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

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Message-ID: <849e57ee-d412-30bd-3cce-47ce3362409d@bytedance.com>
Date:   Thu, 10 Mar 2022 20:57:54 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Miroslav Benes <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>
Cc:     jpoimboe@...hat.com, jikos@...nel.org, joe.lawrence@...hat.com,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2] livepatch: Don't block removal of
 patches that are safe to unload

Hi,

On 2022/3/9 1:49 上午, Miroslav Benes wrote:
> 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.

At first glance, I thought both way is right. But after looking at the case
you mentioned above, they are not the same indeed. The original patch
treat ->replace and not ->replace patches the same in KLP_UNPATCHED transition,
and only set all patches to forced in the atomic replace transition.

Thanks.

> 
> However, I may got lost somewhere along the way.
> 
> Regards
> Miroslav