linux-kernel - Re: [PATCH v2 1/2] kprobes: propagate error from arm_kprobe

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Message-ID: <20171103133312.1a0646e2@vmware.local.home>
Date:   Fri, 3 Nov 2017 13:33:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Jessica Yu <jeyu@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Ingo Molnar <mingo@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] kprobes: propagate error from
 arm_kprobe_ftrace()

On Fri, 3 Nov 2017 09:53:37 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > > -static void arm_kprobe_ftrace(struct kprobe *p)
> > > +static int arm_kprobe_ftrace(struct kprobe *p)
> > >  {
> > > -	int ret;
> > > +	int ret = 0;
> > >  
> > >  	ret = ftrace_set_filter_ip(&kprobe_ftrace_ops,
> > >  				   (unsigned long)p->addr, 0, 0);
> > > -	WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret);
> > > -	kprobe_ftrace_enabled++;
> > > -	if (kprobe_ftrace_enabled == 1) {
> > > +	if (WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret))
> > > +		return ret;
> > > +
> > > +	if (kprobe_ftrace_enabled == 0) {
> > >  		ret = register_ftrace_function(&kprobe_ftrace_ops);
> > > -		WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret);
> > > +		if (WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret))
> > > +			goto err_ftrace;
> > >  	}
> > > +
> > > +	kprobe_ftrace_enabled++;
> > > +	return ret;
> > > +
> > > +err_ftrace:
> > > +	ftrace_set_filter_ip(&kprobe_ftrace_ops, (unsigned long)p->addr, 1, 0);  
> > 
> > Hmm, this could have a very nasty side effect. If you remove a function
> > from the ops, and it was the last function, an empty ops means to trace
> > *all* functions.  
> 
> But this error path only runs when register_ftrace_function() fails, in
> which case the ops aren't live anyway, right?

I was thinking that if there was more than one function that is going
to be registered, that only this one would be black listed. But yeah,
if there was only one function in the hash, then it probably wouldn't
matter if it was cleared, because it failed. But I'm paranoid about
things like this, and prefer to be more robust than to depend on the
design to enforce correctness than to have each individual function
being contained and do what is expected of it.

-- Steve