linux-kernel - Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

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Date:   Thu, 18 Nov 2021 08:15:23 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        live-patching@...r.kernel.org, nstange@...e.de
Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

On Tue, 16 Nov 2021, Josh Poimboeuf wrote:

> On Mon, Nov 15, 2021 at 01:59:36PM +0100, Miroslav Benes wrote:
> > On Fri, 12 Nov 2021, Josh Poimboeuf wrote:
> > 
> > > If the child schedules out, and then the parent gets patched, things can
> > > go off-script if the child later jumps back to the unpatched version of
> > > the parent, and then for example the old parent tries to call another
> > > patched function with a since-changed ABI.
> > 
> > ...
> >  
> > > I don't know about other patch creation tooling, but I'd imagine they
> > > also need to know about .cold functions, unless they have that
> > > optimization disabled.  Because the func and its .cold counterpart
> > > always need to be patched together.
> > 
> > We, at SUSE, solve the issue differently... the new patched parent would 
> > call that another patched function with a changed ABI statically in a live 
> > patch. So in that example, .cold child would jump back to the unpatched 
> > parent which would then call, also, the unpatched function.
> 
> So if I understand correctly, if a function's ABI changes then you don't
> patch it directly, but only patch its callers to call the replacement?

Yes.
 
> Is there a reason for that?

Not really. There were a couple of cases in the past where this was safer 
to implement and then it became a habbit, I guess.

[ Nicolai CCed, if he wants to add more details ]

Miroslav