Replace rules: was: Re: [PATCH v7 for-next 3/8] livepatch: Implement replace set for scoped atomic replace

[Petr Mladek <pmladek@suse.com>]

On Tue 2026-08-25 19:46:36, Yafang Shao wrote:
> The current bool replace flag is too coarse: it is either all or
> nothing. A livepatch with .replace=true replaces ALL existing
> livepatches, which is safe but inflexible. There is no way to have
> multiple independent livepatch sets coexist on the same system.
> 
> Replace it with a more flexible model using two new fields in
> struct klp_patch:
> 
> - provides: an unsigned int id identifying the patch replace set.
>   By default (provides=0), any livepatch replaces any other livepatch.
>
> - obsoletes: an optional array of unsigned int ids specifying
>   additional provides ids to be replaced. This allows a new patch
>   to explicitly obsolete patches from different replace sets.

> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -123,7 +123,8 @@ struct klp_state {
>   * @mod:	reference to the live patch module
>   * @objs:	object entries for kernel objects to be patched
>   * @states:	system states that can get modified
> - * @replace:	replace all actively used patches
> + * @provides:	only one active livepatch per id
> + * @obsoletes:	replace given livepatch id(s)
>   * @list:	list node for global list of actively used patches
>   * @kobj:	kobject for sysfs resources
>   * @obj_list:	dynamic list of the object entries
> @@ -137,7 +138,9 @@ struct klp_patch {
>  	struct module *mod;
>  	struct klp_object *objs;
>  	struct klp_state *states;
> -	bool replace;
> +	unsigned int provides;
> +	unsigned int *obsoletes;
> +	unsigned int nr_obsoletes;

This is a different sematic in compare with the other arrays.
I guess that you wanted to allow obsoleting livepatch with '0' ID.

But '0' is special. It obsoletes anything. Maybe, we could
make it even more special and say that it can't obsoleted.
Then we would be able to use it as the trailing element
in the array...

I do not have strong opinion about this. It is just an idea.


Another question:

Should we allow to enable a livepatch when its provides id
is obsoleted by a currently enabled livepatch?

For example, let's have:

  + Livepatch A: provides:1
  + Livepatch B: provides:2, obsoletes:1

Now, two scenarios:

  1. Livepatch A can be replaced by livepatch B. This is easy.
  2. Can livepatch B get replaced by livepatch A?

The current code would allow to replace B with A when there is
_no_ real conflict in the livaptched objects, functions, and states.

But does it make sense?

Reasoning: Livepatch B obsoleted the livepatch A for a reason.
	It sounds like they should not be enabled at the same time.


Special case:

Should we allow to install a livepatch with non-zero provides
when a livepatch with zero provides is installed.

For example, let's have:

  + Livepatch A: provides:1
  + Livepatch B: provides:0

Now, two scenarios:

  1. Livepatch A can be replaced by livepatch B. This is easy.
  2. Can livepatch A be installed in parallel with B?

Reasoning: The livepatch B replaces everything because it wants
	to be the only installed livepatch. It sounds weird
	to "break" it by installing A in parallel later again.


Best Regards,
Petr