linux-kernel - Re: [RFC][PATCH] x86/ftrace: Have ftrace trampolines turn read-only at the end of system boot up

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Date:   Fri, 1 May 2020 10:13:10 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC][PATCH] x86/ftrace: Have ftrace trampolines turn read-only
 at the end of system boot up

On Fri, May 01, 2020 at 09:24:04AM -0400, Steven Rostedt wrote:
> On Fri, 1 May 2020 00:17:06 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > > Would it be easier to just call a new __text_poke_bp() which skips the
> > > SYSTEM_BOOTING check, since you know the trampoline will always be
> > > read-only?
> > > 
> > > Like:  
> > 
> > early_trace_init() is called after mm_init(), so I thought it might
> > work, but I guess not:
> 
> Yeah, I was about to say that this happens before mm_init() ;-)

It happens *after* mm_init().  But now text_poke() has a dependency on
poking_init(), has a dependency on proc_caches_init(), which has a
dependency on kmem_cache_init_late(), etc.

So how early do you need early_trace_init()?  I'm assuming moving it to
after kmem_cache_init_late() would be too late.

> It's why we already have magic for enabling function tracing the first time.
> 
> Do you see anything wrong with this current solution? It probably needs
> more comments, but I wanted to get acceptance on the logic before I go and
> pretty it up and send a non RFC patch.

Assuming we can't get text_poke() working earlier, it seems reasonable
to me.

-- 
Josh