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

From: Josh Poimboeuf
Date: Tue Nov 16 2021 - 16:27:56 EST

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?

Is there a reason for that?