Re: [RFC PATCH 3/4] livepatch: Add "replaceable" attribute to klp_patch

Yafang Shao
2026-04-07T03:17:33Z

On Tue, Apr 7, 2026 at 10:54 AM Song Liu <song@kernel.org> wrote:
>
> On Mon, Apr 6, 2026 at 2:12 PM Joe Lawrence <joe.lawrence@redhat.com> wrote:
> [...]
> > > > > - The regular livepatches are cumulative, have the replace flag; and
> > > > > are replaceable.
> > > > > - The occasional "off-band" livepatches do not have the replace flag,
> > > > > and are not replaceable.
> > > > >
> > > > > With this setup, for systems with off-band livepatches loaded, we can
> > > > > still release a cumulative livepatch to replace the previous cumulative
> > > > > livepatch. Is this the expected use case?
> > > >
> > > > That matches our expected use case.
> > >
> > > If we really want to serve use cases like this, I think we can introduce
> > > some replace tag concept: Each livepatch will have a tag, u32 number.
> > > Newly loaded livepatch will only replace existing livepatch with the
> > > same tag. We can even reuse the existing "bool replace" in klp_patch,
> > > and make it u32: replace=0 means no replace; replace > 0 are the
> > > replace tag.
> > >
> > > For current users of cumulative patches, all the livepatch will have the
> > > same tag, say 1. For your use case, you can assign each user a
> > > unique tag. Then all these users can do atomic upgrades of their
> > > own livepatches.
> > >
> > > We may also need to check whether two livepatches of different tags
> > > touch the same kernel function. When that happens, the later
> > > livepatch should fail to load.

That sounds like a viable solution. I'll look into it and see how we
can implement it.

> > >
> > > Does this make sense?
> > >
> >
> > I haven't been following the thread carefully, but could the Livepatch
> > system state API (see Documentation/livepatch/system-state.rst) be
> > leveraged somehow instead of adding further replace semantics?
>
> AFAICT, system state will not help Yafang's use case.

Right.

--
Regards
Yafang