Re: [PATCH 20/24] objtool: Another static block fail

[Peter Zijlstra]

On Wed, Jan 31, 2018 at 10:07:06AM +0000, David Woodhouse wrote:
> On Wed, 2018-01-31 at 11:01 +0100, Peter Zijlstra wrote:
> > On Tue, Jan 30, 2018 at 09:12:21PM -0600, Josh Poimboeuf wrote:
> > > 
> > > Or, maybe we should just forget the whole thing and just stick with the
> > > dynamic IBRS checks with lfence.  Yes, it's less ideal for the kernel,
> > > but adding these acrobatics to objtool also has a cost.
> > 
> > For now, IBRS seems off the table entirely. But no, I really don't want
> > to have to unconditionally eat the LFENCE cost in all those sites.
> 
> There's also alternatives. And without the IBRS-on-kernel-entry bits
> there aren't that many call sites that really need this anyway and
> don't have *other* conditionals that really are runtime-only (like
> dumpable etc.).

There is that.. So I think people wanted to use jump_labels for IBRS so
that we could runtime enable it when people loaded dodgy modules or
something. But even that we could all write in inline asm if needed I
suppose, it'll be ugly, but it should work.