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

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Message-ID: <20180131102750.GN2269@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Jan 2018 11:27:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 20/24] objtool: Another static block fail

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.