linux-kernel - Re: [PATCH v2] livepatch: Remove immediate feature

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Message-ID: <nycvar.YFH.7.76.1801111059400.11852@cbobk.fhfr.pm>
Date:   Thu, 11 Jan 2018 11:00:42 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>
cc:     jpoimboe@...hat.com, jeyu@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, pmladek@...e.com, jbaron@...mai.com
Subject: Re: [PATCH v2] livepatch: Remove immediate feature

On Wed, 10 Jan 2018, Miroslav Benes wrote:

> Immediate flag has been used to disable per-task consistency and patch
> all tasks immediately. It could be useful if the patch doesn't change any
> function or data semantics.
> 
> However, it causes problems on its own. The consistency problem is
> currently broken with respect to immediate patches.
> 
> func            a
> patches         1i
>                 2i
>                 3
> 
> When the patch 3 is applied, only 2i function is checked (by stack
> checking facility). There might be a task sleeping in 1i though. Such
> task is migrated to 3, because we do not check 1i in
> klp_check_stack_func() at all.
> 
> Coming atomic replace feature would be easier to implement and more
> reliable without immediate.
> 
> Thus, remove immediate feature completely and save us from the problems.
> 
> Note that force feature has the similar problem. However it is
> considered as a last resort. If used, administrator should not apply any
> new live patches and should plan for reboot into an updated kernel.
> 
> The architectures would now need to provide HAVE_RELIABLE_STACKTRACE to
> fully support livepatch.
> 
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> ---
> v2 changes:
> - 2/2 from v1 dropped
> - documentation

It's very refreshing to have to handle something non-toxic (no PTI, no 
retpolines) finally :)

Applied to for-4.16/remove-immediate, thanks.

-- 
Jiri Kosina
SUSE Labs