Re: [PATCH net-next v2 3/8] x86/jump_label: expect default_nop if static_key gets enabled on boot-up

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Sat, Oct 05, 2013 at 08:05:58PM -0400, Steven Rostedt wrote:
> >  	if (type == JUMP_LABEL_ENABLE) {
> > -		/*
> > -		 * We are enabling this jump label. If it is not a nop
> > -		 * then something must have gone wrong.
> > -		 */
> > -		if (unlikely(memcmp((void *)entry->code, ideal_nop, 5) != 0))
> > -			bug_at((void *)entry->code, __LINE__);
> > +		if (init) {
> > +			/*
> > +			 * Jump label is enabled for the first time.
> > +			 * So we expect a default_nop...
> > +			 */
> > +			if (unlikely(memcmp((void *)entry->code, default_nop, 5)
> > +				     != 0))
> > +				bug_at((void *)entry->code, __LINE__);
> > +		} else {
> > +			/*
> > +			 * ...otherwise expect an ideal_nop. Otherwise
> > +			 * something went horribly wrong.
> > +			 */
> > +			if (unlikely(memcmp((void *)entry->code, ideal_nop, 5)
> > +				     != 0))
> > +				bug_at((void *)entry->code, __LINE__);
> > +		}
> 
> I don't know if I like this change. This is similar to a bug we had
> with the Xen folks, where they didn't realize that jump labels are not
> suppose to be used (or set) before jump_label_init() is called.
> 
> I'll have to take a deeper look at this on Monday.

Yes, I understand and saw the commit to call jump_label_init
earlier. Maybe the default could be to insert illegal instructions by
default if we try to replace them with nops or branches afterwards anyway.

insn_sanity programs would have to be tought about that, then.

Greetings,

  Hannes