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding
 weak function

On Thu, 26 May 2022 04:50:17 +0200
Ingo Molnar <mingo@...nel.org> wrote:


> > The real fix would be to fix kallsyms to not show address of weak
> > functions as the function before it. But that would require adding code in
> > the build to add function size to kallsyms so that it can know when the
> > function ends instead of just using the start of the next known symbol.  
> 
> Yeah, so I actually have a (prototype...) objtool based kallsyms 
> implementation in the (way too large) fast-headers tree that is both faster 
> & allows such details in principle:

Nice.

Will this work for other architectures too?

> > 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.
> > 
> > [1] https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/
> > 
> > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>  
> 
> LGTM.
> 
> I suppose you'd like to merge this via the tracing tree? If so:

Yeah, I'll pull it through my tree.

> 
>   Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks a lot Ingo for reviewing it. I really appreciate it!

-- Steve