linux-kernel - Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

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Message-ID: <20200219164638.GE2935@paulmck-ThinkPad-P72>
Date:   Wed, 19 Feb 2020 08:46:38 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, mingo@...nel.org,
        joel@...lfernandes.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

On Wed, Feb 19, 2020 at 05:34:09PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 19, 2020 at 08:27:47AM -0800, Paul E. McKenney wrote:
> > On Wed, Feb 19, 2020 at 11:12:28AM -0500, Steven Rostedt wrote:
> > > On Wed, 19 Feb 2020 17:04:42 +0100
> > > Peter Zijlstra <peterz@...radead.org> wrote:
> > > 
> > > > > -		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> > > > > +		for (i = 0; i < count; i++) {
> > > > > +			int idx = (dst <= src) ? i : count - i;  
> > > > 
> > > > That's an off-by-one for going backward; 'count - 1 - i' should work
> > > > better, or I should just stop typing for today ;-)
> > > 
> > > Or, we could just cut and paste the current memmove and make a notrace
> > > version too. Then we don't need to worry bout bugs like this.
> > 
> > OK, I will bite...
> > 
> > Can we just make the core be an inline function and make a notrace and
> > a trace caller?  Possibly going one step further and having one call
> > the other?  (Presumably the traceable version invoking the notrace
> > version, but it has been one good long time since I have looked at
> > function preambles.)
> 
> One complication is that GCC (and others) are prone to stick their own
> implementation of memmove() (and other string functions) in at 'random'.
> That is, it is up to the compiler's discretion wether or not to put a
> call to memmove() in or just emit some random giberish they feel has the
> same effect.
> 
> So if we go play silly games like that, we need be careful (or just call
> __memmove I suppose, which is supposed to avoid that IIRC).

Urgh, good point.  :-/

							Thanx, Paul