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

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Date:   Wed, 19 Feb 2020 17:34:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 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).