linux-kernel - Re: [RFC] ftrace: Add support to keep some functions out of ftrace

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Date:   Mon, 15 Aug 2022 08:49:11 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, 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>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace

On Mon, Aug 15, 2022 at 8:41 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Aug 15, 2022 at 05:28:02PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 15, 2022 at 08:17:42AM -0700, Alexei Starovoitov wrote:
> > > It's hiding a fake function from ftrace, since it's not a function
> > > and ftrace infra shouldn't show it tracing logs.
> > > In other words it's a _notrace_ function with nop5.
> >
> > Then make it a notrace function with a nop5 in it. That isn't hard.
> >
> > The whole problem is that it isn't a notrace function and you're abusing
> > a __fentry__ site.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=x86/fineibt&id=8d075bdf11193f1d276bf19fa56b4b8dfe24df9e

Brand new stuff.
Awesome. That should fit perfectly.

> foo.c:
>
> __attribute__((__no_instrument_function__))
> __attribute__((patchable_function_entry(5)))

Interesting. Didn't know about this attribute.

> void my_func(void)
> {
> }
>
> void my_foo(void)
> {
> }

Great.
Jiri, could you please revise your patch with this approach?