linux-kernel - Re: [PATCH v4] livepatch: fix race between fork and KLP transition

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Date:   Tue, 02 Aug 2022 16:07:08 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, kernel-team@...com,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Breno Leitao <leitao@...ian.org>
Subject: Re: [PATCH v4] livepatch: fix race between fork and KLP transition

On Thu, 2022-07-28 at 17:37 +0200, Petr Mladek wrote:
> On Wed 2022-07-27 10:24:37, Rik van Riel wrote:
> > v4: address changelog comments by Josh (thank you)
> > 
> > ---8<---
> > When a KLP fails to apply, klp_reverse_transition will clear the
> > TIF_PATCH_PENDING flag on all tasks, except for newly created tasks
> > which are not on the task list yet.
> 
> It actually is not true. klp_reverse_transtion() clears
> TIF_PATCH_FLAG only
> temporary when it waits until all processes leave the ftrace
> handler. It sets TIF_PATCH_FLAG once again for all tasks by calling
> klp_start_transition().
> 
> The difference is important. The WARN_ON_ONCE() in
> klp_complete_transition() will be printed when fork() copied
> TIF_PATCH_FLAG before it was set again.
> 
> Anyway, the important thing is that TIF_PATCH_FLAG and task-
> >patch_state
> might be incompatible because fork() copies them at different times.
> 
> klp_copy_process() must make sure that they are in sync. And
> it must be done under tasklist_lock when the child is added
> to the global task list.

Hmmm, how should this be addressed in the changelog?

Should I just remove most of that paragraph and leave it
at "there can be a race"?

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