linux-kernel - Re: [PATCH] livepatch: Add KLP

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Date: Thu, 4 Apr 2024 17:17:57 +0200
From: Petr Mladek <pmladek@...e.com>
To: Joe Lawrence <joe.lawrence@...hat.com>
Cc: zhangwarden@...il.com, jpoimboe@...nel.org, mbenes@...e.cz,
	jikos@...nel.org, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: Add KLP_IDLE state

On Tue 2024-04-02 09:52:31, Joe Lawrence wrote:
> On Tue, Apr 02, 2024 at 11:09:54AM +0800, zhangwarden@...il.com wrote:
> > From: Wardenjohn <zhangwarden@...il.com>
> > 
> > In livepatch, using KLP_UNDEFINED is seems to be confused.
> > When kernel is ready, livepatch is ready too, which state is
> > idle but not undefined. What's more, if one livepatch process
> > is finished, the klp state should be idle rather than undefined.
> > 
> > Therefore, using KLP_IDLE to replace KLP_UNDEFINED is much better
> > in reading and understanding.
> > ---
> >  include/linux/livepatch.h     |  1 +
> >  kernel/livepatch/patch.c      |  2 +-
> >  kernel/livepatch/transition.c | 24 ++++++++++++------------
> >  3 files changed, 14 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > index 9b9b38e89563..c1c53cd5b227 100644
> > --- a/include/linux/livepatch.h
> > +++ b/include/linux/livepatch.h
> > @@ -19,6 +19,7 @@
> >  
> >  /* task patch states */
> >  #define KLP_UNDEFINED	-1
> > +#define KLP_IDLE       -1
> 
> Hi Wardenjohn,
> 
> Quick question, does this patch intend to:
> 
> - Completely replace KLP_UNDEFINED with KLP_IDLE
> - Introduce KLP_IDLE as an added, fourth potential state
> - Introduce KLP_IDLE as synonym of sorts for KLP_UNDEFINED under certain
>   conditions
> 
> I ask because this patch leaves KLP_UNDEFINED defined and used in other
> parts of the tree (ie, init/init_task.c), yet KLP_IDLE is added and
> continues to use the same -1 enumeration.

Having two names for the same state adds more harm than good.

Honestly, neither "task->patch_state == KLP_UNDEFINED" nor "KLP_IDLE"
make much sense.

The problem is in the variable name. It is not a state of a patch.
It is the state of the transition. The right solution would be
something like:

  klp_target_state -> klp_transition_target_state
  task->patch_state -> task->klp_transition_state
  KLP_UNKNOWN -> KLP_IDLE

But it would also require renaming:

  /proc/<pid>/patch_state -> klp_transition_state

which might break userspace tools => likely not acceptable.


My opinion:

It would be nice to clean this up but it does not look worth the
effort.

Best Regards,
Petr