linux-kernel - Re: [PATCH v3] livepatch: Don't block removal of patches that are safe to unload

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Message-ID: <alpine.LSU.2.21.2203161536330.6444@pobox.suse.cz>
Date:   Wed, 16 Mar 2022 15:48:25 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Chengming Zhou <zhouchengming@...edance.com>
cc:     jpoimboe@...hat.com, jikos@...nel.org, pmladek@...e.com,
        joe.lawrence@...hat.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, songmuchun@...edance.com,
        qirui.001@...edance.com
Subject: Re: [PATCH v3] livepatch: Don't block removal of patches that are
 safe to unload

On Sat, 12 Mar 2022, Chengming Zhou wrote:

> module_put() is not called for a patch with "forced" flag. It should
> block the removal of the livepatch module when the code might still
> be in use after forced transition.
> 
> klp_force_transition() currently sets "forced" flag for all patches on
> the list.
> 
> In fact, any patch can be safely unloaded when it passed through
> the consistency model in KLP_UNPATCHED transition.
> 
> By other words, the "forced" flag must be set only for livepatches

s/By/In/

> that are being removed. In particular, set the "forced" flag:
> 
>   + only for klp_transition_patch when the transition to KLP_UNPATCHED
>     state was forced.
> 
>   + all replaced patches when the transition to KLP_PATCHED state was
>     forced and the patch was replacing the existing patches.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
> Changes in v3:
>  - rewrite more clear commit message by Petr.
> 
> Changes in v2:
>  - interact nicely with the atomic replace feature noted by Miroslav.
> ---
>  kernel/livepatch/transition.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 5683ac0d2566..7f25a5ae89f6 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -641,6 +641,18 @@ 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;
> +	/*
> +	 * Only need to set forced flag for the transition patch
> +	 * when force transition to KLP_UNPATCHED state, but
> +	 * have to set forced flag for all replaced patches
> +	 * when force atomic replace transition.
> +	 */

How about something like

/*
 * Set forced flag for patches being removed, which is the transition
 * patch in KLP_UNPATCHED state or all replaced patches when forcing
 * the atomic replace transition.
 */

?

> +	if (klp_target_state == KLP_UNPATCHED)
> +		klp_transition_patch->forced = true;
> +	else if (klp_transition_patch->replace) {
> +		klp_for_each_patch(patch) {
> +			if (patch != klp_transition_patch)
> +				patch->forced = true;
> +		}
> +	}

Looks good to me.

Miroslav