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

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Message-ID: <20200501121916.310942b8@gandalf.local.home>
Date:   Fri, 1 May 2020 12:19:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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, 1 May 2020 10:13:10 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> 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.

People have asked to move it even earlier. The point of having it early is
to allow tracing to debug early boot up.

> 
> > 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.
> 

Thanks.

Peter, what about you?

-- Steve