Re: [PATCH] samples/livepatch: Add BPF struct_ops integration sample

[Petr Mladek <pmladek@suse.com>]

On Sun 2026-04-19 11:19:19, Yafang Shao wrote:
> On Fri, Apr 17, 2026 at 11:52 PM Song Liu <song@kernel.org> wrote:
> >
> > On Fri, Apr 17, 2026 at 6:20 AM Petr Mladek <pmladek@suse.com> wrote:
> > >
> > > On Thu 2026-04-16 09:32:46, Song Liu wrote:
> > [...]
> > > Let' use the code from this patch:
> > >
> > > static int __init livepatch_bpf_init(void)
> > > {
> > >         int ret;
> > >
> > >         ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
> > >                                         &klp_bpf_kfunc_set);
> > >         ret = ret ?: register_bpf_struct_ops(&bpf_klp_bpf_cmdline_ops,
> > >                                              klp_bpf_cmdline_ops);
> > >         if (ret)
> > >                 return ret;
> > >
> > > --->    /*
> > > --->     * We would need to wait here until the BPF program gets loaded.
> > > --->     * for the new bpf_struct_ops in this new livepatch.
> > > --->     */
> 
> No waiting is necessary. If the BPF program is not attached, the
> default logic can be executed instead.

But it means a regression. I guess that you need the BPF program
for a reason. The default logic is not good enough indeed.

> Consider Song's test case: we can handle it as follows.
> 
> static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
> {
>     struct klp_bpf_cmdline_ops *ops = READ_ONCE(active_ops);
> 
>     if (ops && ops->set_cmdline)
>         return ops->set_cmdline(m);
> 
>     // If no BPF program is attached, the default kernel function runs.
>     return cmdline_proc_show(m, v);
> }
> 
> However, as Song explained below, if we want atomic replace to work,
> we may need to wait for the new BPF program here. But that would make
> the combination of livepatch and BPF more complex.
> 
> Currently, on our production servers, we handle this through a user
> script, such as:
> 
>   stop_traffic_relying_on_livepatch_bpf
>   kpatch load new-livepatch-bpf-module.ko
>   reattach_the_bpf_program
>   start_the_traffic_again
> 
> Although this approach requires restarting the affected traffic, other
> services running on the same server remain unaffected.

We put a lot of effort to make livepatching as less disruptive
as possible. The atomic replace is supposed to work without
any disruption.

> > >         return klp_enable_patch(&patch);
> > > }
> >
> > Yes, something in this direction is needed to make atomic replace work.
> > We have no plan to use this in production. I will let Yafang figure out
> > his plan.
> >
> > > Or maybe, the bpf_struct_ops can be _allocated dynamically_ and
> > > the pointer might be _passed via shadow variables_.
> > >
> > > One problem is that shadow variables would add another overhead
> > > and need not be suitable for hot paths.
> > >
> > >
> > > Anyway, I think that I have similar feelings as Miroslav.
> > > The combination of livepatches and BPF programs increases
> > > the complexity for all involved parties: core kernel maintainers,
> > > livepatch and BPF program authors, and system maintainers.
> > >
> > > Do we really want to propagate it?
> > > Is there any significant advantage in combining these two, please?
> > > Is it significantly easier to write BPF program then a livepatch?
> > > Is it significantly easier to update BPF programs then livepatches?
> 
> This is an important feature for avoiding server restarts,
> particularly in a VM host environment. Since only one VM on the host
> may be affected by this feature, we can deploy it rapidly without
> impacting other VMs on the same host.

This does not answer the question why you need the combination
of livepatch + BPF. Why a livepatch is not enough?

Let me repeat the questions:

Is it significantly easier to write BPF program then a livepatch?
Is it significantly easier to install BPF programs then livepatches?

> > > Would the support of different replace tags help?
> > > They would allow to replace only livepatches with the same tag.
> 
> Right, it will help.

Would this make a rapid update of livepatches easy enough so that
you won't need the BPF part?

Best Regards,
Petr