Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults

[Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
> 
> > -	if (notify_page_fault(regs))
> > -		return;
> >  	if (unlikely(kmmio_fault(regs, address)))
> >  		return;
> > 
> > @@ -634,6 +632,9 @@ void __kprobes do_page_fault(struct pt_r
> >  		if (spurious_fault(address, error_code))
> >  			return;
> > 
> > +		/* kprobes don't want to hook the spurious faults. */
> > +		if (notify_page_fault(regs))
> > +			return;
> >  		/*
> >  		 * Don't take the mm semaphore here. If we fixup a prefetch
> >  		 * fault we could otherwise deadlock.
> > @@ -641,6 +642,9 @@ void __kprobes do_page_fault(struct pt_r
> >  		goto bad_area_nosemaphore;
> >  	}
> > 
> > +	/* kprobes don't want to hook the spurious faults. */
> > +	if (notify_page_fault(regs))
> > +		return;
> 
> I dont know - this spreads that callback to two places now. Any
> reason why kprobes cannot call spurious_fault(), if there's a
> probe active?
> 
> Also, moving that would remove the planned cleanup of merging these
> two into one call:
> 
>  	if (notify_page_fault(regs))
>  		return;
>   	if (unlikely(kmmio_fault(regs, address)))
>   		return;
> 
> We should reduce the probing cross section, not increase it,
> especially in such a critical codepath as the pagefault handler.
> 
> Btw., why cannot kprobes install a dynamic probe to the fault
> handler itself? That way the default path would have no such
> callbacks and checks at all.
> 

Or we could simply merge my 2 LTTng page fault handler tracepoints per
architecture and be done with it ?

I'd need to clean up the patchset a little bit to fold a few patches,
but that would be straightforward enough.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68