linux-kernel - Re: [PATCH] kcsan, debugfs: Move debugfs file creation out of early init

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Date:   Wed, 3 Mar 2021 11:18:06 +0100
From:   Marco Elver <elver@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     rafael@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] kcsan, debugfs: Move debugfs file creation out of early init

On Wed, 3 Mar 2021 at 10:57, Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Wed, Mar 03, 2021 at 10:38:45AM +0100, Marco Elver wrote:
> > Commit 56348560d495 ("debugfs: do not attempt to create a new file
> > before the filesystem is initalized") forbids creating new debugfs files
> > until debugfs is fully initialized. This breaks KCSAN's debugfs file
> > creation, which happened at the end of __init().
>
> How did it "break" it?  The files shouldn't have actually been created,
> right?

Right, with 56348560d495 the debugfs file isn't created anymore, which
is the problem. Before 56348560d495 the file exists (syzbot wants the
file to exist.)

> > There is no reason to create the debugfs file during early
> > initialization. Therefore, move it into a late_initcall() callback.
> >
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > Cc: stable <stable@...r.kernel.org>
> > Fixes: 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
> > Signed-off-by: Marco Elver <elver@...gle.com>
> > ---
> > I've marked this for 'stable', since 56348560d495 is also intended for
> > stable, and would subsequently break KCSAN in all stable kernels where
> > KCSAN is available (since 5.8).
>
> No objection from me, just odd that this actually fixes anything :)

56348560d495 causes the file to just not be created if we try to
create at the end of __init(). Having it created as late as
late_initcall() gets us the file back.

When you say "fixes anything", should the file be created even though
it's at the end of __init()? Perhaps I misunderstood what 56348560d495
changes, but I verified it to be the problem by reverting (upon which
the file exists as expected).

> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

Thanks! Would it be possible to get this into 5.12?

-- Marco