linux-kernel - Re: [PATCH v2] livepatch: Don't block removal of patches that are safe to unload

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Date:   Tue, 8 Mar 2022 18:49:28 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Chengming Zhou <zhouchengming@...edance.com>, jpoimboe@...hat.com,
        jikos@...nel.org, joe.lawrence@...hat.com,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] livepatch: Don't block removal of patches that are
 safe to unload

On Tue, 8 Mar 2022, Petr Mladek wrote:

> On Thu 2022-03-03 18:54:46, Chengming Zhou wrote:
> > module_put() is currently never called for a patch with forced flag, to block
> > the removal of that patch module that might still be in use after a forced
> > transition.
> > 
> > But klp_force_transition() will set all patches on the list to be forced, since
> > commit d67a53720966 ("livepatch: Remove ordering (stacking) of the livepatches")
> > has removed stack ordering of the livepatches, it will cause all other patches can't
> > be unloaded after disabled even if they have completed the KLP_UNPATCHED transition.
> > 
> > In fact, we don't need to set a patch to forced if it's a KLP_PATCHED forced
> > transition. It can still be unloaded safely as long as it has passed through
> > the consistency model in KLP_UNPATCHED transition.
> 
> It really looks safe. klp_check_stack_func() makes sure that @new_func
> is not on the stack when klp_target_state == KLP_UNPATCHED. As a
> result, the system should not be using code from the livepatch module
> when KLP_UNPATCHED transition cleanly finished.
> 
> 
> > But the exception is when force transition of an atomic replace patch, we
> > have to set all previous patches to forced, or they will be removed at
> > the end of klp_try_complete_transition().
> > 
> > This patch only set the klp_transition_patch to be forced in KLP_UNPATCHED
> > case, and keep the old behavior when in atomic replace case.
> > 
> > Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> > ---
> > v2: interact nicely with the atomic replace feature noted by Miroslav.
> > ---
> >  kernel/livepatch/transition.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index 5683ac0d2566..34ffb8c014ed 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -641,6 +641,10 @@ void klp_force_transition(void)
> >  	for_each_possible_cpu(cpu)
> >  		klp_update_patch_state(idle_task(cpu));
> >  
> > -	klp_for_each_patch(patch)
> > -		patch->forced = true;
> > +	if (klp_target_state == KLP_UNPATCHED)
> > +		klp_transition_patch->forced = true;
> > +	else if (klp_transition_patch->replace) {
> > +		klp_for_each_patch(patch)
> > +			patch->forced = true;
> 
> This works only because there is should be only one patch when
> klp_target_state == KLP_UNPATCHED and
> klp_transition_patch->forced == true.

I probably misunderstand, but the above is not generally true, is it? I 
mean, if the transition patch is forced during its disablement, it does 
not say anything about the amount of enabled patches.

> But it is a bit tricky. I would do it the other way:
> 
> 	if (klp_transition_patch->replace) {
> 		klp_for_each_patch(patch)
> 			patch->forced = true;
> 	} else if (klp_target_state == KLP_UNPATCHED) {
> 		klp_transition_patch->forced = true;
> 	}
> 
> It looks more sane. And it makes it more clear
> that the special handling of KLP_UNPATCHED transition
> is done only when the atomic replace is not used.

But it is not the same. ->replace being true only comes into play when a 
patch is enabled. If it is disabled, then it behaves like any other patch.

So, if there is ->replace patch enabled (and it is the only patch present) 
and then more !->replace patches are loaded and then if ->replace patch is 
disabled and forced, your proposal would give a different result than what 
Chengming submitted, because in your case all the other patches will get 
->forced set to true, while it is not the case in the original. It would 
be an unnecessary restriction if I am not missing something.

However, I may got lost somewhere along the way.

Regards
Miroslav